  The Linux Text-Terminal-HOWTO
  David S. Lawyer  <mailto:dave@lafn.org>
  v1.10, 22 December 1999

  This document explains what text terminals are, how they work, how to
  install and configure them, and provides some info on how to repair
  them.  It may be of some use even if you don't have a terminal manual.
  While it's written for real terminals on a Linux system, some of it is
  also applicable to terminal emulation and may be helpful for non-Linux
  systems.
  ______________________________________________________________________

  Table of Contents





















































  1. Introduction

     1.1 Copyright, Trademarks, Disclaimer, & Credits
        1.1.1 Copyright
        1.1.2 Disclaimer
        1.1.3 Trademarks
        1.1.4 Credits
     1.2 Future Plans: You Can Help
     1.3 New Versions of this HOWTO
     1.4 Related HOWTO's
     1.5 Terminology Used in this Document
     1.6 What is a Terminal ?

  2. Types of Terminals

     2.1 Dumb Terminals
     2.2 Text Terminals
     2.3 Graphics Terminals
        2.3.1 Serial Line Graphics Terminals
        2.3.2 Fast Graphics Terminals (often known by other names)
     2.4 Network Computers (NCs)
     2.5 Emulation on a PC

  3. Quick Install

  4. Why Use a Terminal ?

     4.1 Intro to Why Use a Terminal
     4.2 Lower Hardware Costs ?
     4.3 Control of Software
     4.4 Hardware Upgrades
     4.5 Other Advantages of Terminals
     4.6 Major Disadvantages of Terminals
     4.7 Are Text Terminals Obsolete ?

  5. Overview of How Terminals Work (in Linux)

     5.1 Device Names
     5.2 Login/Logout
     5.3 Half/Full Duplex
     5.4 Terminal Memory
     5.5 Commands for the Terminal
     5.6 Lack of Standardization Solved by Terminfo
     5.7 The Interface
     5.8 Emulation
     5.9 The Console

  6. Terminal Special Files such as /dev/tty

     6.1 Serial Port Terminals
     6.2 Pseudo Terminals
     6.3 The Controlling Terminal /dev/tty
     6.4 /dev/ttyIN "Terminals"
     6.5 The Console: /dev/ttyN
     6.6 Creating a Device with "mknod"

  7. Some Details on How Terminals Work

     7.1 Terminal Memory Details
     7.2 Early Terminals
     7.3 Escape Sequences and Control Codes (intro)
        7.3.1 Control Codes
        7.3.2 Escape Sequences
     7.4 Display Attributes & Magic Cookies

  8. Special Features of Some Terminals
     8.1 Color
     8.2 Multiple Sessions
     8.3 Printer/Auxiliary Port
     8.4 Pages
     8.5 Character-Sets
     8.6 Fonts
     8.7 Keyboards & Special Keys

  9. Terminal Emulation; the Console

     9.1 Intro to Terminal Emulation
     9.2 Don't Use TERM For Emulation
     9.3 Communication (Dialing) programs
        9.3.1 Emulation under X-Windows
        9.3.2 Real Terminals Better
     9.4 Testing Terminal Emulation
     9.5 The Linux Console
     9.6 Emulation Software
        9.6.1 Make a Linux PC a terminal
        9.6.2 Make a non-Linux PC a terminal

  10. Flow Control (Handshaking)

     10.1 Why Is Flow Control Needed ?
     10.2 Padding
     10.3 Overrunning a Serial Port
     10.4 Stop Sending
     10.5 Keyboard Lock
     10.6 Resume Sending
     10.7 Hardware Flow Control (RTS/CTS etc.)
        10.7.1 RTS/CTS, DTR, and DTR/DSR Flow Control
        10.7.2 Connecting Up DTR or DTR/DSR Flow Control
        10.7.3 Old RTS/CTS Handshaking Is Different
        10.7.4 Reverse Channel
     10.8 Is Hardware Flow Control Done by Hardware ?
     10.9 Obsolete ?? ETX/ACK or ENQ/ACK Flow Control

  11. Physical Connection

     11.1 Multiport I/O Cards (Adapters)
     11.2 Direct Cable Connection.
        11.2.1 Null Modem Cable Pin-out (3, 4, or 5 conductor)
        11.2.2 Standard Null Modem Cable Pin-out (7 conductor)
        11.2.3 Length Limitations
        11.2.4 Hardware Flow Control Cables
        11.2.5 Cable Tips
        11.2.6 A Kludge using Twisted-Pair Cable
        11.2.7 Cable Grounding
     11.3 Modem Connection
        11.3.1 Dialing Out From a Terminal
        11.3.2 Terminal Gets Dialed Into
     11.4 Terminal Server Connection
     11.5 Connector and Adapter Types
        11.5.1 Sex of Connector/Adapters
        11.5.2 Types of Adapters
        11.5.3 DB Connectors
        11.5.4 RJ Modular Connectors
     11.6 Making or Modifying a Cable
        11.6.1 Buy or Make ?
        11.6.2 Pin Numbers
        11.6.3 Installing DB Connectors on Cable Ends
        11.6.4 Installing RJ Connectors

  12. Set-Up (Configure) in General

     12.1 Intro to Set-Up
     12.2 Terminal Set-Up (Configure) Overview
     12.3 Computer Set-Up (Configure) Overview
     12.4 Many Options
     12.5 Communication Interface Options
        12.5.1 Speed
        12.5.2 Parity & Should You Use It ?
        12.5.3 Bits/Character
        12.5.4 Which Flow Control (Handshaking) ?
        12.5.5 Port Select
     12.6 Quick Attempt

  13. Terminal Set-Up (Configure) Details

     13.1 Send Escape Sequences to the Terminal
     13.2 Older Terminals Set-Up
     13.3 Getting Into Set-Up (Configuration) Mode
     13.4 Communication Options
     13.5 Saving the Set-up
     13.6 Set-Up Options/Parameters
     13.7 Emulation {Personality} {{Terminal Modes}}
     13.8 Display Options
        13.8.1 Character Cell Size {Char Cell}
        13.8.2 Columns/Lines
        13.8.3 Cursor
        13.8.4 Display Attributes (Magic Cookies)
        13.8.5 Display Control Characters {Monitor}
        13.8.6 Double Width/Height
        13.8.7 Reverse Video {Display} (Background Light/Dark)
        13.8.8 Status Line
        13.8.9 Upon 80/132 Change: Clear or Preserve?
     13.9 Page Related Options
        13.9.1 Page Size
        13.9.2 Coupling (of cursor & display)
     13.10 Reporting and Answerback
        13.10.1 Answerback Message (String)
        13.10.2 Auto Answerback
        13.10.3 Answerback Concealed
        13.10.4 Terminal ID {ANSI ID}
     13.11 Keyboard Options
        13.11.1 Keyclick
        13.11.2 Caps Lock {Keylock}
        13.11.3 Auto Repeat {Repeat}
        13.11.4 Margin Bell
        13.11.5 Remapping the Keys
        13.11.6 Corner Key (for Wyse only)
        13.11.7 Numeric Keypad or Arrow Keys Sends
        13.11.8 What does shifted-del and shifted-bs send?
        13.11.9 PC Scan Codes
        13.11.10 Alternate Characters
     13.12 Meaning of Received Control Codes
        13.12.1 Auto New Line {Newline}
        13.12.2 Auto Line Feed {Rcv CR}
        13.12.3 Recognize Del (Wyse Only ??) or Null
     13.13 Where New Text Goes
        13.13.1 Line Wrap
        13.13.2 Scrolling
        13.13.3 New Page?
     13.14 Function Keys
     13.15 Block Mode Options
        13.15.1 Forms Display
        13.15.2 Send Entire Block ?
        13.15.3 Region to Send
        13.15.4 Block/Page terminator
     13.16 Locks
     13.17 Screen Saver {Scrn Saver}
     13.18 Printer
  14. Computer Set-Up (Configure) Details

     14.1 Getty (in /etc/inittab)
        14.1.1 Agetty (may be named getty)
           14.1.1.1 Agetty's detection of parity
           14.1.1.2 Agetty's parity with 8-bit data bytes
        14.1.2 getty (part of getty_ps)
        14.1.3 mgetty
     14.2 Stty & Setserial
     14.3 Setserial
        14.3.1 Introduction
        14.3.2 Probing
        14.3.3 Can Linux Configure The Serial Devices Automagically?
        14.3.4 Boot-time Configuration
        14.3.5 New configuration method using /etc/serial.conf
        14.3.6 Old configuration method: edit a script
        14.3.7 IRQs
     14.4 Stty
        14.4.1 Introduction
        14.4.2 Using stty for a "foreign" terminal
        14.4.3 Two interfaces at a terminal
        14.4.4 Where to put the stty command ?
     14.5 Terminfo & Termcap (brief)
     14.6 Setting TERM and TERMINFO
        14.6.1 What is the terminfo name of my terminal ?
     14.7 Rarely Needed /etc/ttytype File
     14.8 Login Restrictions
     14.9 Run Command Only If TERM=my_term_type
        14.9.1 Example for ls Function

  15. Terminfo and Termcap (detailed)

     15.1 Intro to Terminfo
     15.2 Terminfo Database
        15.2.1 Introduction
        15.2.2 Where is the database located ?
           15.2.2.1 Compiled database locations
           15.2.2.2 Source-code database locations
        15.2.3 Terminfo Compiler (tic)
        15.2.4 Look at Your Terminfo
        15.2.5 Deleting Data Not Needed
     15.3 Bugs in Existing Terminfo Files (and Hardware)
     15.4 Modifying Terminfo Files
     15.5 Init String
     15.6 TERM Variable
     15.7 Terminfo/Termcap Documents

  16. Using the Terminal

     16.1 Intro to Using Terminal
     16.2 Starting Up the Terminal
     16.3 Terminal (Serial) Device Driver
     16.4 Problems with Editors
        16.4.1 emacs and ^S, ^Q
        16.4.2 vi and Cursor-Keys
     16.5 Color ls Corruption
     16.6 Display Freezes (hung terminal)
     16.7 Corrupted Terminal Interface
        16.7.1 Symptoms
        16.7.2 Sent Terminal Binary Characters
        16.7.3 Abnormally Exited a Program
     16.8 Special (Control) Characters
        16.8.1 Command-Line Editing
        16.8.2 Interrupting (& Quit, Suspend, EOF, Flush)
        16.8.3 Stop/Start Scrolling
        16.8.4 Take Next Character Literally
     16.9 Viewing Latin-1 Files on a 7-bit Terminal
     16.10 Inspecting the Interface
     16.11 Changing the Terminal Settings
        16.11.1 setterm
        16.11.2 tput
        16.11.3 echo
        16.11.4 Saving Changes
     16.12 Make a Terminal the Console
        16.12.1 For Kernels 2.2 or higher
        16.12.2 For Kernels before 2.2
        16.12.3 Can I Run Linux without a Monitor (PC Console) ?
     16.13 Multiple Sessions
     16.14 Logging Out
     16.15 Chatting between Terminals, Spying

  17. Trouble-Shooting (software)

     17.1 Terminal Was Working OK
     17.2 Terminal Newly Installed
     17.3 Is the Terminal OK ?
     17.4 Missing Text
     17.5 Getty Respawning Too Rapidly
        17.5.1 Serial Module Not Loaded
        17.5.2 No Modem Control Voltage
        17.5.3 Key Shorted
     17.6 Fails Just After Login
     17.7 Can't Login
     17.8 Garbled Login Prompt
     17.9 No Sign of any Login Prompt
        17.9.1 Diagnose Problem from the Console
        17.9.2 Measure Voltages
     17.10 Slow: pauses of several seconds between bursts of characters
     17.11 Serial Monitoring/Diagnostics
     17.12 Local Mode
     17.13 Serial Electrical Test Equipment
        17.13.1 Breakout Gadgets, etc.
        17.13.2 Measuring Voltages
        17.13.3 Taste Voltage

  18. Repair & Diagnose

     18.1 Repair Books & Websites
        18.1.1 Books
        18.1.2 Websites
     18.2 Safety
     18.3 Appearance of Display
     18.4 Diagnose
        18.4.1 Terminal Made a Noise
        18.4.2 Terminal Made No Noise
     18.5 Error Messages on the Screen
        18.5.1 Keyboard Error
        18.5.2 Checksum Error in NVR
     18.6 Capacitors
     18.7 Keyboards
        18.7.1 Interchangeability
        18.7.2 How They Work
        18.7.3 One Press Types 2 Different Characters
        18.7.4 Modern vs Old Keyboards
        18.7.5 Keyboard Doesn't Work At All
        18.7.6 Typing b Displays bb, etc. (doubled)
        18.7.7 The Keyboard Types By Itself
        18.7.8 Liquid Spilled on the Keyboard
        18.7.9 Cleaning Keyboard Contacts
           18.7.9.1 Keyboards with Membranes
           18.7.9.2 Keyboards with Individual Switches

  19. Appendix A: General

     19.1 List of Linux Terminal Commands
        19.1.1 Sending a Command to the Terminal
        19.1.2 Configuring the Terminal Device Driver
        19.1.3 Terminfo
        19.1.4 Other
     19.2 The Internet and Books
        19.2.1 Terminal Info on the Internet
        19.2.2 Books Related to Terminals
        19.2.3 Entire Books on Terminals
        19.2.4 Books with Chapters on Terminals
     19.3 Non-Linux OS's

  20. Appendix B: Escape Sequence Commands Terminology

     20.1 Esc Sequence List
     20.2 8-bit Control Codes
     20.3 Printer Esc
     20.4 Reports
     20.5 Cursor Movements
     20.6 Pages (definition)

  21. Appendix C: Serial Communications on EIA-232 (RS-232)

     21.1 Intro to Serial Communication
     21.2 Voltages
        21.2.1 Voltage for a Bit
        21.2.2 Voltage Sequence for a Byte
     21.3 Parity Explained
     21.4 Forming a Byte (Framing)
     21.5 Limitations of EIA-232
        21.5.1 Low Speed & Short Distance
        21.5.2 Successors to EIA-232
        21.5.3 Line Drivers
     21.6 Synchronization & Synchronous
        21.6.1 How "Asynchronous" is Synchronized
        21.6.2 Defining Asynchronous vs Synchronous
        21.6.3 Synchronous Communication
     21.7 Block Mode
        21.7.1 Intro to Block Mode
        21.7.2 Types of Block Modes, Forms
        21.7.3 Efficiency
     21.8 EIA-232 (RS-232) Books
     21.9 Serial Software

  22. Appendix D: Notes by Brand Name

     22.1 CIT
     22.2 IBM Terminals
        22.2.1 IBM 3153
     22.3 Teletypes
     22.4 VT (DEC)
     22.5 Wyse
        22.5.1 Wyse 60
        22.5.2 Wyse 99-GT
        22.5.3 Wyse 150


  ______________________________________________________________________

  1.  Introduction

  For a quick attempt to install a terminal see ``Quick Install''.


  1.1.  Copyright, Trademarks, Disclaimer, & Credits

  1.1.1.  Copyright

  Copyright 1998-9 by David S. Lawyer.

  Please freely copy and distribute (sell or give away) this document in
  any format.  Forward any corrections and comments to the document
  maintainer.  You may create a derivative work and distribute it
  provided that you:


  1. Send your derivative work (in the most suitable format such as
     sgml) to the LDP (Linux Documentation Project) or the like for
     posting on the Internet.  If not the LDP, then let the LDP know
     where it is available.  Except for a translation, send a copy to
     the previous maintainer's url as shown in the latest version.

  2. License the derivative work in the spirit of this license or use
     GPL.  Include a copyright notice and at least a pointer to the
     license used.

  3. Give due credit to previous authors and major contributors.

  If you're considering making a derived work other than a translation,
  it's requested that you discuss your plans with the current
  maintainer.


  1.1.2.  Disclaimer

  While I haven't intentionally tried to mislead you, there are likely a
  number of errors in this document.  Please let me know about them.
  Since this is free documentation, it should be obvious that I cannot
  be held legally responsible for any errors.



  1.1.3.  Trademarks

  If certain words are trademarks, the context should make it clear to
  whom they belong.  For example "MS Windows NT" implies that "Windows
  NT" belongs to Microsoft (MS). Mac is by Apple Computer.  Trademarks
  belong to their respective owners.


  1.1.4.  Credits

  Much of the section "Physical Connection" is from Serial-HOWTO v.
  1.11 by Greg Hankins (with his permission).  His "How Do I Set Up A
  Terminal Connected To My PC?" was incorporated into v1.00 at various
  places.  v1.09 has about 25 changes (and error corrections) suggested
  by Alessandro Rubini who reviewed this HOWTO.


  1.2.  Future Plans: You Can Help

  Please let me know of any errors in facts, opinions, logic, spelling,
  grammar, clarity, links, etc.  But first, if the date is over a few
  months old, check to see that you have the latest version.  Please
  send me any info that you think belongs in this document.

  Starting with version 1.00, a first attempt was made to help people
  set up terminals without recourse to a terminal manual.  Much more is
  needed in this respect.  One way to solve this problem would be if
  terminal manufacturers put their manuals on the Internet.  I suggest
  that you encourage them to do so.  The task of providing information
  on how to configure most terminals in this HOWTO is daunting.  There
  are so many different terminals, but there are far fewer models than
  there used to be in the 1980,s so the task is not totally infeasible.

  Please send me any surplus terminal manuals which you may have,
  especially on terminals made within the past 10 years (but I'll accept
  older ones also).  Also, you might want to write up something on a
  certain terminal to put in the Appendix D: Notes by Brand Name.


  1.3.  New Versions of this HOWTO

  New versions of the Text-Terminal-HOWTO will be available to browse
  and/or download at LDP mirror sites.  For a list of mirror sites see:
  <http://metalab.unc.edu/LDP/mirrors.html>.  Various formats are
  available.  If you only want to quickly check the date of the latest
  version look at  <http://metalab.unc.edu/LDP/HOWTO/Text-Terminal-
  HOWTO.html>.  The version your are currently reading is v1.09, 28
  November 1999.


  1.4.  Related HOWTO's

  Go to the websites shown above to get these.

  o  Serial-HOWTO has info on Multiport Serial Cards used for both
     terminals and banks of modems.  It has general technical info on
     the serial port including troubleshooting it.

  o  Modem-HOWTO

  o  Serial-Programming-HOWTO

  o  Xterminal-HOWTO (unmaintained).  It's at
     <http://sunsite.unc.edu/pub/Linux/docs/HOWTO/unmaintained/mini/Xterminal>


  1.5.  Terminology Used in this Document

  Configuration means the same as set-up.  While Linux commands take
  options (using - symbols), options in a broader sense include various
  other types of choices.  Install in the broad sense includes setting
  up (configuring) software and hardware.  A statement that I suspect is
  true (but may not be) ends with 2 question marks: ??  If you know for
  sure, let me know.


  1.6.  What is a Terminal ?

  A terminal consists of a screen and keyboard that one uses to
  communicate remotely with a (host) computer.  One uses it just like it
  was a personal computer but the terminal is remote from its host
  computer (on the other side of the room or even on the other side of
  the world).  Programs execute on the host computer but the results
  display on the terminal screen.  Its computational ability is
  relatively low (otherwise it would be a computer and not a terminal).
  This computational ability is generally limited to the ability to
  display what is sent to it (possibly including full-screen graphics)
  and the ability to send to the host what is typed at the keyboard.

  In the days of mainframes from the mid 1970's to the mid 1980's, most
  people used terminals to communicate with computers.  They typed in
  programs, ran programs, wrote documents, issued printing commands,
  etc.  A cable connected the terminal to the computer (often
  indirectly).  It was called a terminal since it was located at the
  terminal end of this cable.

  If you've been using Linux (except for X-Window use) with a computer
  monitor and keyboard you already know what a terminal is because you
  have been using one (or more precisely a "virtual terminal").  The
  monitor (along with the keyboard) is emulating a terminal.  In X-
  Windows the programs xterm, rxvt, and zterm emulate terminals.

  A real terminal is different from a monitor because it's a different
  electronic setup.  A text terminal is often connected to a serial port
  of the computer via a long cable.  Thus, in contrast to a monitor
  which is usually located right next to the computer, a terminal may be
  quite a distance away from its host computer.  The video card inside a
  computer stores the video image seen on the monitor screen.  For a
  terminal, the equivalent of this video card is built right into the
  terminal but since text terminals are often monochrome without much
  graphics, the capabilities of its "video card" are rather weak.  Also,
  most text terminals do not have mice.

  In network client-server terminology, one might think that the
  terminal is the client and that the host computer is the server.  The
  terminal has been called a "thin client" by some.  But it is not
  actually a "client" nor is the host a "server".  The only "service"
  the host provides is to receive every letter typed at the keyboard and
  react to this just like a computer would.  The terminal is like a
  window into the computer just like a monitor (and keyboard) are.  You
  may have already used virtual terminals in Linux (by pressing Left
  Alt-F2, etc.).  A real terminal is just like running such a virtual
  terminal but you run it on its own terminal screen instead of having
  to share the monitor screen.  In contrast to using a virtual terminal
  at the console (monitor), this allows another person to sit at the
  real terminal and use the computer simultaneously with others.


  2.  Types of Terminals

  2.1.  Dumb Terminals

  There are various conflicting definitions of "dumb terminal" but as
  time goes by, more and more terminals are called dumb.  This document
  mainly covers text terminals which display only text on the screen.
  It might be titled "Dumb-Terminal-HOWTO" but in some magazines
  articles any terminal, no matter how smart, including ones which
  present a full graphical user interface (GUI), are called dumb.  If
  all terminals are "dumb" then there is no point of prefixing the word
  "dumb" to terminal (except as a sales pitch to sell computers or the
  like in place of "smart" terminals).  Due to the ambiguous meaning of
  "dumb terminal" it is not classified here as a type of terminal.


  2.2.  Text Terminals

  For a text terminal, a 2-way flow of information between the computer
  and the terminal takes place over the cable that connects them
  together.  This flow is in ASCII bytes where each byte usually
  represents a character.  Bytes typed at the keyboard go to the
  computer and most bytes from the computer are displayed on the
  terminal screen.  Special bytes (or sequences of bytes) from the
  computer tell the terminal where to move the cursor to, what to erase,
  where to begin and end underlining and/or blinking and/or bold, etc.
  There are often hundreds of such special commands and many terminals
  can even change fonts.

  The communication uses characters (letters) encoded using a code chart
  for the character set being used.  Usually, the first 128 bytes out of
  256 possible bytes use ASCII codes.  Terminals for Unix-like systems,
  normally connect to computers via a cable running between the
  asynchronous serial ports (RS-232-C = EIA-232-D) of the host computer
  and terminal.  Sometimes the connection is via modem or terminal
  server, etc.

  Other names for text terminals are "serial terminal", "character-cell
  terminal", "ASCII/ANSI terminal", "asynchronous terminal", "data
  terminal", "video terminal" and "video display terminal" (VDT).  In
  olden days "video display unit" (VDU) was used for terminals but
  strictly speaking, it excludes the keyboard.

  "Block mode" was used exclusively by old IBM mainframe terminals but
  many modern terminals also have this capability (which is not used
  much).  The characters you type are temporarily retained in the
  terminal memory (and may possibly be edited by a built-in editor at
  the terminal).  Then when the send key (or the like) is pressed, a
  block of characters (sometimes just a line of characters) is sent to
  the computer all at once.  Block mode (as of late 1998) is not
  supported by Linux.  See section ``Block Mode''.


  2.3.  Graphics Terminals

  To a limited degree some ASCII symbols can provide graphics on text
  terminals.  One may form  arrows <--- and draw boxes with _ and |.
  With special graphic character sets, even more is possible.  None of
  these are really graphics terminals.  However, the term "graphics
  terminal" is sometimes applied to all text-only terminals since text
  is a limited form of graphics.

  There are two basic types of graphics displays: raster and vector
  (rarely used).  Raster graphics (bit-mapped) puts dots on the screen
  by horizontal scan lines drawn by an electron beam (or by activating
  pixels or dots on a flat screen).  Vector graphic displays are usually
  for monochrome screens that don't have any dots.  They use smart
  electronics to draw lines and curves with an electron beam that can
  move in any direction (at any angle and location).  Vector graphics
  draws high quality lines without zig-zags but is both rare and
  expensive.  Raster graphics is almost universally used today.  For
  PC's, images encoded in vector graphic format are sometimes used but
  they are translated to raster graphics format for display (with a drop
  in image quality).


  2.3.1.  Serial Line Graphics Terminals

  Most of this document also applies to these.  Most of these can also
  function as text terminals.  The protocols for such graphics include:
  Tektronix Vector Graphics, ReGIS (DEC), Sixel (DEC), and NAPLPS (North
  American Presentation Level Protocol Syntax).


  2.3.2.  Fast Graphics Terminals (often known by other names)

  None of these covered in this document.  A terminal that deserves to
  be called smart is a graphics terminal which can rapidly display full-
  screen graphics just like a PC monitor.  It will also have a mouse.
  Bytes sent to it often represent bit-maps for pictures (and other
  graphics).  It will often use a high-speed connection to its host
  computer using twisted pair or coax cable.  X-Window terminals are
  such devices.  See the link to Xterminal-HOWTO at ``Related HOWTO's''

  For displaying a MS-Windows GUI there are at various types of
  interfaces and terminals: Winterm using WinFrame software from Citrix
  is one.  Another (based in part on Citrix's code) is Hydra (code name)
  by Microsoft, also known as "Windows Terminal Server" which works with
  versions 4 or higher of MS Windows NT.  Citrix uses its ICA protocol
  and has created an add-on to Hydra known as pICAsso so that WinFrame
  (ICA) based terminals can use the Hydra system.  Hydra is also multi-
  user.  There is also the "MultiConsole Personal Terminal" by Unbounded
  Technologies and Tektronix had its own multi-user interface but will
  now support Hydra.  A magazine article in 1997 called Winterm a "dumb
  terminal" but it's really pretty smart.  Such terminals are often
  called "thin clients", but some thin clients are more that just
  terminals as they can execute Java code sent to them, etc.


  2.4.  Network Computers (NCs)

  These are neither true computers nor terminals but are something in-
  between.  One type of network computer (NC's) is a computer with a CPU
  but no hard Disk.  They are full-graphics and connect to a server
  computer.  They are different from terminals since the programs they
  run execute on their own CPU chips.  Java code may be sent to them for
  execution.  IBM calls this a "Network Station".  They should work on
  IP networks and might work under a server running Linux.  Wintel
  established a "NetPC" which, unlike the above, is almost a PC
  computer.  However, it has no removable disks so users can't install
  their own software or obtain copies of anything.

  Although the promoters of NCs and related Window-Terminals projected
  that they would replace millions of PCs, it hasn't happened.  A major
  reason is that PCs have come down in price in recent years so that
  they are often even cheaper than NCs, etc.  Thus for terminals, the
  Text-Terminal still predominates.


  2.5.  Emulation on a PC

  Since a PC has a screen and keyboard (as does a terminal) but also has
  much more computing power, it's easy to use some of this computing
  power to make the PC computer behave like a text terminal.  This is
  called "terminal emulation".  They usually emulate text-terminals.
  See ``Terminal Emulation''


  3.  Quick Install

  This is a quick procedure to install a terminal without going through
  a ``Setup'' procedure for both the terminal and the host computer.  It
  probably will not work right if the terminal happens to have been set
  up incompatible with the computer.  If you don't understand some of it
  you'll need to consult other parts of this document for more info.

  To install a terminal, first look in /etc/termcap or terminfo.src to
  find an entry for it (see ``Terminfo and Termcap (detailed)'').
  Figure out what serial port you'll connect it to and what the tty
  designation is for that port (e.g. ttyS1, see ``Device Names'').  As
  the root user, edit /etc/inittab and add a getty command next to the
  other getty commands.  The format of the getty command depends on
  which getty program you use.  agetty (called just getty in the Debian
  distribution) is the easiest (no configuration file).  See the "info"
  or "man re getty.  For getty parameters use the terminfo (or termcap)
  name (such as vt100) for your terminal.  Type in a baud-rate that the
  terminal supports.  But if you set the baud too high you may need to
  use (See``Flow Control'').

  Then physically connect the main serial port of the terminal to the
  chosen serial port of the computer with a null-modem cable and turn on
  the terminal.  Don't expect most ready-made cables to be wired
  correctly for hardware flow control.  Make sure the baud-rate of the
  terminal is set the same as you gave to getty and that its "data bits"
  is 8.  Then at the computer console type "init q" to apply the changes
  you made to the inittab file.  You should now see a login prompt at
  the terminal.  If you don't, tap the terminal's return key.  If this
  doesn't work read more of this document and/or see ``Trouble-
  Shooting''.


  4.  Why Use a Terminal ?

  4.1.  Intro to Why Use a Terminal

  PC's are so powerful today that just one PC can often support several
  persons using it at once, especially if they are doing low-load tasks
  such as text editing, data entry, etc.  One way to do this is to
  connect a number of terminals to a single PC (or other host computer)
  by modems or direct cable connection.  To do this, it's usually best
  to have a multi-user operating system such as Linux so that each user
  at a terminal can use the computer independently.  This has been
  called "time sharing" but it's not good terminology today since
  "distributed" computing over a network is also a type of time sharing.
  It might be better described as "centralized" computing.  But the
  central computer may be connected to the rest of the world via a
  network so that terminal users may send email, browse the Internet
  with the "lynx" browser, etc.  So it's not exactly "centralized"
  either.

  Terminals have seldom been used with PC's because the popular
  operating systems used for them (Windows, DOS, and Mac) were not
  multiuser until 1998 (available for MS Windows NT) and previously
  could not support terminals very well.  Now that Linux, a multiuser
  operating system, is freely available for PC's, the use of terminals
  with PC's becomes more feasible.  The drawback is that text terminals
  are not smart enough to support the type of graphical user interface
  (GUI) that many computer users today normally expect.


  4.2.  Lower Hardware Costs ?

  When Computers (including PCs) were quite expensive, lower hardware
  costs was a significant advantage of using terminals.  Today with
  cheap PCs, the cost savings is problematical.  Here's what I wrote
  years ago when PCs were more expensive.  It's still true today but of
  less significance.

  If several people use the same computer as the same time, there is a
  reduction in the amount of hardware needed for the same level of
  service.  One type of savings is due to code sharing.  The application
  files on hard disks are shared as well as shared libraries in memory
  (even when people are running different programs provided they use
  some of the same functions in their code).  Another type of savings is
  due to reduction of peak load.  The hardware of a single PC may be
  idle most of the time as people slowly type in information, think,
  talk, or are away from their desks.  Having several people on the same
  computer at once makes good use of much of this idle time which would
  otherwise be wasted.

  These savings are substantial.  One may roughly estimate (using
  statistical theory) that for 9 persons (8 terminals & 1 console) the
  shared PC only needs only about 3 times as much capacity (in memory,
  disk storage, CPU power, etc.) as a single PC in order to provide the
  same level of service per person.  Thus the computational hardware for
  such a shared system should only cost about 1/3 as much per user.
  However, the cost of the display hardware (CRT's, keyboards, video
  electronics, etc.) is about the same for both cases.  The terminals
  have the added cost of requiring additional serial ports at the host
  computer.
  For a fair comparison with PC's, the terminals should have the same
  capabilities as the PC monitors.  Unfortunately, color graphic
  terminals for Linux (X-windows) with high speed communication is a
  niche market with high prices so in this case there is not likely to
  be any savings in hardware costs.  But for text terminals there will
  be some savings, especially if the terminals are obtained used at low
  cost.


  4.3.  Control of Software

  For centralized computing, software (and the updates to software) only
  need be installed and configured on one host computer instead of
  several.  The person in charge of this computer may control and
  configure the software which is installed on it.  This is advantageous
  if the person controlling the host computer does an excellent job and
  knows about the needs and preferences of the other users.  Users can
  be restricted in playing games or surfing the Internet by not
  installing the software (or by otherwise restricting access to it).
  Whether or not centralized control is desirable depends on the
  situation.


  4.4.  Hardware Upgrades

  With terminals, the computer hardware upgrades take place on only one
  computer instead of many.  This saves installation labor effort.
  While the cost of the hardware for the host computer upgrade will be
  more than that for a single PC (since the host needs more computing
  power than a PC), the cost will be significantly less than upgrading
  the hardware of a number of PC's being used instead of terminals.


  4.5.  Other Advantages of Terminals



  o  The elimination of noise from fans and disk drives provided the
     terminals are not close to the computer.

  o  The users of the terminals can share data and files and send e-mail
     to each other.  It's similar to a local network.


  4.6.  Major Disadvantages of Terminals



  o  Text terminals have no high-speed graphic display (or high
     resolution graphics) although they can often use graphic character
     sets to draw boxes, etc.  This lack limits the software that may be
     used on it.

  o  If the host computer goes down, then no one can use the terminals
     either (unless there is a "standby" host computer to connect to).


  4.7.  Are Text Terminals Obsolete ?

  Text terminals are technologically obsolete because for a slightly
  higher cost of hardware, one could build a smart terminal (with the
  same quality of display).  This wasn't always the the case since
  around 1980 memory cost thousands of dollars per megabyte.  Today with
  low costs for memory and processors, one could make a text terminal
  smart for only about a 10% or 20% increase in hardware cost.

  The reasons that text terminals are not yet obsolete are:

  o  There is no satisfactory standard interface for smart graphics
     terminals.  The MS Hydra system is for MS Windows NT, while X-
     Windows is not as efficient as it should be (and X-Windows
     terminals are too costly).

  o  Many people don't need full screen graphics.

  o  Text terminals are low in cost and allegedly take longer to become
     obsolete, yet can give access to a much newer (and powerful)
     computer.

  o  Since running a text-terminal (in contrast to a full-graphics
     terminal) doesn't consume much of a modern PC's resources, a large
     number of terminals may be efficiently run from one PC.


  5.  Overview of How Terminals Work (in Linux)

  See also section ``Some Details on How Terminals Work''


  5.1.  Device Names

  Each terminal is connected to a serial port on the host computer
  (often just a PC).  The ports have names: ttyS0, ttyS1, ttyS2 etc.
  These are represented by special files found in the /dev (device)
  directory.  /dev/ttyS0 corresponds to COM1 in DOS or Windows.  ttyS1
  is COM2, etc.  See ``Terminal Special Files'' for details on these and
  related "devices" such as cua.


  5.2.  Login/Logout

  When the host computer starts up it runs the program getty.  The getty
  program runs the "login" program to log people in.  See ``Getty (in
  /etc/inittab)''.  A "login:" prompt appears on the screen.  People at
  the terminals log in (after giving their passwords) and then have
  access to the computer.  When it's time to shut the terminal down, one
  normally logs out and turns the terminal off.  See ``Login
  Restrictions'' regarding restricting logins (including allowing the
  root user to log in at terminal).


  5.3.  Half/Full Duplex

  If one watches someone typing at a terminal, the letters one types
  simultaneously appear on the screen.  A naive person might think that
  what one types is being sent directly from the keyboard to the screen
  with a copy going to the computer (half-duplex like, see next
  paragraph).  What is usually going on is that what is typed at the
  keyboard is directly sent only to the host computer which in turn
  echoes back to the terminal each character it receives (called full-
  duplex).  In some cases (such as passwords or terse editor commands)
  the typed letters are not echoed back.

  Full-duplex means that there are two (dual) one-way communication
  links.  Full-duplex is the norm for terminals.  Half-duplex is half of
  a duplex, meaning that there is only a single one-way communication
  link.  This link must be shared by communications going in both
  directions and only one direction may be used at a time.  In this case
  the computer would not be able to echo the characters you type (and
  send to it) so the terminal would need to also send each character you
  type directly to the terminal screen.  Some terminals have a half-
  duplex mode of operation which is seldom used.
  5.4.  Terminal Memory

  The image on a CRT tube will fade away almost instantly unless it is
  frequently redrawn on the screen by a beam of electrons shot onto the
  face of the tube.  Since text sent to a terminal needs to stay on the
  screen, the image on the screen must be stored in the memory chips of
  the terminal and the electron beam must repeatedly scan the screen
  (say 60 times per second) to maintain the image.  See ``Terminal
  Memory Details'' for more details.


  5.5.  Commands for the Terminal

  The terminal is under the control of the computer.  The computer not
  only sends the terminal text to display on the screen but also sends
  the terminal commands which are acted on.  These are ``Control Codes''
  (bytes) and ``escape sequences''.  For example, the CR (carriage
  return) control code moves the cursor the the left hand edge of the
  screen.  A certain escape sequence (several bytes where the first byte
  is the "escape" control code) can move the cursor to the location on
  the screen specified by parameters placed inside the escape sequence.

  The ``first terminals'' had only a few such commands but modern
  terminals have hundreds of them.  The appearance of the display may be
  changed for certain regions: such as bright, dim, underline, blink,
  and reverse video.   A speaker in a terminal can "click" when any key
  is pressed or beep if a mistake has occurred.  Function keys may be
  programmed for special meanings.  Various fonts may exist.  The
  display may be scrolled up or down.  Specified parts of the screen may
  be erased.  Various types of flow control may be used to stop the flow
  of data when bytes are being sent to the terminal faster than the
  terminal can handle them.  There are many more as you will see from
  looking over an advanced terminal manual or from the Internet links
  ``Esc Sequence List''


  5.6.  Lack of Standardization Solved by Terminfo

  While terminals made for the US all used the same ASCII code for the
  alphabet (except for IBM terminals which used EBCDIC), they
  unfortunately did not all use the same escape sequences.  This
  happened even after various ANSI (and ISO) standards were established
  since these standards were never quite advanced enough.  Furthermore,
  older terminals often lacked the capabilities of newer terminals.
  This might cause problems.  For example, the computer might send a
  terminal an escape sequence telling it to split the screen up into two
  windows of specified size, not realizing that the terminal was
  incapable of doing this.

  To overcome these problems a database called "termcap" (meaning
  "terminal capabilities") was established.  Termcap was superceded by
  "terminfo".  This database resides in certain files on the computer
  and has a section of it (sometimes an entire file) for each model of
  terminal.  For each model (such as VT100) a list of capabilities is
  provided including a list of certain escape sequences available.  For
  example blink=\E5m means that to make the cursor start blinking the
  terminal must be sent: Escape 5 m.  See Section ``Termcap and Terminfo
  (detailed)'' for more details.  Application programs may utilize this
  database by calling certain C-Library functions.  One large set of
  such programs (over 200) is named "ncurses" and are listed in the
  manual page for "ncurses".





  5.7.  The Interface

  The environment variable TERM is the type of terminal Linux thinks you
  are using.  Some application programs use this to look up the
  capabilities in the terminfo database so TERM needs to be set
  correctly.   But there is more to a correct interface than the
  computer knowing about the capabilities of the terminal.

  For bytes to flow from the computer to the terminal the terminal must
  be set to receive the bytes at the same baud rate (bits per second) as
  they are sent out from the terminal.  If the terminal is set to
  receive at 19,200 baud and the computer sends out characters at 9600
  baud, only garbage (or perhaps nothing) will be seen on the screen.
  One selects the baud rate for a terminal (as well as many other
  features) from the terminals "set-up" menus at the terminal.  Most
  terminals have a large number of options in their "set-up" menus  (see
  ``Terminal Set-Up (Configure) Details'').  The computer serial port
  has options also and these options must be set up in a compatible way
  (see ``Computer Set-Up (Configure) Details''.


  5.8.  Emulation

  Most terminals today have more than one emulation (personality or
  "terminal mode").  The terminal model numbers of terminals formerly
  made by DEC (Digital Equipment Corporation now Compaq) start with VT
  (e.g. VT100).  Many other terminals which are not VT100 may be set up
  to emulate a VT100.  Wyse is a major terminal manufacturer and most of
  their terminals can emulate various DEC terminals such at VT100 and
  VT220.  Thus if you want to, say, use a VT320 terminal you may either
  use a real VT320 in "native" personality or possibly use some other
  terminal capable of emulating a VT320.  The "native" personalities
  usually have more capabilities so, other things being equal, "native"
  is usually the best to use.

  The most common type of emulation is to use a PC like it was a vt100
  terminal (or the like).  Programs loaded into the PC's memory permits
  this.  In Linux (unless you're in X-windows) the PC monitor (called
  the console) emulates a terminal of type "Linux" (close to vt100).
  Even certain windows within X-windows emulate terminals.  See
  ``Terminal Emulation''.


  5.9.  The Console

  On a PC, the monitor is normally the console.  It emulates a terminal
  of type "Linux".  One logs on to it as a virtual terminal.  See ``The
  Console: /dev/tty?''.  It receives messages from the kernel regarding
  booting and shutdown progress.  One may have the messages that
  normally go to the console, go to the terminal.  To get this you must
  manually patch the kernel, except that for kernel 2.2 (or higher) it
  is a "make config" option.  See ``Make a Terminal the Console''.


  6.  Terminal Special Files such as /dev/tty

  "tty" is an abbreviation for "Teletype".  The first terminals were
  Teletypes (like remotely controlled typewriters).  See subsection
  ``Teletypes''.  A list of Linux devices (the stuff in the /dev
  directory) may be found in "Linux Allocated Devices" which should be
  included with kernel sources.  It "describes" what each device used
  for in only a word or two but doesn't tell you how to use them.




  6.1.  Serial Port Terminals

  The computer considers each serial port to be a "device".  It's
  sometimes called a terminal device since at one time terminals were
  the common use for the serial port.  For each such serial port there
  is a special file in the /dev (device) directory.  /dev/ttyS0 is the
  special file for the serial port known as COM1 in the DOS/Windows
  world.  To send text to a terminal you may redirect standard output of
  some command-line command to the appropriate special file.  For
  example typing "echo test > /dev/ttyS1" at the command prompt should
  send the word "test" to the terminal on ttyS1 (COM2) provided you have
  write permission on /dev/ttyS1.  Similarly, typing "cat my_file >
  /dev/ttyS0" will send the contents of the file my_file to COM1
  (ttyS0).

  In addition to ttyS0 (/dev/ttyS0), ttyS1, ttyS2, etc. (the "S" stands
  for Serial port) there is also a "cua" series: cua0, cua1, cua2, etc.
  cua0 is the same port as ttyS0, etc.  The "cu" of cua stands for
  CalloUt.  The ttyS series are Posix compliant while using cua may
  permit the opening of a port that the modem control lines say is not
  ready.  Starting with kernel version 2.2 cua is obsolete and a warning
  message is issued when you attempt to use it (although it still
  works).  For the past few years it has only been included with Linux
  for backwards compatibility.  A programmer can arrange things so that
  ttyS can behave just like cua, so cua is not really needed.


  6.2.  Pseudo Terminals

  Pseudo terminals have no physical connector on the computer.  They are
  used to emulate a serial port.  They have no IO address nor IRQ.  For
  example, if someone connects via telnet to your computer over a
  network, they may wind up connected to the device /dev/ptyp2 (a pseudo
  terminal port).  In X-Windows, the terminal emulator program, xterm
  (or rxvt), uses pseudo terminals.  Ham radio programs under Linux also
  use them.  Using certain application software it is possible to have 2
  or more pseudo terminals attached to the same physical serial port.

  Pseudo terminals come in pairs such as ttyp3 and ptyp3.  The pty... is
  the master or controlling terminal and the tty... is the slave.  ttyq5
  is also a pseudo terminal as is ttysc (c is a hexadecimal digit).
  More precisely, pseudo master terminals are /dev/pty[p-s]N and the
  corresponding slaves are /dev/tty[p-s]N where N is a hexadecimal digit
  0-f.  The slave /dev/ttyp  looks just like a serial port to an
  application program that reads and writes to it.  However the
  application program is really communicating with whatever program is
  writing and reading the corresponding master /dev/pty.  Thus the
  master and slave are really the same "port" but one is used by the
  application program and the other is used by a network program (or the
  like) which supplies (and gets) data to/from the "port".

  Unix98 doesn't use the above but instead uses a "pty master" which is
  /dev/ptmx.  This can supply a pty on demand.  Details on using pseudo
  terminals with other programs are in a few manual pages but a manual
  page devoted to only to pseudo terminals is needed for Linux.  Some
  other unix-like systems have such a page named "pty - pseudo terminal
  driver".


  6.3.  The Controlling Terminal /dev/tty

  /dev/tty stands for the controlling terminal (if any) for the current
  process.  To find out which tty's are attached to which processes use
  the "ps -a" command at the shell prompt (command line).  Look at the
  "tty" column.  For the shell process you're in, /dev/tty is the
  terminal you are now using.  Type "tty" at the shell prompt to see
  what it is (see manual pg. tty(1)).  /dev/tty is something like a link
  to the actually terminal device name with some additional features for
  C-programmers: see the manual page tty(4).


  6.4.  /dev/ttyIN "Terminals"

  N stands for an integer.  One use of these in Linux is with the ISDN
  driver package: isdn4linux.  The ttyIN is something like ttySN.  There
  is also a cuiN  which is something like cuaN.  The ttyI and cui
  emulate modems and may be given modem commands.


  6.5.  The Console: /dev/ttyN

  In Linux the PC monitor is usually called the console and has several
  device special files associated with it: tty0, tty1, tty2, etc.  When
  you log in you are on tty1.  To go to tty2 (on the same screen) For
  tty3 use Left Alt-F3, etc.  These (tty1, tty2, tty3, etc.)  are called
  "virtual terminals".  tty0 is just an alias for the current virtual
  terminal and it's where messages from the system are sent.  Thus
  messages from the system will be seen on the console (monitor)
  regardless of which virtual terminal it is displaying.

  You may log in to different virtual terminals and thus have a few
  different sessions with the computer going on at the same time.  Only
  the system or the root user may write to /dev/tty0 to which
  /dev/console is sometimes linked.  For more info on the console see
  ``The Linux Console''.


  6.6.  Creating a Device with "mknod"

  The /dev directory comes supplied with many device special files.  If
  you need something that's not there you may try to create it with the
  "mknod" command.  See the manual page ttys(4) for how to do this for
  serial ports.  To use mknod you must know the major and minor device
  numbers.  You might be able to infer the numbers you need by using the
  "ls -l" command in the /dev directory.  It will display the major and
  minor numbers of existing special files.


  7.  Some Details on How Terminals Work

  If you know almost nothing about terminals, it's suggested that you
  first read ``Introduction'' and also read ``Overview of How Terminals
  Work''.


  7.1.  Terminal Memory Details

  The terminal screen refreshes itself at perhaps 60 times per second
  from an image stored in the memory of the terminal.  For a PC the
  monitor's image is stored on the video card inside the computer but
  for a terminal, the equivalent of the video card is inside the
  terminal.  For a text terminal the storage of the image uses little
  memory.  Instead of putting every dot (pixel) on the screen into
  memory and requiring the storage of about a quarter-million dots, a
  much more efficient method of storage is used.

  A screen-full of text may be represented inside the terminal memory by
  ASCII bytes, one for each character on the screen.   An entire screen
  only takes about 2K ASCII bytes.  To display these characters, the
  terminal must also know the bit-map (the shape) of each of the almost
  100 printable ASCII characters.  With a bit-map of a character using
  say 15 bytes, only about 1.5K of memory is needed for the bit-maps of
  all the ASCII characters (the font).  This ASCII text and font memory
  is scanned so that the resulting image is put on the screen about 60
  times each second.  This is a form of shared memory where a single
  bit-map of a letter such as the letter e, is shared by all of the many
  letter e's which appear on a screen-full of text.  Low memory
  requirements meant low costs to produce monitors in the early 1980's
  when the cost of memory was several thousand times higher than it is
  today (costing then several dollars per kilobyte).


  7.2.  Early Terminals

  The first terminals were something like remotely controlled
  typewriters which could only "display" (print on paper) the character
  stream sent to them from the computer.  The earliest models were
  called ``Teletypes''.  The name "tty" is just an abbreviation for
  "Teletype".   Early terminals could do a line feed and a carriage
  return just like a typewriter and ring a bell when a bell character
  was received.  Due to the lack of significant capabilities this was
  the first type of terminal to be labeled "dumb".  This type of
  terminal interface (using a terminal type called "dumb") is sometimes
  used today when the computer can't figure out what kind of a terminal
  it is communicating with.


  7.3.  Escape Sequences and Control Codes (intro)

  Terminals have many capabilities some of which are always present and
  some of which require commands from the computer to change or
  activate.  To exercise all these capabilities under the control of the
  computer requires that special codes be established so that the
  computer can tell the terminal what to do.  There are two major type
  of such codes: escape sequences and control codes (control
  characters).  There are many times more escape sequences than control
  codes.


  7.3.1.  Control Codes

  The control codes (or control characters) consist of the first 32
  bytes of the ASCII alphabet.  They include the following: carriage-
  return (cursor to far left), line-feed (cursor down one line),
  backspace, escape-character, tab, and bell.  They do not normally show
  on the screen.  There is usually a command which you may give to your
  terminal which will result in them being displayed when they are
  received by the terminal.  It's called something like "Display
  Controls" or "Monitor".  If you do this then the display may look a
  mess since escape sequences, which all start with the ESC (escape)
  control character, are no longer executed.  Words which should appear
  at the top or bottom of the screen will show up in other locations.
  The escape sequences to reposition the cursor display on the screen
  but the cursor doesn't move to where the escape sequence says.


  7.3.2.  Escape Sequences

  Since there are not nearly enough control codes to do everything (and
  for some reason, not all of them are utilized) many escape sequences
  are used.  They consist of the "escape" (ESC) control character
  followed by a sequence of ordinary characters.  Upon receiving an
  escape character, the terminal examines the characters following it so
  that it may interpret the sequence and carry out the intended command
  from the computer.  Once it recognizes the end of a valid sequence,
  further characters received just display on the screen (unless they
  are control codes or more escape sequences).  Some escape sequences
  may take parameters (or arguments) such as the coordinates on the
  screen to move the cursor to.  The parameters become a part of the
  escape sequence.  An ``Esc Sequence List'' is on the web for some
  terminals, but it's terse.

  A list of the escape sequences for your terminal should be in the
  "programmers manual" for the terminal.  Except for very old terminals,
  there may be two or three hundred such sequences.  If you don't have a
  such manual it's not easy to find them.  Some of the sequences are
  available on the Internet.  One link is ``Esc Sequence List''.  By
  searching the Internet for one sequence (such as ESC[5m) you may come
  across a long list of them.

  Another way to determine some of them is to find the terminfo entry
  (termcap) for the terminal and mentally decode it.  See ``Terminfo and
  Termcap (detailed)'' in this document and/or the ``Termcap Manual'' on
  the Internet.  Unfortunately, the terminfo (termcap) for a terminal
  often does not list all of the escape sequences which the terminal has
  available for use, but fortunately, the most important ones are
  usually there.


  7.4.  Display Attributes & Magic Cookies

  Terminals have various methods of generating character attributes such
  as bold, reverse-video, underlining, etc.  There should be no need for
  the user to worry about how how this is done, except that it creates
  problems for some old terminals and there is sometimes an option for
  this in the set-up menu of newer terminals.

  The magic cookie method is obsolete.  It's the simplest (and worst)
  method of defining attributes: Use a certain byte for the start of an
  attribute and another to end that attribute.  For example, a "start
  underlining" magic cookie byte is placed just before the first word to
  be underlined.  These extra bytes are put into the memory of the
  screen page, just like character bytes that display as characters.
  But this might foul up the count of the number of characters per line
  since non-printable magic cookie characters are intermingled with
  other printable characters.  This sometimes causes problems.

  A better method which uses more memory is to assign an attribute byte
  (or half=byte, etc.) to each displayed character.  This method is used
  by PC video cards (for text) for the common PC monitor.


  8.  Special Features of Some Terminals

  8.1.  Color

  While the common monochrome terminal is not a color terminal it may
  have a fixed "color" display other than white such as green or amber.
  All terminals have black (electron beam turned off = zero brightness).
  A real color terminal can change the color of the text and background
  to many different colors while a monochrome terminal can only change
  the brightness of a fixed color.

  However, changing the brightness, etc. gives a lot of possibilities.
  For example, a black and white (monochrome) terminal can have white,
  grey, and black by varying the brightness.  Some words can be black on
  a light grey background while other are highlighted by black on white.
  In addition there is white on black, underlining, and blinking.

  Color works like the color on a computer monitor or TV screen.  The
  CRT has three colors of dots on it with each color controlled by its
  own electron beam (3 beams).  Monochrome has inherently better
  resolution since it doesn't depend on dots permanently fixed on the
  screen.  For text terminals the only use of color is to differentiate
  text and this advantage is not always worth the cost of worse
  resolution.  Thus monochrome may be better since it also costs less.


  8.2.  Multiple Sessions

  For dual sessions the terminal has two serial ports of equal status.
  Each port is connected to a serial port on a different computer.  Thus
  one may log in to two different computers with each session displaying
  in a split-screen window.   Alternatively, each session may run full-
  screen with a "hot" key (or the like) to switch between sessions.  One
  could also connect to two different serial ports on the same computer
  and log in twice (similar to "virtual terminals" at the console).  The
  program "screen" will make any ordinary terminal (single session)
  connected to a single computer run two or more "sessions".


  8.3.  Printer/Auxiliary Port

  Many terminals have a connector on the rear for such a port.  It may
  be labeled as "Aux" or "Printer", etc.  Some printer ports are for
  parallel printers while others are for serial printers.  If a printer
  is connected to the printer or auxiliary port, then pressing certain
  keys will print the screen.  One may also have everything that
  displays on the screen go also to the printer.  If the port is an
  auxiliary port, one may connect this to another computer and almost
  have dual sessions as above.  However, the video memory inside the
  terminal may not retain both sessions so you may need to refresh the
  screen when switching to the other session.  There will likely not be
  a hot key either but possibly a programmable function key may be
  programmed to do this.  There exists various key combinations and
  escape sequences for controlling such a port.  See ``Printer Esc''.

  There is a program called vtprint which is designed to send a print
  job to your terminal to be printed on a printer attached to the
  terminal.  It's homepage is http://people.qualcomm.com/garrett/vtprint
  .  It's also a included (as of 1998) in the Debian distribution of
  Linux.  xprt (also in Debian) seems to do something similar, but only
  for X-Window terminals ??


  8.4.  Pages

  Many terminals permit the storage of more than one page in their video
  memory.  Sometimes the page size is the same as the screen, but
  sometimes it is larger so that scrolling will reveal unseen parts of a
  page.  So when one looks at a screen, there may be hidden text on the
  same page above or below the display.  In addition, if there is more
  than just one page, there may be hidden text on these other pages.
  One use for pages is on terminals that support dual sessions.  Each
  session may have its own page and one may switch back and forth
  between them.

  Even if you only have a one-page-terminal with the page sized equal to
  what is displayed on the screen, you will still see other pages of a
  file (etc.) as the host sends more data to the terminal.  One
  advantage to having additional pages stored in the terminal memory is
  so that you can jump to them instantly without waiting a second or so
  for them to be transmitted from the host.

  Multiple pages is supported by ncurses.  There is also a commercial
  program called "Multiscreen" which supports multiple pages but
  probably not for Linux ??  Multiscreen is reported to be part of SCO
  and is something like the virtual terminals on a Linux PC console.
  The Linux program "screen" makes it look like you have multiple pages
  but they are stored in the computer and but you can have only one
  page-like window for each running program.


  8.5.  Character-Sets

  A character-set is normally represented by a list (or table or chart)
  of characters along with the byte code assigned to each character.
  The codes for a byte range from 0 to 255 (00 to FF in hexadecimal).
  In MS-DOS, character-set tables are called "code-pages".  You should
  examine such a table if you're not familiar with them.  They are
  sometimes included in printer and terminal manuals but may be
  difficult to find.

  ASCII is one of the most common character-sets used on text terminals.
  It is a 7-bit code but can be made into 8-bit if the first bit (high
  order bit) is always set to 0.  Other character-sets are usually
  available (except on very old terminals where the only choice is
  ASCII).  The first half of most character-sets are the conventional
  ASCII characters and the second half (the characters with the high-
  order bit set to 1) belong to a wide variety of character-sets.
  Character sets are often ISO standards.  To get specialized character
  sets on a terminal, you may need to download a soft-font for that
  character-set into the memory of the terminal.

  Besides ASCII, there are some other common character-sets, all 8-bit.
  CP stands for Code Page character sets invented by IBM: CP-437 (DOS
  ECS), CP-850 (Multilingual Latin 1 --not the same as ISO Latin-1),
  ISO-8859-1 (Latin-1), ANSI (derived from Latin-1).  MS Windows uses
  ANSI while the Internet often uses Latin-1.  There are several
  ISO-8859 character sets in addition to Latin-1.  These include Greek
  (-7), Arabic (-6), Eastern European (-2), and Russian (-5).  There are
  many others.  For example, KOI8-R is more commonly used for Russian
  than IS0-8859-5.  Unicode is a very large character-set where each
  character is represented by 2 bytes instead on just one byte.

  More info re character-sets are:

  o  Manual pages: ASCII and latin1

  o  HOWTO's for various languages (likely written in that language).
     See "Cyrillic" for Russian.

  o  <http://www.cc.columbia.edu/kermit/charsets.html> for a short
     listing of various character-set names.

  o  <http://www.pku.edu.cn/on_line/w3html/International/Overview.html>
     for info on character-sets and the Internet.

  o  Languages, Countries and Character Sets
     <http://www.w3.org/International/O-charset-lang.html>

  o  <http://vancouver-webpages.com/multilingual/> name="Using Multiple
     Languages in HTML">

     Once you've found the character set name (or number) you are
     interested in, you may search for more info about it on the
     Internet.


  8.6.  Fonts

  Most terminals made after the mid 1980's can accept downloaded soft-
  font.  This means that they can display almost any character set
  provided that you can find the soft-font for it.  If you can't find
  the needed soft-font, you can always create your own.  A free font
  editor for this is called BitFontEdit (written by the author of this
  document) and (in 1998) was at
  Europe:  <http://www.funet.fi/pub/culture/russian/comp/cyril-term/>
  N. America:
  <ftp://cs.utk.edu/pub/shuford/terminal/BitFontEdit.tar.gz>


  8.7.  Keyboards & Special Keys

  Terminal keyboards often have a number of keys that one doesn't find
  on a PC keyboard.  Few (if any) actual terminals will have all of
  these keys and most will have additional keys not listed here.  Some
  have a large number of special purpose keys such as terminals made for
  use with cash registers.  There are often many more key meanings than
  shown here since these keys often have extended meanings when used in
  conjunction with other keys (such as shift and control).


  o  BREAK sends a very long 0 bit (space = +12 V) of duration 300 to
     700 milliseconds to the host.  The host may interpret this as an
     interrupt if stty has set brkint or ignore it if ignbrk is set.

  o  NO SCROLL stops the screen from scrolling like ^S does.  Depressing
     it again resumes scrolling.  Uses flow control signals to do this.

  o  REPEAT if held down with an other key, forces repeated output of
     that other key even if the auto-repeat option is set to off.

  o  LINE FEED sends the line feed character ^J to the host.  Seldom
     used.

  o  SET-UP allows the manual configuration of the terminal via menus.
     Sometimes purposely disabled by putting a block under it so it
     can't be pressed down.  Sometimes another key such as shift or
     control must be pressed at the same time. See ``Getting Into Set-Up
     (Configuration) Mode''.

  o  LOCAL disconnects the terminal from the host.  In local, what one
     types goes directly to the screen.  Useful for testing.

  o  RETURN is the same as the "enter" key on a PC.  It usually sends a
     carriage return to the host which normally get translated to a new-
     line character by the host's device driver.  On some terminals it
     may be set up to send something else.

  o  F1, F2, ... or PF1, PF2, ... are function keys which usually may be
     programmed to send out a sequence of bytes (characters).  See
     ``Function Keys''


  9.  Terminal Emulation; the Console

  9.1.  Intro to Terminal Emulation

  Since a PC has a screen and keyboard (as does a terminal) but also has
  much more computing power, it's easy to use some of this computing
  power to make the PC computer behave like a text terminal.  This is
  one type of terminal emulation.  Another type of terminal emulation is
  where you set up a real terminal to emulate another brand/model of
  terminal.  To do this you select the emulation you want (called
  "personality" in Wyse jargon) from the terminal's set-up menu.  This
  section is about the first type of emulation: emulating a terminal on
  a PC.

  Emulation software is available for MS Windows and comes built-in with
  recent versions of MS Windows.  Most Linux free software can only
  emulate a VT100, VT102, or VT100/ANSI.  If you find out about any
  others, let me know.  Since most PC's have color monitors but VT100
  and VT102 were designed for a monochrome monitor, the emulation
  usually adds color capabilities (and a choice of colors).  Sometimes
  the emulation is not 100% perfect but this usually causes few
  problems.  For using a Mac computer to emulate a terminal see the
  mini-howto: Mac-Terminal.


  9.2.  Don't Use TERM For Emulation

  Some have erroneously thought that they could create an emulator at a
  Linux console (monitor) by setting the environment variable TERM to
  the type of terminal they would like to emulate.  This does not work.
  The value of TERM only tells an application program what terminal you
  are using.  This way it doesn't need to interactively ask you this
  question.  If you're at the PC monitor it's a terminal of type "Linux"
  and your can't change this.  So you must set TERM to "Linux".

  If you set it to something else you are fibbing to application
  programs.  As a result they will incorrectly interpret certain escape
  sequences from the console resulting in a corrupted interface.  Since
  the Linux console behaves almost like a vt100 terminal, it could still
  work almost OK if you falsely claimed it was a vt100 (or some other
  terminal which is something like a vt100).  It may seeming work OK
  most of the time but once in a while will make a mistake when editing
  or the like.


  9.3.  Communication (Dialing) programs

  Dialing programs for making a PPP connection to the Internet don't
  normally include any terminal emulation.  But some other modem dialing
  programs (such as minicom or seyon) do.  Using them one may (for
  example) dial up public libraries to use their catalogs and indexes,
  (or even read magazine articles).  They are also useful for testing
  modems.  Seyon is only for use with X-windows and can emulate
  Tektronix 4014 terminals.

  The communication program kermit doesn't do terminal emulation as it
  is merely a semi-transparent pipe between whatever terminal you are on
  and the remote site you are connected to.  Thus if you use kermit on a
  Linux PC the terminal type will be "Linux".  If you have a Wyse60
  connected to your PC and run kermit on that, you will appear as a
  Wyse60 to the remote computer (which may not be able to handle Wyse60
  terminals).  Minicom emulates a VT102 and if you use it on Wyse60
  terminal the Wyse escape sequences will get translated to VT102 escape
  sequences before the data goes out to the modem.  Kermit can't do
  this.

  Emulators exist under DOS such as telix and procomm work just as well.
  The terminal emulated is often the old VT100, VT102, or ANSI (like
  VT100).


  9.3.1.  Emulation under X-Windows

  Xterm (obsolete ??) may be run under X-Windows which can emulate a
  VT102, VT220, or Tektronix 4014.  There is also an xterm emulation
  (although there is no physical terminal named "xterm").   If you don't
  need the Tektronix 4014 emulation (a vector graphics terminal; see
  ``Graphics Terminals'') you may use eterm.  Predecessors to eterm are
  rxvt and xvt.  eterm supports pixmaps.

  For non-Latin alphabets, kterm is for Kanji terminal emulation (or for
  other non-Latin alphabets) while xcin is for Chinese.  There is also
  9term emulation.  This seems to be more than just an emulator as it
  has a built-in editor and scroll-bars.  It was designed for Plan 9, a
  Unix-like operating system from AT&T.


  9.3.2.  Real Terminals Better

  Unless you are using X-Windows with a large display, a real terminal
  is often nicer to use than emulating one.  It usually costs less, has
  better resolution for text, and has no disk drives to make annoying
  noises.


  9.4.  Testing Terminal Emulation

  For the VT series terminals there is a test program: vttest to help
  determine if a terminal behaves correctly like a vt53, vt100, vt102,
  vt220, vt320, vt420 etc.  There is no documentation but it has menus
  and is easy to use.  To compile it run the configure script and then
  type "make".  It may be downloaded from:
  <ftp://ftp.clark/net:/pub/dickey/vttest/>.  An alternate download site
  is:  <http://sunsite.unc.edu/pub/Linux/utils/console/>


  9.5.  The Linux Console

  The console for a PC Linux system is normally the computer monitor.
  It emulates a terminal of type "Linux".  There is no way (unless you
  want to spend weeks rewriting the kernel code) to get it to emulate
  anything else.  Setting the TERM environment variable to type of
  terminal other than "Linux" will not result in emulating that other
  terminal.  It will only result in a corrupted interface since you have
  falsely declared (via the TERM variable) that your "terminal" is of a
  type different from what it is.  See ``Don't Use TERM For Emulation''

  The "Linux" emulation is flexible and has features which go well
  beyond those of the vt102 terminal which it was intended to emulate.
  These include the ability to use custom fonts and easily re-map the
  keyboard (without patching the source code and recompiling the kernel
  as is required for the case of a real terminal).  These extra features
  reside in the console driver software and not in the emulation
  software but the results are like it was part of the emulation.

  Many commands exist (see Keyboard-and-Console-HOWTO) to utilize these
  added features.   Real terminals, which use neither scan codes nor VGA
  cards, unfortunately can't use most of these features.  One may
  recompile Linux to make a terminal receive the messages which normally
  go to the console.  See ``Make a Terminal the Console''.


  9.6.  Emulation Software

  Emulators often don't work quite right so before purchasing software
  you should try to throughly check out what you will get.


  9.6.1.  Make a Linux PC a terminal

  Unless you want to emulate the standard vt100 (or close to it).  There
  doesn't seem to be much free terminal emulation software available for
  Linux.  The free programs minicom and seyon (only for X-windows) can
  emulate a vt100 (or close to it).   Seyon can also emulate a Tektronix
  4014 terminal.

  TERM (non-free from Century Software)
  <http://www.ecc400.com/censoft/termunix.html> can emulate Wyse60, 50;
  VT 220, 102, 100, 52: TV950, 925, 912; PCTERM; ANSI; IBM3101; ADM-1l;
  WANG 2110.  Block mode is available for IBM and Wyse.  It runs on a
  Linux PC.


  9.6.2.  Make a non-Linux PC a terminal

  Emulators exist which run on non-Linux PCs.  They permit you to use a
  non-Linux-PC as a terminal connected to a Linux-PC.  Under DOS there
  is telix and procomm.   Windows comes with "HyperTerminal" (formerly
  simply called "Terminal" in Windows 3.x and DOS).  Competing with this
  is "HyperTerminal Private Edition"
  <http://www/hilgraeve.com/htpe/index.html> which is non-free to
  business.  It can emulate vt-220.  Turbosoft's (Australia) TTWin
  <http://www.turbosoft.com.au/> can emulate over 80 different terminals
  under Windows.

  For the Mac Computer there is emulation by Carnation Software
  <http://www.webcom.com/carn/carnation/panel-default.html>

  One place to check terminal emulation products is Shuford's site, but
  it seems to lists old products (which may still work OK).  The fact
  that most only run under DOS (and not Windows) indicates that this
  info is dated.  See
  <http://www.cs.utk.edu/~shuford/terminal/term_emulator_products.txt>.


  10.  Flow Control (Handshaking)

  Flow control (= handshaking = pacing)  is to prevent too fast of a
  flow of bytes from overrunning a terminal, computer, modem or other
  device.  Overrunning is when a device can't process what it is
  receiving quickly enough and thus loses bytes and/or makes other
  serious errors.  What flow control does is to halt the flow of bytes
  until the terminal (for example) is ready for some more bytes.  Flow
  control sends its signal to halt the flow in a direction opposite to
  the flow of bytes it wants to stop.  Flow control must both be set at
  the terminal and at the computer.

  There are 2 types of flow control: hardware and software (Xon/Xoff or
  DC1/DC3).  Hardware flow control uses dedicated signal wires such as
  RTS/CTS or DTR/DSR while software flow control signals by sending DC1
  or DC3 control bytes in the normal data wires.  For hardware flow
  control, the cable must be correctly wired.

  The flow of data bytes in the cable between 2 serial ports is bi-
  directional so there are 2 different flows (and wires) to consider:

  1. Byte flow from the computer to the terminal

  2. Byte flow from the terminal keyboard to the computer.


  10.1.  Why Is Flow Control Needed ?

  You might ask: "Why not send at a speed slow enough so that the device
  will not be overrun and then flow control is not needed?"  This is
  possible but it's usually significantly slower than sending faster and
  using flow control.  One reason for this is that one can't just set
  the serial port baud rate at any desired speed such as 14,500, since
  only a discrete number of choices are available.  The best choice is
  to select a rate that is a little higher than the device can keep up
  with but then use flow control to make things work right.

  If one decides to not use flow control, then the speed must be set low
  enough to cope with the worst case situation.  For a terminal, this is
  when one sends escape sequences to it to do complex tasks that take
  more time than normal.  In the case of a modem (with data compression
  but no flow control) the speed from the computer to the modem must be
  slow enough so that this same speed is usable on the phone line, since
  in the worst case the data is random and can't be compressed.  If one
  failed to use flow control, the speed (with data compression turned
  on) would be no faster than without using any compression at all.

  Buffers are of some help in handling worst case situations of short
  duration.  The buffer stores bytes that come in too fast to be
  processed at once, and saves them for processing later.


  10.2.  Padding

  Another way to handle a "worst case" situation (without using flow
  control or buffers) is to add a bunch of nulls (bytes of value zero)
  to escape sequences.  Sometimes DEL's are used instead provided they
  have no other function.  See ``Recognize Del''.

  The escape sequence starts the terminal doing something, and while the
  terminal is busy doing it, it receives a bunch of nulls which it
  ignores.  When it gets the last null, it has completed its task and is
  ready for the next command.  This is called null padding.  These nulls
  formerly were called "fill characters".  These nulls are added just to
  "waste" time, but it's not all wasted since the terminal is usually
  kept busy doing something else while the nulls are being received.  It
  was much used in the past before flow control became popular.  To be
  efficient, just the right amount of nulls should be added and figuring
  out this is tedious.  It was often done by trial and error since
  terminal manuals are of little or no help.  If flow control doesn't
  work right or is not implemented, padding is one solution.  Some of
  the options to the stty command involve padding.


  10.3.  Overrunning a Serial Port

  One might wonder how overrunning is possible at a serial port since
  both the sending and receiving serial ports involved in a transmission
  of data bytes are set for the same speed (in bits/sec) such as 19,200.
  The reason is that although the receiving serial port electronics can
  handle the incoming flow rate, the hardware/software that fetches and
  processes the bytes from the serial port sometimes can't cope with the
  high flow rate.

  One cause of this is that the serial port's hardware buffer is quite
  small.  Older serial ports had a hardware buffer size of only one byte
  (inside the UART chip).  If that one received byte of data in the
  buffer is not removed (fetched) by CPU instructions before the next
  byte arrives, that byte is lost (the buffer is overrun).  Newer
  UART's, namely most 16550's, have 16-byte buffers (but may be set to
  emulate a one-byte buffer) and are less likely to overrun.  It may be
  set to issue an interrupt when the number of bytes in its buffer
  reaches 1, 4, 8, or 14 bytes.  It's the job of another computer chip
  (usually the main CPU chip for a computer) to take these incoming
  bytes out of this small hardware buffer and process them (as well as
  perform other tasks).

  When contents of this small hardware receive buffer reaches the
  specified limit (one byte for old UART'S) an interrupt is issued.
  Then the computer interrupts what it was doing and software checks to
  find out what happened.  It finally determines that it needs to fetch
  a byte (or more) from the serial port's buffer.  It takes these
  byte(s) and puts them into a larger buffer (also a serial port buffer)
  that the kernel maintains in main memory.  For the transmit buffer,
  the serial hardware issues an interrupt when the buffer is empty (or
  nearly so) to tell the CPU to put some more bytes into it to send out.
  Terminals also have serial ports and buffers similar to the computer.
  Since the flow rate of bytes to the terminal is usually much greater
  than the flow in the reverse direction from the keyboard to the host
  computer, it's the terminal that is most likely to suffer overrunning.
  Of course, if you're using a computer as a terminal (by emulation),
  then it is likewise subject to overrunning.

  Risky situations where overrunning is more likely are:  1. When
  another process has disabled interrupts (for a computer).  2. When the
  serial port buffer in main (or terminal) memory is about to overflow.


  10.4.  Stop Sending

  When its appears that the receiver is about to be overwhelmed by
  incoming bytes, it sends a signal to the sender to stop sending.  That
  is flow control and the flow control signals are always sent in a
  direction opposite to the flow of data which they control (although
  not in the same channel or wire).  This signal may either be a control
  character (^S = DC3 = Xoff) sent as an ordinary data byte on the data
  wire (in-band signalling), or a voltage transition from positive to
  negative in the dtr-to-cts (or other) signal wire (out-of-band
  signalling).  Using Xoff is called "software flow control" and using
  the voltage transition in a dedicated signal wire (inside the cable)
  is called hardware flow control.


  10.5.  Keyboard Lock

  With terminals, the most common case of "stop sending" is where the
  terminal can't keep up with the characters being sent to it and it
  issues a "stop" to the PC.  Another case of this is where someone
  presses control-S.  Much less common is the opposite case where the PC
  can't keep up with your typing speed and tells the terminal to stop
  sending.  The terminal "locks" its keyboard and a message or light
  should inform you of this.  Anything you type at a locked keyboard is
  ignored.

  The term "locked" is also sometimes used for the common case of where
  the computer is told to stop sending to a terminal.  The keyboard is
  not locked so that whatever you type goes to the computer.  Since the
  computer can't send anything back to you, characters you type don't
  display on the screen and it may seem like the keyboard is locked.
  Scrolling is locked (scroll lock) but the keyboard is not locked.


  10.6.  Resume Sending

  When the receiver has caught up with its processing and is ready to
  receive more data bytes it signals the sender.  For software flow
  control this signal is the control character ^Q = DC1 = Xon which is
  sent on the regular data line.  For hardware flow control the voltage
  in a signal line goes from negative (negated) to positive (asserted).
  If a terminal is told to resume sending the keyboard is then unlocked
  and ready to use.


  10.7.  Hardware Flow Control (RTS/CTS etc.)

  Some older terminals have no hardware flow control while others used a
  wide assortment of different pins on the serial port for this.  For a
  list of various pins and their names see ``Standard Null Modem Cable
  Pin-out''.  The most popular pin to use seems to be the DTR pin (or
  both the DTR pin and the DSR pin).


  10.7.1.  RTS/CTS, DTR, and DTR/DSR Flow Control

  Linux PC's use RTS/CTS flow control, but DTR/DSR flow control (used by
  some terminals) behaves similarly.  DTR flow control (in one direction
  only and also used by some terminals) is only the DTR part of DTR/DSR
  flow control.

  RTS/CTS uses the pins RTS and CTS on the serial (EIA-232) connector.
  RTS means "Request To Send".  When this pin stays asserted (positive
  voltage) at the receiver it means: keep sending data to me.  If RTS is
  negated (voltage goes negative) it negates "Request To Send" which
  means: request not to send to me (stop sending).  When the receiver is
  ready for more input, it asserts RTS requesting the other side to
  resume sending.  For computers and terminals (both DTE type equipment)
  the RTS pin sends the flow control signal to the CTS pin (Clear To
  Send) on the other end of the cable.  That is, the RTS pin on one end
  of the cable is connected to the CTS pin at the other end.

  For a modem (DCE equipment) it's a different scheme since the modem's
  RTS pin receives the signal and its CTS pin sends.  While this may
  seem confusing, there are valid historical reasons for this which are
  too involved to discuss here.

  Terminals usually have either DTR or DTR/DSR flow control.  DTR flow
  control is the same as DTR/DSR flow control but it's only one-way and
  the DSR pin is not used.  For DTR/DSR flow control at a terminal, the
  DTR signal is like the signal sent from the RTS pin and the DSR pin is
  just like the CTS pin.


  10.7.2.  Connecting Up DTR or DTR/DSR Flow Control

  Some terminals use only DTR flow control.  This is only one-way flow
  control to keep the terminal from being overrun.  It doesn't protect
  the computer from someone typing too fast for the computer to handle
  it.  In a standard null modem cable the DTR pin at the terminal is
  connected to the DSR pin at the computer.  But Linux doesn't support
  DTR/DSR flow control (although drivers for some multiport boards may
  support DTR/DSR flow control.)  A way around this problem is to simply
  wire the DTR pin at the terminal to connect to the CTS pin at the
  computer and set RTS/CTS flow control (stty crtscts).  The fact that
  it's only one way will not affect anything so long as the host doesn't
  get overwhelmed by your typing speed and drop RTS in a vain attempt to
  lock your keyboard.  See ``Keyboard Lock''.  For DTR/DSR flow control
  (if your terminal supports this two-way flow control) you do the
  above.  But you also connect the DSR pin at the terminal to the RTS
  pin at the computer.  Then you are protected if you type too fast.


  10.7.3.  Old RTS/CTS Handshaking Is Different

  What is confusing is that there is the original use of RTS where it
  means about the opposite of the previous explanation above.  This
  original meaning is: I Request To Send to you.  This request was
  intended to be sent from a terminal (or computer) to a modem which, if
  it decided to grant the request, would send back an asserted CTS from
  its CTS pin to the CTS pin of the computer: You are Cleared To Send to
  me.  Note that in contrast to the modern RTS/CTS bi-directional flow
  control, this only protects the flow in one direction: from the
  computer (or terminal) to the modem.

  For older terminals, RTS may have this meaning and goes high when the
  terminal has data to send out.  The above use is a form of flow
  control since if the modem wants the computer to stop sending it drops
  CTS (connected to CTS at the computer) and the computer stops sending.

  10.7.4.  Reverse Channel

  Old hard-copy terminals may have a reverse channel pin (such as pin
  19) which behaves like the RTS pin in RTS/CTS flow control.  This pin
  but will also be negated if paper or ribbon runs out.  It's often
  feasible to connect this pin to the CTS pin of the host computer.
  There may be a dip switch to set the polarity of this signal.


  10.8.  Is Hardware Flow Control Done by Hardware ?

  Some think that hardware flow control is done by hardware but (unless
  you are using an intelligent serial card with several serial ports)
  it's actually done by your operating system software.  UART chips and
  associated hardware usually know nothing at all about hardware flow
  control.  When a hardware flow control signal is received, the signal
  wire flips polarity and the hardware gives an electrical interrupt
  signal to the CPU.  However, the hardware has no idea what this
  interrupt means.  The CPU stops what it was doing and jumps to a table
  in main memory that tells the CPU where to go to find a program which
  will find out what happened and what to do about it.

  There's another way this could have been implemented since when the
  polarity flips the hardware could have been configured to send an
  electrical interrupt signal to the CPU.  Then the CPU would stop what
  it was doing, jump to a service routine of the serial driver, check
  registers in the serial had ware to find out what happened, and make a
  note not to resume the flow after the service routine is exited.  This
  might be a little more efficient, but it seems that Linux doesn't do
  it this way.  I once thought it did.

  Note that with either method the flow of bytes stops nearly instantly.
  However, any bytes (up to 16) which were already in the serial port's
  hardware transmit buffer will still get transmitted.  Using software
  flow control requires that each incoming byte be checked to see if
  it's an "off" byte.  These bytes are delayed by passing thru the
  16-byte receive buffer.  If the "off" byte was the first byte into
  this buffer, there could be a wait while 15 more bytes were received.
  Then the 16 bytes would get read and the "off" byte found.  This extra
  delay doesn't happen with hardware flow control.


  10.9.  Obsolete ?? ETX/ACK or ENQ/ACK Flow Control

  This is also software flow control and requires a device driver that
  knows about it.  Bytes are sent in packets (via the async serial port)
  with each packet terminated by an ETX (End of Text) control character.
  When the terminal gets an ETX it waits till it is ready to receive the
  next packet and then returns an ACK (Acknowledge).  When the computer
  gets the ACK, it then send the next packet.  And so on.  This is not
  supported by Linux ??   Some HP terminals use the same scheme but use
  ENQ instead of ETX.


  11.  Physical Connection

  Multiport boards allow many terminals (or modems) to be connected to
  one PC computer.  A terminal may be connected to its host computer
  either by a direct cable connection, via a modem, or via a terminal
  server.


  11.1.  Multiport I/O Cards (Adapters)

  Additional serial cards may be purchased which have many serial ports
  on them called "multiport boards".  These boards are not covered in
  this HOWTO but there is quite a lot of coverage in the Serial-HOWTO
  One company which had (in 1998) below average prices is ByteRunner
  <http://www.byterunner.com/cgi-bin/goto.cgi?FILE=iocards.html>.


  11.2.  Direct Cable Connection.

  The simplest way to connect a terminal to a host computer is via a
  direct connection to a serial port on the computer.  You may also use
  some the info in this section for connecting one computer to another
  (via the serial port).  Most PC's come with a couple of serial ports,
  but one is usually used by a mouse.  For the EIA-232 port, you need a
  null modem cable that crosses over the transmit and receive wires.  If
  you want hardware flow control, you will probably use the DTR pin (or
  both the DTR and DSR pins).

  Make sure you have the right kind of cable.  A null modem cable bought
  at a computer store may do it (if it's long enough), but it probably
  will not work for hardware flow control.  Such a cable may be labeled
  as a serial printer cable.  See this to help determine if you should
  ``Buy or Make'' your own cable.  Make sure you are connecting to your
  PC's serial port at the male DB25 or the DB9, and not your parallel
  port (female DB25).


  11.2.1.  Null Modem Cable Pin-out (3, 4, or 5 conductor)

  These 3 diagrams are for real text-terminals.  But you could use them
  to connect up 2 PCs if you substitute RTS for DTR and CTS for DSR.
  (Don't use 4-conductors for PC-to-PC).  For terminals, if you only
  have DTR flow control (one-way) you may eliminate the RTS-to-DSR wire.
  If you have no hardware flow control, then you may also eliminate the
  CTS-to-DTR wire.  Then if you have 2@ twisted pairs, you may then use
  2 wires for signal ground per ``A Kludge using Twisted-Pair Cable''.
  For a DB25 connector on your PC, you need:



    PC male DB25                          Terminal DB25
      TxD   Transmit Data       2 --> 3       RxD   Receive Data
      RxD   Receive Data        3 <-- 2       TxD   Transmit Data
      SG    Signal Ground       7 --- 7       SG    Signal Ground
      CTS   Clear To Send       5 <--20       DTR   Data Terminal Ready
      RTS   Request To Send     4 --> 6       DSR   Data Set Ready



  If you have a DB9 connector on your PC, try the following:

          PC DB9                                 Terminal DB25
      RxD   Receive Data        2 <-- 2       TxD   Transmit Data
      TxD   Transmit Data       3 --> 3       RxD   Receive Data
      SG    Signal Ground       5 --- 7       SG    Signal Ground
      CTS   Clear To Send       8 <--20       DTR   Data Terminal Ready
      RTS   Request To Send     7 --> 6       DSR   Data Set Ready   **



  If you have a DB9 connector on both your serial port and terminal:







          PC DB9                                 Terminal DB9
      RxD   Receive Data        2 <-- 3       TxD   Transmit Data
      TxD   Transmit Data       3 --> 2       RxD   Receive Data
      SG    Signal Ground       5 --- 5       SG    Signal Ground
      CTS   Clear To Send       8 <-- 4       DTR   Data Terminal Ready
      RTS   Request To Send     7 --> 6       DSR   Data Set Ready   **



  The above don't have modem control lines so be sure to give a "local"
  option to getty (which is equivalent to "stty clocal").  Also if you
  need hardware flow control it must be enabled at your computer (use a
  -h flag with agetty) ( equivalent to "stty crtscts" ).


  11.2.2.  Standard Null Modem Cable Pin-out (7 conductor)

  The following 3 diagrams show full "standard" null modem cables.  One
  that you purchase is apt to be wired this way.  They will work fine
  for computer-to-computer connections.  They also work for real
  terminals using software (Xon/Xoff) flow control (or no flow control).
  However, they don't work for terminal hardware flow control since most
  real terminals support DTR or DTR/DSR flow control (handshaking) but
  Linux doesn't.


  PC male DB25                            Terminal DB25
  TxD   Transmit Data         2 --> 3     RxD   Receive Data
  RxD   Receive Data          3 <-- 2     TxD   Transmit Data
  RTS   Request To Send       4 --> 5     CTS   Clear To Send
  CTS   Clear To Send         5 <-- 4     RTS   Request To Send
  DSR   Data Set Ready        6
                              |
  DCD   Carrier Detect        8 <-- 20    DTR   Data Terminal Ready
  SG    Signal Ground         7 --- 7     SG    Signal Ground
                                    6     DSR   Data Set Ready
                                    |
  DTR   Data Terminal Ready  20 --> 8     DCD   Carrier Detect



  Alternatively, a full DB9-DB25 null modem cable (will not  work with
  terminal hardware handshaking; see above):

  PC DB9                                  Terminal DB25
  RxD   Receive Data          2 <-- 2     TxD   Transmit Data
  TxD   Transmit Data         3 --> 3     RxD   Receive Data
                                    6     DSR   Data Set Ready
                                    |
  DTR   Data Terminal Ready   4 --> 8     DCD   Carrier Detect
  GND   Signal Ground         5 --- 7     GND   Signal Ground
  DCD   Carrier Detect        1
                              |
  DSR   Data Set Ready        6 <-- 20    DTR   Data Terminal Ready
  RTS   Request To Send       7 --> 5     CTS   Clear To Send
  CTS   Clear To Send         8 <-- 4     RTS   Request To Send
  (RI   Ring Indicator        9 not needed)


  (Yes, the pins 2 and 3 really do have the opposite meanings in DB9
  connectors than in DB25 connectors!)

  Here's how to null-modem connect two DB9's together (but DTR flow
  control will not work):


  PC DB9                                  DB9
  RxD   Receive Data          2 <-- 3     TxD   Transmit Data
  TxD   Transmit Data         3 --> 2     RxD   Receive Data
                                    6     DSR   Data Set Ready
                                    |
  DTR   Data Terminal Ready   4 --> 1     DCD   Carrier Detect
  GND   Signal Ground         5 --- 5     GND   Signal Ground
  DCD   Carrier Detect        1
                              |
  DSR   Data Set Ready        6 <-- 4     DTR   Data Terminal Ready
  RTS   Request To Send       7 --> 8     CTS   Clear To Send
  CTS   Clear To Send         8 <-- 7     RTS   Request To Send
  RI   Ring Indicator         9 (not used)



  Using the above 2 connections provide full modem control signals and
  seemingly allow one to set "stty -clocal".  Then one must turn on the
  terminal first (asserts DTR) before the port may be opened in a normal
  manner by getty, etc.  But there is likely to be trouble if you fail
  to turn on the terminal first (see ``Getty Respawning Too Rapidly'').
  For this reason one should use "stty clocal" which is the default
  (ignores modem control lines) and the additional wires in these cables
  then serve no useful purpose.

  In olden days when it may not have been this easy to ignore modem
  control signals etc, the following "trick" was done for cables that
  lacked conductors for modem control: on your computer side of the
  connector, connect RTS and CTS together, and also connect DSR, DCD and
  DTR together.  This way, when the computer needs a certain handshaking
  signal to proceed, it will get it (falsely) from itself.


  11.2.3.  Length Limitations

  A cable longer than a 50 feet or so may not work properly at high
  speed.  Much longer lengths sometimes work OK, especially if the speed
  is low and/or the cable is a special low-capacitance type and/or the
  electronics of the receiving end are extra sensitive.  It is claimed
  that under ideal conditions at 9600 baud, 1000 feet works OK.  One way
  to cover long distances is to install 2@ line drivers near each serial
  port so as to convert unbalanced to balanced (and conversely) and then
  use twisted pair cabling.  But line drivers are expensive.


  11.2.4.  Hardware Flow Control Cables

  If you expect to use hardware flow control (handshaking) you will
  likely need to make up your own cable (or order one made).  Of course,
  if the connecters on the ends of a used cable remove, you might rewire
  it.  See ``Installing DB Connectors''.  You will need to determine
  whether or not the terminal uses the DTR pin for this, and if not,
  what pin (or pins) it uses.  The set-up menus may give you a clue on
  this since there may be an option for enabling "DTR handshaking" (or
  flow control) which of course implies that it uses the DTR pin.  It
  may also use the DSR pin.  See ``Hardware Flow Control'' for a
  detailed explanation of it.  Older terminals may have no provision for
  hardware flow control.


  11.2.5.  Cable Tips

  The normal "straight thru" cable will not work unless you are using it
  as an extension cable in conjunction with either a null modem cable or
  a null modem adapter.  Make sure that the connectors on the cable ends
  will mate with the connectors on the hardware.  One may use telephone
  cable which is at least 4-conductor (and possibly twisted pair).
  Shielded, special low-capacitance cable computer cable is best.


  11.2.6.  A Kludge using Twisted-Pair Cable

  Although none of the EIA-232 signals are balanced for twisted pair one
  may attempt to use twisted-pair cable with it.  Use one pair for
  transmit and another for receive.  To do this connect signal ground to
  one wire in each of these 2 pair.  Only part of the signal ground
  current flows in the desired wire but it may help.  Due to the lower
  inductance of the twisted pair circuit (as compared to ground return
  current by some other path)  more return (ground) current will confine
  itself to the desired twisted pair than one would expect from only
  resistance calculations.  This is especially true at higher
  frequencies since inductive impedance increases with frequency.  The
  rectangular wave of the serial port contains high frequency harmonics.


  11.2.7.  Cable Grounding

  Pin 1 (of a DB25) should be chassis ground (also earth ground) but on
  cheap serial ports it may not even be connected to anything.  A 9-pin
  connector doesn't even have a chassis ground.  The signal ground is
  pin 7 and is usually grounded to chassis ground.  This means that part
  of the signal current will flow thru the ground wires of the building
  wiring (undesirable).  Cable shields are supposed to be only grounded
  at one end of the cable, but it may be better to ground both ends
  since it's better to have current in the shield than in the building
  wiring ??


  11.3.  Modem Connection

  Using a terminal-modem combination (without a computer) one may
  connect to BBS's.  Some BBS's (such a free-nets) permit Internet
  access via the text browser lynx which will work on text terminals.
  Thus with an old terminal and external modem, one may connect to the
  Internet.  If one connects to a host computer on which one has an
  account, then one may sometimes store ones work (or downloads) there.


  11.3.1.  Dialing Out From a Terminal

  Instead of connecting a terminal (or computer emulating a terminal)
  directly to a host computer using a cable it may be connected to the
  host via a telephone line (or dedicated private line) with a modem at
  each end of the line.  The terminal (or computer) will usually dial
  out on a phone line to a host computer.

  Most people use a PC and modem for dialing out.  The PC could have a
  terminal connected to a serial port and the person at the terminal may
  dial out using the PC.   Connecting a real terminal directly to an
  external modem is more difficult since the real terminal isn't very
  intelligent and doesn't give as much feedback to the user.  For
  dialing out, many terminals can store one or more telephone numbers as
  messages which may be "set-up" into them and are sent out to the modem
  by pressing certain function keys.  Many modems can also store phone
  numbers.  The modem initiation sequence must precede the telephone
  number.  When the outgoing call is answered by another modem at the
  other end of the phone line, the the host computer on this modem may
  run a getty program to enable you to log in.




  11.3.2.  Terminal Gets Dialed Into

  It's common for a computer running Linux to get dialed into.  The
  caller gets a login prompt and logs in.  At first glance, it may seem
  strange how a dumb terminal (not connected to any computer) could
  accept an incoming call, but it can.  One possible reason for doing
  this is to save on phone bills where rates are not symmetric.  Your
  terminal needs to be set up for dial-in: Set the modem at your
  terminal for automatic answer (Register S0 set to 2 will answer on the
  2nd ring).  You turn on the terminal and modem before you expect a
  call and when the call comes in you get a login prompt and log in.

  The host computer that dials out to your terminal needs to do
  something quite unusual.  As soon as your modem answers, it needs to
  run login (getty).   A host may do this by running the Linux program
  "callback" sometimes named "cb".  Callback is for having computer A
  call computer B, and then B hangs up and calls A back.  This is what
  you want if you are using computer A to emulate a terminal.  For the
  case of a real terminal this may be too complex a task so the host may
  utilize only the "back" part of the callback program.  The setup file
  for callback must be properly configured at the host.  Callback makes
  the call to the terminal and then has mgetty run a login on that port.
  Mgetty by itself (as of early 1998) is only for dial-in calls but
  there is work being done to incorporate callback features into it and
  thus make it able to dial-out.  As of early 1999 it didn't seem to
  have been done.


  11.4.  Terminal Server Connection

  One use for terminal servers is to connect many terminals (or modems)
  to a high speed network which connects to host computers.  Of course
  the terminal server must have the computing power and software to run
  network protocols so it is in some ways like a computer.  The terminal
  server may interact with the user and ask what computer to connect to,
  etc. or it may connect without asking.  One may sometimes send jobs to
  a printer thru a terminal server.

  A PC today has enough computing power to act like a terminal server
  for text terminals except that each serial port should have its own
  hardware interrupt.  PC's only have a few spare interrupts for this
  purpose and since they are hard-wired you can't create more by
  software.  A solution is to use an advanced multiport serial card
  which has its own system of interrupts (or on lower cost models,
  shares one of the PC's interrupts between a number of ports).  See
  Serial-HOWTO for more info about such cards.  If such a PC runs Linux
  with getty running on many serial ports it might be thought of as a
  terminal server.  It is in effect a terminal server if it is linked to
  other PC's over a network and if its job is mainly to pass thru data
  and handle the serial port interrupts every 14 (or so) bytes.
  Software called "radius" is sometimes used.

  Terminal servers evolved to serve more than just terminals.  They also
  serve PC's which emulate terminals, and were sometimes connected to a
  bank of modems connected to phone lines.  With the advent of 56k
  digital modems that require a digital connection to service an
  incoming phone call, a digital interface to the telephone company was
  needed.  This (and more) is provided today by "remote access servers"
  which have replaced the terminal server.  Instead of many individual
  telephone line cables connected to a terminal server, one now finds
  just a few cables with many digitized telephone calls on a each cable
  (multiplexed).  The multitude of connectors needed for large numbers
  of terminals or modems is no longer present on a remote access server
  and thus the successor to the terminal server can't readily serve
  text-terminals anymore.

  11.5.  Connector and Adapter Types

  A connector is more-or-less permanently attached to the end of a cable
  or to a hardware unit.  There are two basic types of connectors used
  in serial communications: 1. DBxx with pins (such as DB25) and 2.
  modular telephone-style connectors.

  An adapter looks about like a connector but it has two ends.  It is
  just like a cable that is so short that there is no cable part left at
  all --just different connectors on each end is all that remains.  The
  adapter just plugs in on each side.  It allows two incompatible
  connectors to mate with each other by going in between them.
  Sometimes the purpose of the adapter is to interchange wires.
  Obviously, one may use a special cable (perhaps homemade) as a
  substitute for an adapter.


  11.5.1.  Sex of Connector/Adapters

  Connectors (or one side of adapters) are either male or female.  The
  connectors that have pins are male and the ones that have sockets
  (sometimes also called pins) are female.  For modular connectors, the
  ones with exposed contacts are plugs while the ones with internal
  contacts (not easy to see) are jacks.  Plugs are male; jacks are
  female.


  11.5.2.  Types of Adapters

  There are three basic types of adapters: null modem, gender changers
  and port adapters.  Some adapters perform more than one of these three
  functions.

  o  null modem adapter: Reroutes wires.  Like a null modem cable.

  o  gender changer: Changes the sex of a cable end.  Two connectors of
     the same sex can now connect (mate) with each other.

  o  port adapter: Goes from one type of connector to another (DB9 to DB
     25, etc.)



  11.5.3.  DB Connectors

  (For how to install a DB connector on the ends of a cable see
  ``Installing DB Connectors''.)  These come in 9 or 25 pins.  The
  EIA-232 specs. call for 25 pins but since most of these pins are not
  used on ordinary serial ports, 9 pins is sufficient. See ``DB9-DB25''
  for the pin-out.  The pins are usually numbered if you look closely
  enough or use a magnifying glass.


  11.5.4.  RJ Modular Connectors

  These look like modern telephone connectors but are sometimes not
  compatible with telephone connectors.  See also ``Installing RJ
  Connectors''.  They may be 6, 8, or 10 conductor.  RJ11/14 is a 4-6
  conductor telephone plug.  A look-alike is a MMJ connector
  (6-conductor) used on later model VT (and other) terminals.   MMJ has
  an offset tab and is not compatible with RJ11/14.  However, some
  connectors have been made that are compatible with both MMJ or
  RJ11/14.  The MMJ pin-out is: 1-DTR, 2-TXD, 3-TXD GND, 4-RXD GND,
  5-RXD, 6-DSR.


  A null-modem cable with MMJ (or RJ11/14) connectors will connect: 1-6,
  2-5, and 3-4.  Note that such a cable support DTR/DSR flow control
  which is not supported (yet) by Linux.  Making up your own 6-conductor
  null-modem cable is very simple if you understand that the ordinary
  4-conductor telephone cable from the wall to your telephone, used in
  hundreds of millions of homes, is also a null-modem cable.  Find one
  and wire your cable the same way.

  If you lay such a cable (or your terminal null-modem cable) flat on
  the floor (with no twists) you will note that both plugs on the ends
  have their gold contacts facing up (or both facing down).  Although
  it's symmetrical, it is also null- modem if you think about it a bit.
  One may put a few such cables together with inline couplers and
  everything works OK because each inline coupler is also a null-modem
  adapter.  Two null-modem devices in series result in a straight-thru
  connection.

  RJ45 and RJ48 are 8-conductor (or 10-conductor for some computer
  applications) telephone plugs.  The 10-conductor ones are allegedly
  wider and will not mate with 8-conductor ones.  They are used for both
  flat telephone cable and round twisted pair cable.  The cable end of
  the connector may be different for round and flat cable and both RJ45
  and RJ48 may be 8 or 10 conductor so make sure you get the right one.
  RJ48 has an extra tab so that a RJ48 plug will not push into a RJ45
  jack (but a RJ45 plug will mate with a RJ48 jack).  They're used on
  some multiport serial cards and networks.  Heres the pin numbers for
  an 8-conductor:


    Plug                          Jack
    (Looking at the end           (Looking at the cavity
     end of a cable)               in a wall)
       .__________.                   .__________.
       | 87654321 |                   | 12345678 |
       |__.    .__|                   |__.    .__|
          |____|                         |____|




  11.6.  Making or Modifying a Cable

  11.6.1.  Buy or Make ?

  You may try to buy a short, null modem cable.  They are often labeled
  as serial printer cables (but serial printers are not very popular
  today and neither are the cables).  Unfortunately, they will probably
  not work for hardware flow control.  Make sure the connectors on the
  cable ends will fit the connectors on your computer and terminal.

  But if you need longer cables to connect up terminals or need hardware
  flow control, how do you get the right cables?  The right ready-made
  cables may be difficult to find (you might find them by searching the
  Internet), especially if you want to use a minimum (say 4) of
  conductors.  One option is to get them custom made, which is likely to
  be fairly expensive although you might find someone to make them at
  prices not too much higher than ready-made cable (I did).  Another
  alternative is to make your own.  This may require special tools.  If
  you can find it, used cable can be a real bargain but you will
  probably need to rewire the connectors.  Most connectors that come
  with short cables are permanently molded to the cable and can't be
  rewired but most custom-made and homemade cables have connectors that
  can be rewired.  One advantage of making your own cable is that the
  skills you learn will come in handy if a cable breaks (or goes bad) or
  if you need to make up another cable in a hurry.

  11.6.2.  Pin Numbers

  The numbers of the pins should be engraved in the plastic of the
  connector.  Each pin should have a number next to it.  You may need a
  magnifying glass to read them.


  11.6.3.  Installing DB Connectors on Cable Ends

  See ``DB Connectors'' for a brief description of them.  Unfortunately,
  most cables one purchases today have molded connectors on each end and
  can't be modified.  Others have connectors which unscrew and can be
  rewired.  If you are making up cable or modifying an existing one then
  you need to know about pins.  There are two types: soldered and
  crimped.

  The crimped pins require a special crimping tool and also need an
  "insertion/extraction" tool.  But once you have these tools, making up
  and modifying cable may be faster than soldering.  If you are
  connecting two wires to one pin (also needed if you want to jumper one
  connected pin to another pin) then soldering is faster (for these
  pins).  This is because the crimped pins can only take one wire each
  while the soldered ones can accept more than one wire per pin.

  To insert crimped pins just push them in by hand or with the tool.
  Removing a pin with the tool is a little tricky.  These directions can
  be best understood if you have both the tool and wires in front of
  you.  You must put the tool tip around a wire at the the back of a pin
  and insert it as far as it will go into the hole (about 1 1/2 cm.).
  Some tools have a mark (such as a tiny hole) on them to indicate how
  far to insert it.  The tool tip should have a tapered gap so that you
  may get the tip around the wire by starting it in where the gap is
  wider than the wire.  The tool may have 2 tips.  The one that is the
  most difficult to get around the wire is also the one that removes the
  wire the easiest since it almost completely envelops the wire.

  With the tip properly inserted pull on both the tool and the wire with
  a gentle pull.  If it doesn't come out, the tool was likely not
  inserted correctly so either push it in more or twist it to a
  different position (or both).   Perhaps you should have used the other
  tip that goes more around the pin.  Using this tool, one may readily
  convert a straight-thru cable to a null-modem cable, etc.

  There can be problems using the "insertion/extraction" tool.  If the
  tools will not insert on the back of the pin, it could be that the pin
  was not neatly crimped to the wire and is sort of square where it
  should be round, etc.  If a pin starts to come out but will not pull
  out all the way, the pin may be bent.  Look at it under a magnifying
  glass.  Straightening a pin with needle-nose pliers may damage the
  gold plating.  Sometimes a stuck pin may be pushed out with a thick
  screwdriver blade tip (or the like) but if you push too hard you may
  gouge the plastic hole or bend the pin:.

  Don't try soldering unless you know what you're doing or have read
  about how to do it.


  11.6.4.  Installing RJ Connectors

  These are telephone modular connecters one type of which is used for
  most ordinary telephones.  But there are many different types (see
  ``RJ Modular Connectors'').

  These are not easy to reuse.  You might be able to pull the wires out,
  push in something wedged that would lift up the gold-colored contacts
  and reuse the connector.  There are special crimping tools used to
  install them; a different tool for each type.

  If you don't have a crimping tool, installation is still possible (but
  difficult) using a small screwdriver (and possibly a hammer).  Push in
  the cable wires and then push each gold-colored contact down hard with
  a small screwdriver that will just fit between the insulating ridges
  between the contacts.  You may damage it if you fail to use a
  screwdriver with a head almost the same thickness as the contacts or
  if the screwdriver slips off the contact as you are pushing it down.
  You may also use a small hammer to pound on the screwdriver (push
  first by hand).

  Be sure to not hurt the "remove lever" on the connecter when you push
  in the contacts.  Don't just set it down on a table and push in the
  contacts.  Instead, put a shim (about 1 mm thick) that fits snugly in
  the crevice between the lever and the body.  For such a shim you may
  use thick cardboard, several calling cards, or wood.  Since the bottom
  of the connector (that you will put on the table) isn't level (due to
  the "remove lever), make sure that the table top has something a
  little soft on it (like a sheet of cardboard) to help support the non-
  level connector.  Even better would be to put another 1mm shim under
  the first 6mm of the connector, supporting it just under where you see
  the contacts.  A soft tabletop wouldn't hurt either.  Another method
  (I've never done this) is to hold the connector in a vice but be
  careful not to break the connector.

  As compared to using a crimping tool, installing it per above takes a
  lot longer and is much more prone to errors and failure but it's
  sometimes more expedient and a lot cheaper than buying a special tool
  if you only have one or two connectors to install.


  12.  Set-Up (Configure) in General

  12.1.  Intro to Set-Up

  Configuring (Set-Up) involves both storing a configuration in the non-
  volatile memory of the terminal, and putting commands in start-up
  files (on you hard disk) that will run each time the computer is
  powered on (or possibly only when the run-level changes).  This
  section gives an overview of configuring and covers the configuring of
  the essential communication options for both the terminal and the
  computer.  The next two major sections cover in detail the
  configuration of the terminal (see ``Terminal Set-Up'' and the
  computer (see ``Computer Set-Up (Configure) Details''.


  12.2.  Terminal Set-Up (Configure) Overview

  When a terminal is installed it's necessary to configure the physical
  terminal by saving (in its non-volatile memory which is not lost when
  the terminal is powered off) the characteristics it will have when it
  is powered on.  You might be lucky and have a terminal that has
  already been set-up correctly for your installation so that little or
  no terminal configuration is required.

  There are two basic ways of configuring a terminal.  One is to sit at
  the terminal and go thru a series of set-up menus.  Another is to send
  escape sequences to it from the host computer.  Before you can send
  anything to the terminal (such as the above escape sequences), its
  ``Communication Interface'') options such as the baud rate must be set
  up to match those of the computer.  This can only be done by sitting
  at the terminal since the communications must be set up right before
  the computer and the terminal can "talk" to each other.  See
  ``Terminal Set-Up''.

  12.3.  Computer Set-Up (Configure) Overview

  Besides possibly sending escape sequences from the computer to
  configure the terminal, there is the configuring of the computer
  itself to handle the terminal.  If your lucky, all you need to do is
  to put a "getty" command in the /etc/inittab file so that a "login:"
  prompt will be sent to the terminal when the computer starts up.  See
  ``Getty (in /etc/inittab)'' for details.  this for the computer.

  The computer communicates with the terminal using the device driver
  software (part of the kernel).  The serial device driver has a default
  configuration and is also partly (sometimes fully) configured by the
  getty program before running "login" at each terminal.  However,
  additional configuration is sometimes needed using programs named
  "stty" and "setserial".  These programs (if needed) must be run each
  time the computer starts up since this configuration is lost each time
  the computer powers down.  See ``Computer Set-Up (Configure)
  Details''.


  12.4.  Many Options

  There are a great many configuration options for you to choose from.
  The communication options must be set right or the terminal will not
  work at all.  Other options may be set wrong, but will cause no
  problem since the features they set may not be used.  For example, if
  you don't have a printer connected to the terminal it makes no
  difference how the printer configuration parameters are set inside the
  terminal.  This last statement is not 100% correct.  Suppose that you
  have no printer but the compute (by mistake) sends the terminal a
  command to redirect all characters (data) from the computer to the
  printer only.  Then nothing will display on the screen and your
  terminal will be dead.  Some terminals have a configuration option to
  inform the terminal that no printer is attached.  In this case the
  terminal will ignore any command to redirect output to the "printer"
  and the above problem will never happen.  However, this doesn't help
  much since there are many other erroneous commands that can be sent to
  your terminal that will really foul things up.  This is likely to
  happen if you send the terminal a binary file by accident.

  In some cases a wrong setting will not cause any problem until you
  happen to run a rare application program that expects the terminal to
  be set a certain way.  Other options govern only the appearance of the
  display and the terminal will work fine if they are set wrong but may
  not be as pleasant to look at.

  Some options concern only the terminal and do not need to be set at
  the computer.  For example: Do you want black letters on a light
  background?  This is easier on the eyes than a black background.
  Should a key repeat when held down?  Should the screen wrap when a
  line runs off the right end of the screen?  Should keys click?


  12.5.  Communication Interface Options

  Some of these communication settings (options) are for both the
  terminal and the computer and they must be set exactly the same for
  both: speed, parity, bits/character, and flow control.  Other
  communication options are only set at the terminal (and only a couple
  of these are essential to establish communications).  Still others
  such as the address and interrupt (IRQ) of the physical port ttyS2 are
  set only at the computer using the "setserial" command.  Until all of
  the above essential options are compatibly set up there can be no
  satisfactory serial communication (and likely no communication at all)
  between the terminal and the computer.  For the terminal, one must set
  these options manually by menus at each terminal (or by using some
  sort of special cartridge at each terminal).  The host computer is
  configured by running commands each time the computer is powered up
  (or when people log in).  Sometimes the getty program (found in the
  /etc/inittab file) which starts the login process will take care of
  this for the computer.  See ``Getty (in /etc/inittab)''

  The settings for both the computer and the terminal are:

  o  ``Speed (bits/second) ''

  o  ``Parity''

  o  ``Bits per Character ''

  o  ``Flow Control ''

  Some essential settings for the terminal alone are:

  o  ``Port Select''

  o  Set communication to full duplex (=FDX on Wyse terminals)


  If the  ``Getty (in /etc/inittab)'' program can't set up the computer
  side the way you want, then you may need to use one (or both) of the
  ``Stty & Setserial'' commands.


  12.5.1.  Speed

  These must be set the same on both the terminal and the computer.  The
  speed is the bits/sec (bps or baud rate).  Use the highest speed that
  works without errors.  Enabling flow control may make higher speeds
  possible.  There may be two speeds to set at the terminal: Transmit
  and Receive, sometimes abbreviated T and R.  Usually they are both set
  the same since stty in Linux doesn't seem to have the option yet of
  setting them differently.  (There is an option to do this with the
  "stty" command but it seems to actually set them both the same.)
  Common speeds are 300, 600, 1200, 2400, 4800, 9600, 19200, 38400, ...
  The slower speeds (like 600) are for printers and hard-copy terminals.


  12.5.2.  Parity & Should You Use It ?

  For a definition see ``Parity Explained''.  Parity-disabled is often
  the default.  To enable parity, you must both enable it and then
  select either even or odd parity.  It probably makes no difference if
  it's odd or even.  For terminals there are sometimes settings for both
  transmit and receive parity.  You should set both of these the same
  since stty at the computer doesn't permit setting them differently.
  The PC serial port usually can't support different parities either.
  Some terminal are unable to set receive parity and will simply always
  ignore received parity bits.  On some older terminals if you use
  8-data-bits per byte then parity will not work since there is no room
  in the hardware for the extra parity bit.

  Should you use parity  at all?  Parity, while not really necessary, is
  nice to have.  If you don't have parity, then you may get an incorrect
  letter here and there and wind up trying to correct spelling errors
  that don't really exist.  However parity comes at a cost.  First, it's
  more complicated to set up since the default is usually no parity.
  Secondly, parity will slow down the speed with which bytes travel over
  the serial cable since there will be one more bit per byte.  This may
  or may not slow down the effective speed.


  For example, a hard-copy terminal is usually limited by the mechanics
  of the printing process.  Increasing the bytes/sec when the computer
  (its UART chip) is transmitting only results in more flow-control
  "halt" signals to allow the mechanical printing to catch up.  Due to
  more flow-control waits the effective speed is no better without
  parity than with it.  The situation is similar for some terminals:
  After you implement parity there may be fewer flow-control waits per
  unit time resulting in more bits/sec (average).  However, due to the
  added parity bits the bytes/sec (average) stays the same.

  One option is to install terminals with no parity.  Then if parity
  errors are noticed, it can be implemented later.  To spot possible
  errors with no parity, look for any spelling errors you don't think
  you made.  If you spot such an error, refresh the screen (retransmit
  from the computer).  If the error goes away, then it's likely a parity
  error.  If too many such errors happen (such as more than one every
  few hundred screens) then corrective action is needed such as:  Enable
  parity and/or reduce speed, and/or use a shorter/better cable.
  Enabling parity will not reduce the number of errors but it will tell
  you when an error has happened.

  Just the opposite policy is to initially enable parity.  Then if no
  parity errors (error symbols on the CRT) are ever seen (over a
  reasonable period of time, say a month or two) it may be safely
  disabled.


  12.5.3.  Bits/Character

  This is the character size (the number of data bits per character
  excluding any parity bit).  For ASCII it's 7,  but it's 8 for ISO
  character sets.  If you are only going to use ASCII characters, then
  select 7-bits since it's faster to transmit 7 bits than 8.  Some older
  terminals will only display 7-bit characters.


  12.5.4.  Which Flow Control (Handshaking) ?

  The choice is between "hardware" (for example  dtr/cts) or "software"
  (Xon/Xoff) flow control.  (The Adds terminal menu incorrectly use
  "Xon/Xoff" to mean any kind of flow control.)  While hardware flow
  control may be faster (if the one or two extra wires for it are
  available in the cable and if the terminal supports it) in most cases
  Xon/Xoff should work OK.  Some people report that they solved
  disturbing problems (see below) by converting to hardware flow control
  but software flow control has worked fine at other installations (and
  for me personally).

  If you use software (Xon/Xoff) flow control and have users who don't
  know about it, then they may accidentally send an Xoff to the host and
  lock up their terminal.  While it's locked, they may type frantically
  in a vain attempt to unlock it.  Then when Xon is finally sent to
  restore communication, all that was typed in haste gets executed,
  perhaps with unexpected results.  They can't do this with hardware
  flow control.  See ``Flow Control'' for an explanation of flow
  control.


  12.5.5.  Port Select

  Since most terminals have two or more connectors on the back, it is
  usually possible to assign one of these connecters to connect to the
  host computer and assign another connector to be the printer port.
  The connector may have a name next to it (inspect it) and this name
  (such as Aux, Serial 2, or Modem) may be assigned to either be the
  main host connection or the printer connection (or the like).
  12.6.  Quick Attempt

  While all the above may seem overly complex, to get a terminal working
  is often fairly simple.  The ``Quick Install'' section describes a
  simple way to try to do this.  But if that doesn't work or if you want
  to make the display look better and perform better, more reading will
  be needed.


  13.  Terminal Set-Up (Configure) Details

  Except for the next subsection on sending escape sequences to the
  terminal, this section mainly presents the details of setting up the
  terminal manually by sitting at the terminal and going thru menus.  If
  you haven't already done so, you should read ``Terminal Set-Up
  (Configure) Overview''.  It's best if you have a terminal manual, but
  even it you don't there is information here on many of the options
  which you might possibly need to set.

  The communication parameters such as its baud rate must always be set
  up at the terminal since if this is not done there can be no
  communication with the terminal.   Once communication is established
  you have two choices for doing the rest the terminal configuration.
  You may continue to configure manually at the terminal and save the
  results in the terminal's non-volatile memory or you may do this by
  sending escape sequences to the terminal from the computer each time
  the terminal is powered on (or the like).

  If you know how to set up and save a good configuration inside the
  terminal it may be the best way.  If you don't, you might want to just
  send the init string from terminfo to your terminal each time you use
  the terminal.  Perhaps doing nothing will still give you a usable
  terminal.  You (or an application program) can always change things by
  sending certain escape sequences to the terminal.


  13.1.  Send Escape Sequences to the Terminal

  Once the communication interface is established, the rest of the
  configuration of the terminals may sometimes be done by sending escape
  sequences to the terminals from the computer.  If you have a large
  number of terminals, it may be worthwhile to write (or locate) a shell
  script to automatically do this.  There may (or may not) be a command
  you can send to a terminal to tell it to save its current set-up in
  its non-volatile memory so that it will be present the next time the
  terminal is powered on.

  There is an simple way to send these escape sequences and a complex
  way.  Using the simple way, you never look up escape sequences but
  issue commands that automatically find an appropriate escape sequence
  in the terminfo database and send that.  Unfortunately, not all the
  escape sequences which you might want to send are always in the
  terminfo database.  Thus the more complex (but possibly better) way is
  to directly send escape sequences.

  For this complex method you'll need an advanced manual.  Old terminal
  manuals once included a detailed list of escape sequences but newer
  ones usually don't.  To find them you may need to purchase another
  manual called the "programmers manual" (or the like) which is not
  supplied with the terminal.  A ``Esc Sequence List'' for some
  terminals is on the Internet but it's terse and likely incomplete.

  Even without a manual or the like, you may still send commands to
  configure the terminal by using the programs "tput" and "setterm".
  See ``Changing the Terminal Settings''.  You could just send the
  terminal an init string from the terminfo entry if the init string
  sets up the terminal the way want it.  See ``Init String''.  Unless
  you plan to have these sequences sent from the computer to the
  terminal each time the terminal is powered on, you must somehow save
  the settings in the non-volatile memory of the terminal.


  13.2.  Older Terminals Set-Up

  On older terminals look at the keyboard for labels just above the top
  row of numeric keys.  If they exist, these labels may be what these
  keys do in set-up mode.  Some older terminals may have only one "set-
  up" menu.  Still older ones have physical switches.  In some cases not
  all the switches are well labeled but they may be well concealed.  Of
  course, if you set something with a switch, it's "saved" and there is
  no need to save the setting in non-volatile memory.


  13.3.  Getting Into Set-Up (Configuration) Mode

  To select options (configure) at the terminal, you must first enter
  "set-up" mode and then select options (i.e. configure) using menus
  stored inside the terminal and displayed on the screen.  To do this,
  the terminal does not even need to be connected to a computer.  How to
  get into set-up mode is covered in the terminal's manual, but here's
  some hints that may help:

  If there's a "set-up" key try pressing it.  Also try it shifted.

  o  Wyse: First try the shifted "Select" key; then substitute Ctrl for
     shifted in all of the above.

  o  VT, Dorio: F3 may be the set-up key.  On VT420 and later models
     this key may have been programmed to do something else so turn off
     the power.  When you turn on the power again, hit the F3 key as
     soon as you get an initial screen message.

  o  IBM: 3151: Ctrl-ScrollLock.  3153: Ctrl-Minus_on_Keypad (or like
     3151)

  To move around in the set-up menus, try the arrow keys.  Use Return,
  Space, or a special key ("toggle" on old terminals) to select.  To
  exit set-up mode select exit from a menu (or on some older terminals
  press the set-up key again).


  13.4.  Communication Options

  For the terminal to work at all, speed, parity, bits/character, and
  communication mode must be set correctly.  Incorrect flow control may
  cause loss and/or corruption of data seen on the screen. The essential
  communication options were dealt with (for both the terminal and
  computer) in another section:  See ``Communication Interface''.  The
  following list provides some links to that section, as well as some
  additional communication options set only at the terminal.


  o  ``Speed (bits/second) '' (baud rate): 9600, 19200, etc.

  o  ``Parity'' none, even, odd, mark, space

  o  ``Bits per Character '' {Data}: 7 or 8

  o  ``Flow Control:'' or Handshake {Hndshk}: none, Xon-Xoff, or
     hardware (DTR, etc).


  o  Receiver Handshake {Rcv Hndshk} protects data being Received by the
     terminal by transmitting flow-control signals to the host.

  o  Transmitter Handshake {Xmt Hndshk} is protection of data being
     Transmitted by the terminal.   The terminal receives flow-control
     signals (and locks/unlocks the keyboard).  Includes "Incoming
     Xon/Xoff".

  o  number of stop bits: 1 or 2. See ``Voltage Sequence for a Byte''

  o  Flow control level {Rcv Hndshk Level} {{Xoff at ...}}: Flow control
     will send "stop" when this number of bytes in the terminal's buffer
     is exceeded.

  o  ``Communication Mode'' {Comm}: ``Full Duplex {FDX}, Half Duplex
     {HDX}'' {{Local Echo}}, ``Local Mode'' {{Online/Local}}

  o  Transmit Rate (Speed) Limit {Xmt Lim}: limits the transmit rate to
     the specified cps (chars/sec) even though the baud rate setting may
     be at a higher speed.

  o  Function-Key Rate Limit: as above but for function key messages.

  o  ``Port Select'': Which physical connecter is for the host {Host
     Port} ?


  13.5.  Saving the Set-up

  Your set-up must be saved in the non-volatile memory of the terminal
  so that it will be effective the next time you turn on the terminal.
  If you fail to save it, then the new settings will be lost when you
  turn off the terminal.  Before you go to the trouble of setting up a
  terminal, make sure that you know how to save the settings.  For
  modern terminals the save command is done via a menu.  In some older
  terminals, only the manual tells how to save.  For many of these you
  press Ctrl-S to save.


  13.6.  Set-Up Options/Parameters

  See the Teemworld's Set-Up <http://www.pericom-
  usa.com/twdocs/doc/twusec7.htm> for a description of many of these
  parameter as used in terminal emulation.  Emulation is often a little
  different than an actual terminal.

  What follows in this section describes some of the options which are
  available in the set-up menus of many terminals.  Options are also
  called parameters or features.  Many options may be called "modes".
  Setting options is often called "configuring".  Many of these options
  may also be set by sending certain escape sequences to the terminal.
  Different models and brands of terminals have various options and the
  same option may be called by different names (not all of which are
  given here) Terse names used by Wyse are enclosed in {...}.  Names
  used mostly for VT terminals are enclosed in {{...}}.


  13.7.  Emulation {Personality} {{Terminal Modes}}

  Most modern terminals can emulate several other terminals.  The
  terminal can likely do more if it is set to emulate itself (actually
  no emulation) {native personality}.   Sometimes there are 2 different
  emulations for the same model of terminal.  For example VT220-7
  emulates a VT200 with 7-bits/byte while VT220-8 emulates a VT220 with
  8-bits/byte (256 possible characters).

  Older models of terminals usually have fewer features than newer
  models.  Suppose one wanted to emulate an old terminal but also wanted
  some of the advanced capabilities of the later model terminal they are
  sitting at.  This is sometimes possible (to some degree).  This
  feature is sometimes called {Enhance} (or Enhanced ??).


  13.8.  Display Options

  13.8.1.  Character Cell Size {Char Cell}

  This is the size of the cell in which a character fits.  It is
  measured in pixels (=tiny dots).  The more dots, the better the
  resolution.   10x16 is 10 dots wide by 16 dots high (16 rows and 10
  columns).  Note the notation is inverted as compared to the notation
  for matrix dimensions which gives rows (height) first..  Also, the
  character cell includes rows and columns of pixels allocated for the
  space between adjacent characters so the cell size which defines the
  boundaries of an actual character may be smaller.


  13.8.2.  Columns/Lines

  Usually 80 columns and 24 or 25 lines.  This means that there may be
  up to 80 characters in a row (line) on the screen.   Many terminals
  have a 132 column option but unless you have a large screen, the tiny
  characters may be hard to read. {{Set 132 column mode}}


  13.8.3.  Cursor

  The cursor may be set to appear as a rectangle (= block) {Blk}.  Other
  options are underline {Line} or blinking.  I prefer non-blinking
  {Steady} block since it's big enough to find quickly but there is no
  distractive blinking.  If you set it invisible (an option on some
  terminals) it will disappear but new letters will appear on the screen
  as you type at the invisible cursor.


  13.8.4.  Display Attributes (Magic Cookies)

  ``Display Attributes'' may either be magic cookies or be attribute
  bytes assigned to each character.  For magic cookies, there is a limit
  to their extent: Are they in effect to the end of the line or to the
  end of the page?  It's best to use attribute bytes (which could
  actually be half-bytes = nibbles).


  13.8.5.  Display Control Characters {Monitor}

  May be called various names such as "Display Controls".  When off
  (normal) it's "Interpret Controls".  When set on, you see the escape
  sequences from the host (which you normally never see on the screen).
  So that these sequences may be viewed in sequence on a line, they are
  not acted upon (interpreted) by the terminal.  Except that a CR LF
  sequence creates a new line.  See ``Control Codes''.


  13.8.6.  Double Width/Height

  Some terminals can have their characters double width and/or double
  height.  This feature is seldom needed.  When changing a line to
  double width (DW) the right half (RH) is pushed off the screen and
  there is the question of whether or not to delete (erase) it.
  "Preserve" means to keep the RH of DW lines.  When in double height
  mode, it may be necessary to send each such line twice (the 2nd time
  down one row) in order to get a double-height line on the screen.


  13.8.7.  Reverse Video {Display} (Background Light/Dark)

  Normal video is light (white, green, amber) letters (foreground) on a
  dark (black) background.  Reverse video {Display Light} is the
  opposite: black text on a light background.  This is easier on the
  eyes (unless the room is dark).


  13.8.8.  Status Line

  A status line is a line at the top or bottom of the screen that
  displays info about the application program you are running.  It's
  often highlighted in some way.  With a status line enabled, an
  application can send the terminal a special escape sequence which
  means that the text that follows is for the status line.  However,
  many applications don't use this feature but instead only simulate a
  real status line by direct cursor positioning.  The ordinary user
  looking at it doesn't know the difference.


  13.8.9.  Upon 80/132 Change: Clear or Preserve?

  When switching the number of columns from 80 to 132 (or conversely)
  should the data displayed in the old format be erased (cleared) or
  preserved? {80/132 Clr} {{Screen Width Change}}.  It should make no
  difference how you set this option since if an application program
  uses 132 columns, it should set this option appropriately via a
  control sequence.


  13.9.  Page Related Options

  For a Wyse terminal to be able to access multiple pages of display
  memory {Multipage} must be set to on.


  13.9.1.  Page Size

  The terminal memory may be divided up into a number of pages.  See
  ``Pages'' and ``Pages (definition)'' for explanations of pages.  You
  may partition the page memory into a number of pages of selected
  length.  Linux applications don't seem to use pages at present so it
  shouldn't make much difference how you set this up.


  13.9.2.  Coupling (of cursor & display)

  The terminal memory may be divided up into a number of pages.  See
  ``Pages'' and ``Pages'' for explanations of pages.  When the cursor is
  moved to a location in video memory not currently displayed (such as
  another page, or on the same page but to a location not displayed on
  the screen) should the display change to let one view the new cursor
  location?  If so, this is called "Coupling".  For cursor movement
  within the same page there is "Vertical Coupling" and "Horizontal
  Coupling".  For movement to another page there is "Page Coupling".


  13.10.  Reporting and Answerback

  The terminal will identify itself and its state, or send out a pre-
  recorded message in response to certain escape sequences.


  13.10.1.  Answerback Message (String)

  You may write a short message during set-up which may optionally be
  sent to the host at power-up or be sent to the host in response to a
  request from the host (perhaps the ENQ (inquire) control character).


  13.10.2.  Auto Answerback

  If set, sends the answerback message to the host at power-on without
  the host asking for it.  Do any "getty" processes look for this ??


  13.10.3.  Answerback Concealed

  If set, will never let anyone see the answerback message (except of
  course the host computer).  If it needs to be changed, deselect
  "answerback concealed" and the formerly concealed message will be
  destroyed so you then may enter a new message (but you don't get to
  see the old one).


  13.10.4.  Terminal ID {ANSI ID}

  The terminal send this reply in answer to a request for identity.


  13.11.  Keyboard Options

  13.11.1.  Keyclick

  When set, pressing any key makes a click (broadcast by a tiny
  loudspeaker in the keyboard).  These clicks annoy some people and I
  think it's best to set keyclick off.


  13.11.2.  Caps Lock {Keylock}

  When the Caps-Lock key is down, should only the alphabetic keys
  generate shifted characters?  If set to {Caps} or upper-case-only then
  hitting a number key with the Caps-Lock on will type the number.  To
  get the symbol above the number one must manually hold down the shift
  key.  This is the normal mode.  If set to {Shift} then all keys type
  the shifted character when Caps-Lock is on (hitting the 5 key should
  type % without holding down Shift, etc.).


  13.11.3.  Auto Repeat {Repeat}

  If a key is held down then that key is repeatedly "typed".  This is
  handy for repeatedly typing the same character to create a line across
  the page.


  13.11.4.  Margin Bell

  When the cursor is 8 columns away from the right side of the display,
  a bell is rung (like on an old typewriter).  Almost all editors will
  automatically create a new line if needed (no need to hit the Return
  key) so this feature is seldom needed.


  13.11.5.  Remapping the Keys

  The code sent to the host when a key is pressed is normally the ASCII
  code for that key (depends also on Shift and Control key).  On some
  terminals you may make any key send any code you wish.  That is, you
  may completely remap the keyboard by setting up the terminal that way.
  This may be useful for some foreign languages and Dvorak keyboard
  layouts, etc. which permit one to type faster.


  13.11.6.  Corner Key (for Wyse only)

  Wyse terminals have a key near the lower left corner which may be set
  to do various things.  Its may be labelled "Funct", "Compose
  Character", "Alt", "Hold" or "Scroll Lock".  Early models don't have
  all of the following options:  When set to {Hold} No-Scroll it stops
  the flow of data (using flow control) to the terminal.  Hitting the
  key again restores normal flow.  When set to {Compose} it permits one
  to generate a limited number of pre-defined non-Latin characters.
  When set to Meta, it makes it a meta shift key which sets the high-
  order bit on each byte.  When set to {Funct} (and pressed) any
  alphanumeric key pressed gets a header (SOH) and trailer (CR) byte
  framing the ASCII byte code.  When set to {Kpd Compose} (and pressed)
  then typing a decimal number on the numeric keys (followed by "enter")
  sends out the same number in hexadecimal ??


  13.11.7.  Numeric Keypad or Arrow Keys Sends

  The numeric keypad (the rectangle of mostly numeric keys to the right
  of the main part of the keyboard) can be set to send special codes
  which will do special things in certain application programs.  Ditto
  for the arrow keys.  There is thus a "normal" mode where they send
  what is shown on the keycap (or the normal code sequence for an arrow-
  key) and an "application" mode where a special escape sequence is
  sent.  In some cases there is a "hex" numeric mode which is almost
  like normal numeric mode except that 6 non-numeric keys send the
  letters A-F.  Thus one may type for example "B36F" on the numeric
  keypad.


  13.11.8.  What does shifted-del and shifted-bs send?

  Depending on how they're set up shifted-del sometimes sends the
  control character CAN and shifted backspace sometimes sends DEL.


  13.11.9.  PC Scan Codes

  Newer terminals can emulate a PC keyboard by sending PC scan codes
  (see Keyboard-and-Console-HOWTO) instead of ASCII codes.  This would
  be used if you were directly connected to a PC running Dos/Windows.
  Set {Keycode} to {Scan}.  Emulating the Dec "PCTerm" should do this
  and more.  A serial port under Linux can't cope with such scan codes.


  13.11.10.  Alternate Characters

  Some keys may have alternative letters on them.  When keys is set to
  "Typewriter" they send what they would normally send on a typewriter.
  When keys is set to something else the alternative characters are
  sent.


  13.12.  Meaning of Received Control Codes

  13.12.1.  Auto New Line {Newline}

  In this case "New Line" means a new line starting at the left margin
  below the current line.  In Linux and C "new line" (NL) may have a
  different meaning: the line-feed control character LF also called new-
  line or NL.  This is because in Linux text files, the LF character
  means a "new line starts here" so it's labeled NL.  Normally, a LF
  (NL) sent to a terminal only results in the cursor jumping down one
  line below where is was and does not result in a new line.

  If Auto New Line is set, the above "normal" situation is canceled and
  a physical new line is created on the display upon receiving a LF from
  the host.  This is exactly what one wants in Linux.  Except that (when
  Auto New Line is set) the Return (or Enter) key sends a CR LF sequence
  to the host (for Wyse and VT100, but for VT420 ??).  Since Linux uses
  LF as a "new line" marker in files, Linux would like only a LF to be
  sent (and not a CR LF).  Thus the "New Line" option is seldom used.
  Instead, the required translations are made by the serial port device
  driver by default.  It is as if one gave the command "stty onlcr
  icrnl" but you don't need to do this since it's the default.


  13.12.2.  Auto Line Feed {Rcv CR}

  This is just another type of "Auto New Line".  When a CR (carriage
  return) character is received, a LF (line feed) action is added
  resulting in a new line being displayed.  Since Linux marks the end of
  lines with LF, this option is not used.


  13.12.3.  Recognize Del (Wyse Only ??) or Null

  If off, the DEL character received by the terminal is ignored.  If on
  the DEL performs a destructive backspace.  Null characters are usually
  ignored in any case.  Both DEL and NULL are sometimes used for
  padding.  See ``Padding''


  13.13.  Where New Text Goes

  13.13.1.  Line Wrap

  Also called "Auto Wrap(around)".  What happens when the right edge of
  the screen is reached (col. 80, etc) and no return character (or the
  like) has been sent from the host?  If Line Wrap is set, then the rest
  of the line displays on the line below, etc.  Otherwise, the rest of
  the line is lost and is not seen on the screen.  Any good application
  should provide "return" characters so that wrap happens before the end
  of the screen is reached.  But a raw copy command (and other
  situations) may not do this so it's usually best to set line wrap.

  For an 80 col. screen, most terminals only wrap if the 81st character
  from the host is a graphic (printable) character.  This allows for the
  case where 81st character from the host might be "return" or a
  "newline" (non-graphic characters) which means that the application is
  handing the wrapping OK and intervention by the terminal is not
  needed.


  13.13.2.  Scrolling

  Scrolling {Scrl} is where all the lines on the screen move up or down.
  Its also called "panning" which includes movement sideways.  In
  ordinary scrolling lines roll off the bottom or top of the screen and
  disappear, and new lines from the host appear at the opposite edge
  (top or bottom).  There are 3 types of this: smooth, jump, or burst.
  Burst is not really scrolling since its an instant replacement of an
  old screenfull by a new one (although some lines on the new screen may
  be from the old screen).  Jump is where new lines jump into view one
  at a time.  Smooth {Smth} is where the text moves at a steady speed
  upward or downward.  If the smooth scroll rate is slow enough, one may
  read the newly visible lines when they are still scrolling (in
  motion).

  Smooth scrolling on slow terminals was once useful since one could
  continue reading as the display was scrolling.  But with higher baud
  rates, jump scroll is so fast that little time is lost as the new
  display appears.  Since it takes a little longer to read scrolling
  text than fixed text, it may actually waste more time if smooth
  scrolling is selected.

  If (auto)scrolling {Autoscrl} is disabled, then new text from the host
  must go somewhere so it is put at the top of the display.  If the old
  text is not erased, the new text merges (nonsensically) into the old.
  If the old text is erased, then the new text is out of context.  So
  keep (auto)scrolling enabled.


  13.13.3.  New Page?

  See ``Pages'' and ``Pages'' for explanations of pages.  When the
  current page is full (the last line is finished) should the page
  scroll, or should a new page be created (leaving the previous page
  stored in the terminal's display memory).  If {Autopage} is set, then
  a new page is created.  Since you are probably not using pages, you
  should probably set this to off.


  13.14.  Function Keys

  These are the keys labeled F1, F2, etc.  On older terminals they may
  be labeled PF1, PF2, etc. where the P stands for Programmable.  Some
  keyboards have both.  One may program (redefine) these keys to send
  out a string of user-defined bytes.  They may often be easily
  "programmed" using a certain set-up menu {FKey}.  On some terminals,
  one may also specify where this string is sent to when the key is
  pressed.  In "normal" mode, pressing the key is just like typing the
  string at the keyboard.  In "local" mode pressing the key sends it to
  the terminal (just like if the terminal was in local mode).  This may
  be used to send escape sequences to the terminal so as to configure it
  in a special way.  In "remote" mode, the string is always sent out the
  serial port to the host computer (even if the terminal is in local
  mode).


  13.15.  Block Mode Options

  Some options are only for the case of ``Block Mode''.  This option is
  powerful since it provides forms and takes load off the host computer
  by transmitting in bursts.  But it's more complicated to set up and is
  thus not used too much.


  13.15.1.  Forms Display

  In block mode some regions of the screen are for the text of forms and
  are thus write-protected "Prot" {WPRT}.  Options may set the
  characters in these regions to appear dim, reverse video {WPRT Rev},
  and/or underlined {WPRT Undrln}.  {WPRT Intensity} may be set to dim,
  normal, or even blank (invisible)


  13.15.2.  Send Entire Block ?

  Should write-protected text (the original text in the form) be sent to
  the host upon transmission of a block: {Send All} or is write-
  protected text also read-protected: {Send Erasable}


  13.15.3.  Region to Send

  Should the entire screen be sent or just the scrolling region?  {Send
  Area}.  Should the sending stop when the current cursor position is
  reached?  If {Xfer Term} is set to Cursor, only the data on the screen
  up to the cursor is sent.


  13.15.4.  Block/Page terminator

  What is the termination symbol to be appended to a block of data?
  {Blk End} or at the end of a page {Send Term}ination.


  13.16.  Locks

  There are various types of Locks.  One is the Locked keyboard due to
  flow control.  See ``Keyboard Lock'' Another lock {Feature Lock} is
  that which prohibits the host computer from changing the terminal set-
  up by sending certain escape sequences to the terminal.  Placing such
  a lock may result in unexpected behavior as application programs send
  escape sequences to the terminals that are ignored.  Not all set-up
  parameters lock.  Unless you have a good reason to do so, you should
  not enable such locking.

  A Function Key lock will prohibit the computer from redefining what a
  programmable function key sends.  You may want to use this if you have
  something important programmed into the function keys.


  13.17.  Screen Saver {Scrn Saver}

  Also called "CRT Saver".  This turns off (or dims) the screen after
  the terminal is not used for a period of time.  It prolongs the life
  of the screen and may save some energy.  Hitting any key will usually
  restore the screen and may "execute" that key so it's best to hit the
  shift-key, etc.


  13.18.  Printer

  For Wyse, if there is no {Printer Attached} set it to Off.  It's not
  essential to do this, but if you do it any escape sequence to send
  text to the printer (instead of the terminal) will be ignored.

  Setting up the printer port is about the same (usually simpler) as
  setting up the communications on the main port.  There are a couple of
  options specific to the printer.  Is the printer a serial or parallel
  printer?  If it's parallel it should be designated as such in setup
  and connected to the parallel port on the terminal (if there is one).
  Should a FF (form feed) be sent to the printer at the end of a print
  job?  If {Print Term} is set to FF, this will happen.


  14.  Computer Set-Up (Configure) Details

  There are various files to edit to set up the computer for terminals.
  If you're lucky, you'll only need to edit /etc/inittab.  One does this
  by editing at the console (or from any working terminal).




  14.1.  Getty (in /etc/inittab)

  In order to have a login process run on a serial port when the
  computer starts up (or switches run levels) a getty command must be
  put into the /etc/inittab file.  Getty GETs a TTY (a terminal) going.
  Each terminal needs its own getty command.  There is also at least one
  getty command for the console in every /etc/inittab file.  Find this
  and put the getty commands for the real terminals next to it.  This
  file may contain sample getty lines for text terminals that are
  commented out so that all you need to do is to uncomment them (remove
  the leading #) and change a few arguments.

  The arguments which are permitted depend on which getty you use:
  Two gettys best for directly connected terminals are:

  1. agetty (sometimes just called getty): Very easy to set up. No
     config files.  See ``Agetty''

  2. See ``getty (part of getty_ps)''

     Two gettys best for modems (avoid for terminals) are:

  1. mgetty: the best one for modems; works for terminals too but
     inferior

  2. uugetty: for modems only; part of the getty_ps package

     A simple getty to use only for console login is:

  1. mingetty: for consoles only


  If you don't have the getty you want check other distributions and the
  alien program to convert between RPM and Debian packages.  The source
  code may be downloaded from Serial Software
  <http://sunsite.unc.edu/pub/Linux/system/serial/>.

  If you are not using modem control lines (for example if you only use
  the minimum number of 3 conductors: transmit, receive, and common
  signal ground) you should let getty know this by using a "local" flag.
  The format of this depends on which getty you use.


  14.1.1.  Agetty (may be named getty)

  An example line in /etc/inittab:


       S1:23:respawn:/sbin/getty -L 19200 ttyS1 vt102




  S1 is from ttyS1.  23 means that getty is run upon entering run levels
  2 or 3.  respawn means that if getty is killed, it will automatically
  start up (respawn) again.  /sbin/getty is the getty command.  The -L
  means Local (ignore modem control signals).  -h (not shown in the
  example) enables hardware flow control (same as stty crtscts).  19200
  is the baud rate.  ttyS1 means /dev/ttyS1 (COM2 in MS-DOS).  vt102 is
  the type of terminal and this getty will set the environment variable
  TERM to this value.  There are no configuration files.  Type "init q"
  on the command line after editing getty and you should see a login
  prompt.



  14.1.1.1.  Agetty's detection of parity

  The agetty program will auto-detect any parity set inside the
  terminal.  Except it will not work if you use 8-bit data bytes with
  1-bit parity.  See ``Agetty's parity with 8-bit data bytes''.  If you
  use stty to set parity, agetty will automatically unset it since it
  wants the parity bit to come thru as if it was a data bit.  This is
  because it needs to get the last bit (possibly a parity bit) as you
  type your login-name so that it can auto-detect parity.  Thus if you
  use parity, enable it only at the terminals and let agetty auto-detect
  it and set it at the computer.  If your terminal supports received
  parity, the login prompt will look garbled until you type something so
  that getty can detect the parity.  The garbled prompt will deter
  visitors, etc. from trying to login.  That could be just what you
  want.

  There is sometimes a problem with auto detection of parity.  This
  happens because after you first type your login name, agetty uses the
  login program to finish logging you in.  If the first login attempt
  fails, login runs again to handle all future attempts at logging in
  (including the type-in of your login-name).  The problem is that only
  agetty can detect parity while the login program doesn't detect
  parity.  So if for some reason you wind up in the login program and
  the correct parity hasn't yet been detected, you're in trouble since
  the login program can't detect the parity.  With wrong parity, the
  login program can't correctly read what you type and you can't log in.
  If your terminal supports received parity, you will continue to see a
  garbled screen.

  One may get into this "login loop" in various ways.  Suppose you only
  type a single letter or two for your login name and then hit return.
  If these letters are not sufficient for parity detection, then login
  runs before parity has been detected.  Sometimes this problem happens
  if you don't have the terminal on and connected when agetty first
  starts up.  If you get stuck in this "login" loop a solution is to
  just wait about a minute until agetty runs again due to "timeout".


  14.1.1.2.  Agetty's parity with 8-bit data bytes

  Unfortunately, agetty can't detect this parity.  It (as of late 1999)
  has no option for disabling the auto-detection of parity and thus will
  detect incorrect parity.  The result is that the login process will be
  garbled and parity will be set wrong.  Thus it doesn't seem feasible
  to try to use 8-bit data bytes with parity.


  14.1.2.  getty (part of getty_ps)

  (This is from the old Serial-HOWTO by Greg Hankins)
  Add entries for getty to use for your terminal in  the configuration
  file /etc/gettydefs if there they aren't already there:


  # 38400 bps Dumb Terminal entry
  DT38400# B38400 CS8 CLOCAL # B38400 SANE -ISTRIP CLOCAL #@S @L login: #DT38400

  # 19200 bps Dumb Terminal entry
  DT19200# B19200 CS8 CLOCAL # B19200 SANE -ISTRIP CLOCAL #@S @L login: #DT19200

  # 9600 bps Dumb Terminal entry
  DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #DT9600




  If you want, you can make getty print interesting things in the login
  banner.  In my examples, I have the system name and the serial line
  printed.  You can add other things:


       @B    The current (evaluated at the time the @B is seen) bps rate.
       @D    The current date, in MM/DD/YY.
       @L    The serial line to which getty is attached.
       @S    The system name.
       @T    The current time, in HH:MM:SS (24-hour).
       @U    The number of currently signed-on users.  This is  a
             count of the number of entries in the /etc/utmp file
             that have a non-null ut_name field.
       @V    The value of VERSION, as given in the defaults file.
       To display a single '@' character, use either '\@' or '@@'.




  When you are done editing /etc/gettydefs, you can verify that the
  syntax is correct by doing:


       linux# getty -c /etc/gettydefs




  Make sure there is no getty or uugetty config file for the serial port
  that your terminal is attached to (/etc/default/{uu}getty.ttySN or
  /etc/conf.{uu}getty.ttySN), as this will probably interfere with
  running getty on a terminal.  Remove the file if it exits.

  Edit your /etc/inittab file to run getty on the serial port
  (substituting in the correct information for your environment - port,
  speed, and default terminal type):


       S1:23:respawn:/sbin/getty ttyS1 DT9600 vt100




  Restart init:


       linux# init q




  At this point, you should see a login prompt on your terminal.  You
  may have to hit return to get the terminal's attention.


  14.1.3.  mgetty

  The "m" stands for modem.  This program is primarily for modems and as
  of mid 1999 doesn't always work very well for text-terminals.  It's
  poorly documented for terminals and you may need to wade thru much
  documentation for modems in order to figure out how to use it for
  terminals.  Look at the last lines of /etc/mgetty/mgetty.config for an
  example of configuring it for a terminal.  It will only support
  software (Xon/Xoff) flow control (used by many terminals) if you
  recompile it.  This will be hopefully be fixed in the future.  It
  would be nice to use the same getty for terminals as for modems but
  mgetty needs a little fixing to fill the bill.


  14.2.  Stty & Setserial

  There is both a "stty" command and a "setserial" command for setting
  up the serial ports.   Some (or all) of the needed stty settings can
  be done via getty and there may be no need to use setserial so you may
  not need to use either command.  These two commands (stty and
  setserial) set up different aspects of the serial port.  Stty does the
  most while setserial configures the low-level stuff such as interrupts
  and port addresses.  To "save" the settings, these commands must be
  written in certain files (shell scripts) which run each time the
  computer starts up.  Distributions of Linux often supply a shell
  script which runs setserial but seldom supply one which runs stty
  since on seldom need it.


  14.3.  Setserial

  14.3.1.  Introduction

  Don't ever use setserial with Laptops (PCMCIA).  setserial is a
  program which allows you to tell the device driver software the I/O
  address of the serial port, which interrupt (IRQ) is set in the port's
  hardware, what type of UART you have, etc.  It can also show how the
  driver is currently set.  In addition, it can  probe the hardware (if
  certain options are given).

  If you only have one or two serial ports, they will usually get set up
  correctly without using setserial.  Otherwise (or if there are
  problems with the serial port) you will likely need to deal with
  setserial.  Besides the manual for setserial, check out info in
  /usr/doc/setserial.../ or the like.  It should tell you how setserial
  is handled in your distribution of Linux.

  Setserial is often run automatically at boot-time by a start-up shell-
  script.  It will only work if the serial module is loaded.  If you
  should for some reason unload the serial module later on, the changes
  previously made by setserial will be forgotten by the kernel (but not
  by /etc/serial.conf).  So setserial must be run again to reestablish
  them.  In addition to running via a start-up script, something akin to
  setserial runs when the serial module is loaded.  Thus when you watch
  the start-up messages on the screen it may look like it ran twice, and
  in fact it has.

  Setserial can set the time that the port will keep operating after
  it's closed (in order to output any characters still in its buffer in
  main RAM).  This is needed at slow baud rates of 1200 or lower.  It's
  also needed at higher speeds if there are a lot of "flow control"
  waits.  If your serial port is Plug-and-Play you may need to consult
  other HOWTOs such as Plug-and-Play or Serial.

  With appropriate options, setserial can probe (at a given I/O address)
  for a serial port but you must guess the I/O address.  If you ask it
  to probe for /dev/ttyS2 for example, it will only probe at the address
  it thinks ttyS2 is at.  If you tell setserial that ttyS2 is at a
  different address, then it will probe at that address, etc.  See
  ``Probing''

  Setserial does not set either IRQ's nor I/O addresses in the serial
  port hardware itself.  It's set in the hardware either by jumpers or
  by plug-and-play.  You must tell setserial these identical values that
  have been set in the hardware.  Do not just invent some values that
  you think would be nice to use and then tell them to setserial.
  However, if you know the I/O address but don't know the IRQ you may
  command setserial to attempt to determine the IRQ.

  You can see a list of possible commands to use (but not the one-letter
  options such as -v for verbose --which you should normally use when
  troubleshooting) by just typing setserial with no arguments.  Note
  that setserial calls an I/O address a "port".  If you type:

  setserial-g /dev/ttyS*


  you'll see some info about how that device driver is configured for
  your ports.  Add a "v" to the option "-g" to see more.  But this
  doesn't tell you if the hardware actually has these values set in it.
  If fact, you can run setserial and assign a purely fictitious I/O
  address, any IRQ, and whatever uart type you would like to have.  Then
  the next time you type "setserial ..." it will display these bogus
  values without complaint.  Note that assignments made by setserial are
  lost when the PC is powered down so it is usually run automatically
  somewhere each time that Linux is booted.


  14.3.2.  Probing

  In order to try to find out if you have a certain piece of serial
  hardware you must first know (or guess) its I/O address (or the device
  driver must have an I/O address for it, likely previously set by
  setserial).  To try to detect the physical hardware use the -v
  (verbose) and autoconfig command to setserial.  If the resulting
  message shows a uart type such as 16550A, then you're OK.  If instead
  it shows "unknown" for the uart type, then there is supposedly no
  serial port at all at that I/O address.  Some cheap serial ports don't
  identify themselves correctly so if you see "unknown" you still might
  have a serial port there.

  Besides auto-probing for uart type, setserial can auto-probe for IRQ's
  but this doesn't always work right either.  In versions of setserial
  >= 2.15, your last probe test may be saved and put into the
  configuration file /etc/serial.conf which will be used next time you
  start Linux.  The script that runs setserial at boot-time does not
  usually probe, but you could change it so that it does.  See the next
  section.


  14.3.3.  Can Linux Configure The Serial Devices Automagically?

  Yes, but ...  Your distribution may already do this on startup.  But
  you may want to customize it.  It's easy to do for setserial < 2.15.
  Just add some lines to the file that runs setserial on start-up.  See
  ``Old configuration method: edit a script''   For example, for ttyS3
  you would add:



       /sbin/setserial /dev/ttyS3 auto_irq skip_test autoconfig




  to the file that runs setserial on startup.  Do this for every serial
  port you want to auto configure.  Be sure to give a device name that
  really does exist on your machine.  In some cases this will not work
  right due to the hardware so you may want to assign it an irq and/or a
  uart type.  For example



  /sbin/setserial /dev/ttyS3 irq 5 uart 16550A  skip_test





  For versions >= 2.15 (provided your distribution implemented the
  change, Redhat didn't) it's much harder to do since the file that runs
  setserial on startup, /etc/init.d/setserial or the like was not
  intended to be edited by the user.  There may be no helpful comments
  in it like there were in earlier versions.


  14.3.4.  Boot-time Configuration

  When the kernel loads the serial module (or if the "module" is built
  into the kernel) then only ttyS{0-3} are auto-detected and the driver
  is set to IRQs 4 and 3 (regardless of what the hardware is actually
  set at).  You see this as a boot-time message just like as if
  setserial had been run..  If you use 3 or more ports, this may result
  in IRQ conflicts.

  To fix such conflicts by telling setserial the true IRQs (or for other
  reasons) there may be a file somewhere that runs setserial again.
  This happens early at boot-time before any process uses the serial
  port.  In fact, your distribution may have set things up so that the
  setserial program runs automatically from a start-up script at boot-
  time.  More info about how to handle this situation should be found in
  /usr/doc/setserial.../ or the like.



  14.3.5.  New configuration method using /etc/serial.conf

  Prior to setserial version 2.15, the way to configure setserial was to
  manually edit the shell-script that ran setserial at boot-time.
  Starting with version 2.15 (1999) of setserial this shell-script is
  not edited but instead gets its data from a configuration file:
  /etc/serial.conf.  Furthermore not even serial.conf is intended to be
  edited.  Instead just use setserial on the command line.

  Normally, what you changed with the setserial command is saved to the
  configuration file (serial.conf) when you shutdown (normally) or
  reboot.  This only works if "###AUTOSAVE### or the like is on the
  first line of serial.conf.  If you should use setserial experimentally
  and it doesn't work out right, then don't forget to redo it so that
  the experimental settings don't get saved by mistake.  The file most
  commonly used to run setserial at boot-time (in conformance with the
  configuration file) is now /etc/init.d/setserial (Debian) or
  /etc/init.d/serial (Redhat), or etc.,  but it also should not normally
  be edited.

  To disable a port, use setserial to set it to "uart none".  The format
  of /etc/serial.conf appears to be just like that of the parameters
  placed after "setserial" on the command line with one line for each
  port.  If you don't use autosave, you may edit /etc/serial.conf
  manually.  For 2.15, the Debian distribution installs the system with
  autosave enabled, but Redhat 6.0 just had a file
  /usr/doc/setserial-2.15/rc.serial which you have to move to
  /etc/init.d/.

  BUG: As of July 1999 there is a bug/problem since with ###AUTOSAVE###
  only the setserial parameters displayed by "setserial -G /dev/ttyS?"
  (where ? = 0, 1, 2, ...) get saved but the other parameters don't get
  saved.  This will only affect a small minority of users since the
  parameters not saved are seldom used anyway.  It's been reported as a
  bug and may be fixed by now.

  In order to force the current settings set by setserial to be saved to
  the configuration file (serial.conf) without shutting down, do what
  normally happens when you shutdown: Run the shell-script
  /etc/init.d/{set}serial stop.  The "stop" command will save the
  current configuration but the serial ports still keep working OK.

  I some cases you may wind up with both the old and new configuration
  methods installed but hopefully only one of them runs at boot-time.
  Debian labeled obsolete files with "...pre-2.15".


  14.3.6.  Old configuration method: edit a script

  Prior to 2.15 (1999) there was no /etc/serial.conf file to configure
  setserial.   Thus you need to find the file that runs "setserial" at
  boot time and edit it.  If it doesn't exist, you need to create one
  (or place the commands in a file that runs early at boot-time).   If
  such a file is currently being used it's likely somewhere in the /etc
  directory-tree.  But Redhat <6.0 has supplied it in
  /usr/doc/setserial/ although you need to move it to the /etc tree
  before using it.   You might use "locate" to try to find such a file.
  For example, you could type: locate "*serial*".

  What you are looking for could be named  rc.serial, or 0setserial
  (Debian).  If such a file is supplied, it should contain a number of
  commented-out examples.  By uncommenting some of these and/or
  modifying them, you should be able to set things up correctly.  Make
  sure that you are using a valid path for setserial, and a valid device
  name.  You could do a test by executing this file manually (just type
  its name as the super-user) to see if it works right.  Testing like
  this is a lot faster than doing repeated reboots to get it right.  Of
  course you can also test a single setserial command by just typing it
  on the command line.

  The script /etc/rc.d/rc.serial was commonly used in the past.  The
  Debian distribution used /etc/rc.boot/0setserial.  Another file once
  used is /etc/rc.d/rc.local but it's not a good idea to use this since
  it may not be run early enough.  It's been reported that other
  processes may try to open the serial port before rc.local runs
  resulting in serial communication failure.


  14.3.7.  IRQs

  By default, both ttyS0 and ttyS2 share IRQ 4, while ttyS0 and ttyS3
  share IRQ 3.  But sharing serial interrupts is not permitted unless
  you: 1. have kernel 2.2 or better, and 2. you've complied in support
  for this, and 3. your serial hardware supports it.  See Serial-HOWTO:
  Interrupt sharing and Kernels 2.2+.


  If you only have two serial ports, ttyS0 and ttyS1, you're still OK
  since IRQ sharing conflicts don't exist for non-existent devices.

  If you add an internal modem and retain ttyS0 and ttyS1, then you
  should attempt to find an unused IRQ and set it both on your serial
  port (or modem card) and then use setserial to assign it to your
  device driver.  If IRQ 5 is not being used for a sound card, this may
  be one you can use for a modem.  To set the IRQ in hardware you may
  need to use isapnp, a PnP BIOS, or patch Linux to make it PnP.  To
  help you determine which spare IRQ's you might have, type "man
  setserial" and search for say: "IRQ 11".


  14.4.  Stty

  14.4.1.  Introduction

  stty does much of the configuration of the serial port but since
  application programs (and the getty program) often handle it, you may
  not need to use it much.  It's handy if your having problems or want
  to see how the port is set up.  Try typing ``stty -a'' at your
  terminal/console to see how it's now set.  Also try typing it without
  the -a (all) for a short listing which shows how it's set different
  than normal.  Don't try to learn all the setting unless you want to
  become a serial guru.  Most of the defaults should work OK and some of
  the settings are needed only for certain obsolete dumb terminals made
  in the 1970's (but not much after that)

  Whereas setserial only deals with actual serial ports, stty is used
  both for serial ports and for virtual terminals such as the standard
  Linux text interface at a PC monitor.  For the PC monitor, many of the
  stty settings are meaningless.  Changing the baud rate, etc. doesn't
  appear to actually do anything.

  Here are some of the items stty configures: speed (bits/sec), parity,
  bits/byte, # of stop bits, strip 8th bit?, modem control signals, flow
  control, break signal, end-of-line markers, change case, padding, beep
  if buffer overrun?, echo, allow background tasks to write to
  terminal?, define special (control) characters (such as what key to
  press for interrupt).  See the stty man or info page for more details.
  Also see the man page: termios which covers the same options set by
  stty but (as of mid 1999) covers features which the stty man page
  fails to mention.  For use of some special characters see ``Special
  (Control) Characters''

  With some implementations of getty (getty_ps package), the commands
  that one would normally give to stty are typed into a getty
  configuration file: /etc/gettydefs.  Even without this configuration
  file, the getty command line may be sufficient to set things up so
  that you don't need stty."')

  One may write C programs which change the stty configuration, etc.
  Looking at some of the documentation for this may help one better
  understand the use of the stty command (and its many possible
  arguments).  Serial-Programming-HOWTO is useful.  The manual page:
  termios contains a description of the C-language structure (of type
  termios) which stores the stty configuration in computer memory.  Many
  of the flag names in this C-structure are almost the same (and do the
  same thing) as the arguments to the stty command.


  14.4.2.  Using stty for a "foreign" terminal

  Using stty to inspect or configure the terminal that you are currently
  using is easy.  Doing it for a different (foreign) terminal or serial
  port is tricky.  For example, let's say you are at the PC monitor
  (tty1) and want to use stty for the serial port ttyS2.  You need to
  use the redirection operator <.  First, be warned that if there is a
  terminal on ttyS2 and a shell running on that terminal, then what you
  see will likely be deceptive and trying to set it will not work.  See
  ``Two Interfaces at a Terminal'' to understand it.

  Type ``stty -a < /dev/ttyS2'' to look at the settings of ttyS2.  Use
  the same redirection operator < to set ttyS2.  This makes ttyS2 the
  standard input to stty.  This gives the stty program a link to the
  "file" ttyS2 so that it may "read" it.  But instead of reading the
  bytes sent to ttyS2 as one might expect, it uses the link to find the
  configuration settings of the port so that it may read or change them.
  Some people try to use ``stty ... > /dev/ttyS2'' to set the terminal.
  This will not do it.  Instead, it takes the message normal displayed
  by the stty command for the terminal you are on (say tty1) and sends
  this message to ttyS2.  But it doesn't change any settings for ttyS2.

  Here's another problem with the redirection operator.  Sometimes when
  trying to use stty, the command hangs and nothing happens (you don't
  get a prompt for a next command even after hitting <return>).  This is
  likely due to the port being stuck because it's waiting for one of the
  modem control lines to be asserted.  For example, unless you've set
  "clocal" to ignore modem control lines, then if no CD signal is
  asserted the port will not open and stty will not work for it.  A
  similar situation seems to exist for hardware flow control.  If the
  cable for the port doesn't even have a conductor for the pin that
  needs to be asserted then there is no easy way to stop the hang.

  One way to try to get out of the above hang is to use a program on the
  port that will force it to operate even if the control lines say not
  to.  Then hopefully this program might set the port so it doesn't need
  the control signal in the future in order to open: clocal or -crtscts.
  To use "minicom" to do this you have to reconfigure minicom for
  another ttyS, etc, and then exit it and restart it.  Since you then
  have to reconfigure minicom again, it may be simpler to just reboot
  the PC.

  Versions starting with 1.17 (still not released as of mid 1999) will
  no longer need to use redirection (<) but instead will use ``stty ...
  -F /dev/ttyS2'' (or --file instead of F) etc.  This should force the
  port to open and avoid the 2nd problem with redirection.


  14.4.3.  Two interfaces at a terminal

  When using a shell (such as bash) with command-line-editing enabled
  there are two different terminal interfaces (what you see when you
  type stty -a).  When you type at the command line you have a temporary
  "raw" interface (or raw mode) where each character is read by the
  command-line-editor as you type it.  Once you hit the <return> key,
  the command-line-editor is exited and the terminal interface is
  changed to the nominal "cooked" interface (cooked mode) for the
  terminal.  This cooked mode lasts until the next prompt is sent to the
  terminal.  Note that one never types anything to this cooked mode but
  what was typed in raw mode becomes cooked mode as soon as one hits the
  <return> key.

  When a prompt is sent to the terminal the terminal goes from "cooked"
  to "raw" mode (just like it does when you start an editor since you
  are starting the command-line editor).  The settings for the "raw"
  mode are based only on the basic settings taken from the "cooked"
  mode.  Raw mode keeps these setting but changes several other settings
  in order to change the mode to "raw".  It is not at all based on the
  settings used in the previous "raw" mode.  Thus if one uses stty to
  change settings for the raw mode, such settings will be lost as soon
  as one hits the <return> key at the terminal that has supposedly been
  "set".

  Now when one types stty to look at the terminal interface, one may
  either get a view of the cooked mode or the raw mode.  You need to
  figure out which one you're looking at.  It you use stty from another
  terminal to deal with a terminal that is displaying a command line,
  then the view is that of the raw mode.  Any changes made will only be
  made to the raw mode and will be lost when someone presses <return> at
  the terminal you tried to "set".  But if you type a stty command at
  your terminal (without using < for redirection) and then hit <return>
  it's a different story.  The <return> puts the terminal in cooked
  mode.  Your changes are saved and will still be there when the
  terminal goes back into raw mode (unless of course it's a setting not
  allowed in raw mode).

  This situation can create problems.  For example, suppose you corrupt
  your terminal interface and to restore it you go to another terminal
  and "stty sane <dev/ttyS1" to restore it.  It will not work!  Of
  course you can try to type "stty sane ..." at the terminal that is
  corrupted but you can't see what you typed.  All the above not only
  applies to dumb terminals but to virtual terminals used on a PC
  Monitor as well as to the terminal windows in X.  In other words, it
  applies to almost everyone who uses Linux.  Luckily, a file that runs
  stty at boot-time will likely deal with a terminal (or serial port
  with no terminal) that has no shell running on it so there's no
  problem.


  14.4.4.  Where to put the stty command ?

  Should you need to have stty set up the serial interface each time the
  computer starts up then you need to put the stty command in a file
  that will be executed each time the computer is started up (Linux
  boots).  It should be run before the serial port is used (including
  running getty on the port).  There are many possible places to put it.
  If it gets put in more than one place and you only know about (or
  remember) one of those places, then a conflict is likely.  So make
  sure to document what you do.

  One place to put it would be in the same file that runs setserial when
  the system is booted.  The location is distribution and version
  dependent.  It would seem best to put it after the setserial command
  so that the low level stuff is done first.  If you have directories in
  the /etc tree where every file in them is executed at boot-time
  (System V Init) then you could create a file named "stty" for this
  purpose.




  14.5.  Terminfo & Termcap (brief)

  See ``Terminfo and Termcap (detailed)'' for a more detailed discussion
  of termcap.  Many application programs that you run use the terminfo
  (formerly termcap) data base.  This has an entry (or file) for each
  model or type (such as vt100) of terminal and tells what the terminal
  can do, what codes to send for various actions, and what codes to send
  to the terminal to initialize it.

  Since many terminals (and PC's also) can emulate other terminals and
  have various "modes" of operation, there may be several terminfo
  entries from which to choose for a given physical terminal.  They
  usually will have similar names.  The last parameter of getty (for
  both agetty and getty_ps) should be the terminfo name of the terminal
  (or terminal emulation) that you are using (such as vt100).

  The terminfo does more than just specify what the terminal is capable
  of doing and disclose what codes to send to the terminal to get it to
  do those things.  It also specifies what "bold" will look like (will
  it be reverse video or will it be high intensity, etc.), what the
  cursor will look like, if the letters will be black, white, or some
  other color, etc.  In PC terminology these are called "preferences".
  It also specifies initialization codes to send to the terminal
  (analogous to the init strings sent to modems).  Such strings are not
  automatically sent to the terminal by Linux.  See ``Init String''.  If
  you don't like the way the display on the screen looks and behaves you
  may need to edit (and then update) the terminfo (or termcap) file.
  See ``Terminfo Compiler (tic)'' for how to update.

  14.6.  Setting TERM and TERMINFO

  These are two environment variables for terminals: TERM and TERMINFO,
  but you may not need to do anything about them.  TERM must always be
  set to the  type of the terminal you are using (such as vt100).  If
  you don't know the type (name) see ``What is the terminfo name of my
  terminal ?''.  TERMINFO contains the path to the terminfo data base,
  but may not be needed if the database is in a default location (or
  TERMINFO could be set automatically by a file that comes with your
  distribution of Linux).  You may want to look at `` Compiled  database
  locations''.

  Fortunately, the getty program usually sets TERM for you just before
  login.  It just uses the terminal type that was specified on getty's
  command line (in /etc/inittab).  This permits application programs to
  find the name of your terminal and then look up the terminal
  capabilities in the terminfo data base.  See ``TERM Variable'' for
  more details on TERM.

  If your terminfo data base can't be found you may see an error message
  about it on your terminal.  If this happens it's time to check out
  where terminfo resides and set TERMINFO if needed.  You may find out
  where the terminfo database is by searching for a common terminfo file
  such as "vt100" using the "locate" command.  Make sure that your
  terminal is in this database.  An example of setting TERMINFO is:
  export TERMINFO=/usr/share/terminfo (put this in /etc/profile or the
  like).  If the data for your terminal in this data base is not to your
  liking, you may need to edit it.  See ``Terminfo & Termcap (brief)''.


  14.6.1.  What is the terminfo name of my terminal ?

  You need the exact name in order to set the TERM environment variable
  or to give to getty.  The same name should be used by both the termcap
  and terminfo databases so you only need to find it once.  A terminal
  usually has alias names but if more than one name is shown, use the
  first one.

  To find it, try looking at the /etc/termcap... file (if you have it).
  If not, then either look at the terminfo trees (see `` Compiled
  database locations'') or try to find the terminfo source code file
  (see ``Source-code database locations''.


  14.7.  Rarely Needed /etc/ttytype File

  The configuration file /etc/ttytype is used to map /dev/ttySn's to
  terminal names per terminfo.  tset uses it, but if the TERM
  environment variable is already set correctly, then this file is not
  needed.  Since the Linux getty sets TERM for each tty, you don't need
  this file.  In other Unix-like systems such as FreeBSD, the file
  /etc/ttys maps ttys to much more, such as the appropriate getty
  command, and the category of connection (such as "dialup").  An
  example line of Linux ttytype: vt220 ttyS1


  14.8.  Login Restrictions

  By default, the root user may not login from a terminal.  To permit
  this you must create (or edit) the file /etc/securetty per the manual
  page "securetty".  But use of this may be be distribution specific as
  the Suse distribution doesn't use /etc/securetty.  To restrict logins
  of certain users and/or certain terminals, etc. edit /etc/login.access
  (this replaces the old /etc/usertty file ??).  /etc/login.def
  determines if /etc/securetty is to be used and could be edited so as
  to make /etc/securetty not needed (or not used).  /etc/porttime
  restricts the times at which certain ttys and users may use the
  computer.  If there are too many failed login attempt by a user, that
  user may be prohibited from ever logging in again.  See the man page
  "faillog" for how to control this.


  14.9.  Run Command Only If TERM=my_term_type

  Sometimes there are commands that one wants to execute at start-up
  only for a certain type of terminal.  To do this for the stty command
  is no problem since one uses the redirection operator < to specify
  which terminal the command is for.  But what about shell aliases or
  functions?  You may want to make a function for the ls command so it
  will color-code the listing of directories only on color terminals or
  consoles.  For monochrome terminals you want the same function name
  (but a different function body) which will use symbols as a substitute
  for color-coding.  Where to put such function definitions that are to
  be different for different terminals?

  You may put them inside an "if" statement in /etc/profile which runs
  at startup each time one logs on.  The conditional "if" statement
  defines certain functions, etc. only if the terminal is of a specified
  type.


  14.9.1.  Example for ls Function

  While much of what this if statement does could be done in the
  configuration file for dircolors, here's an example for the case of
  the bash shell:


  ______________________________________________________________________
  if [ "$TERM" = linux ]; then
      eval `dircolors`;
  elif [ "$TERM" = vt220 ]; then
      ls () { command ls -F $* ; }# to export the function ls():
      declare -xf ls
  else echo "From /etc/profile: Unknown terminal type $TERM"
  fi
  ______________________________________________________________________




  15.  Terminfo and Termcap (detailed)

  15.1.  Intro to Terminfo

  Terminfo (formerly Termcap) is a database of terminal capabilities and
  more.  For every (well almost) model of terminal it tells application
  programs what the terminal is capable of doing.  It tells what escape
  sequences (or control characters) to send to the terminal in order to
  do things such as move the cursor to a new location, erase part of the
  screen, scroll the screen, change modes, change appearance (colors,
  brightness, blinking, underlining, reverse video etc.).  After about
  1980, many terminals supported over a hundred different commands (some
  of which take numeric parameters).

  The normal way in which terminfo gives the its information to an
  application program is via the "ncurses" functions that a programmer
  may put into a C program.  For example if a program wants to move the
  cursor to row 3, col 6 it simply calls: move(3,6).  The move()
  function (part of ncurses) knows how to do this for your terminal (it
  can read terminfo).  So it sends the appropriate escape sequence to
  the terminal to make this particular move for a certain terminal.
  The terminfo abbreviations are usually longer than those of termcap
  and thus it's easier to guess what they mean.  The manual pages for
  terminfo are more detailed (and include the old termcap
  abbreviations).  Thus, unless you are already committed to working
  with termcap, it's suggested you use terminfo.


  15.2.  Terminfo Database

  15.2.1.  Introduction

  The terminfo database is compiled and thus has a source part and a
  compiled part.  The old termcap database has only a source part but
  this source can, by a single command, be both converted to terminfo
  source and then compiled.  Thus you may get by without having any
  terminfo source since the termcap source can create the compiled
  terminfo database.  To see a display of the database for the terminal
  you're now using (including a PC monitor) type "infocmp" and you
  should see the source terminfo "file" for it.

  To see if your terminal (say vt100) is in the terminfo data base type
  "locate vt100".  If you don't know what your terminal name is, explore
  the listing of files in the compiled database or see ``What is the
  terminfo name of my terminal ?''


  15.2.2.  Where is the database located ?

  15.2.2.1.  Compiled database locations

  Typing "locate vt100" may show /usr/lib/terminfo/v/vt100,
  /usr/share/terminfo/v/vt100, /home/you/.terminfo/v/vt100, and/or
  /etc/terminfo/v/vt100.  All these are possible locations of the
  compiled terminfo files.  Although the /etc/terminfo directory is not
  a standard location for it, having a few terminal types there could be
  useful in case the /usr directory is not accessible.  For example /usr
  could be on a separate disk or partition that failed to mount.
  Normally, programs that use your main terminfo data base are able to
  find it if it's in at least one of the locations mentioned above.
  Otherwise the environment variable TERMINFO may be set to the path to
  this database.  Example: TERMINFO=/usr/share/terminfo

  If the compiled terminfo is in more than one location, everything is
  usually OK until someone gets a new terminfo file(s) (from a newer
  distribution, from the net, by editing the old one, etc.).  The newer
  terminfo needs to be put in all the existing locations (or redundant
  locations need to be abolished).  If you don't ensure this gets done,
  then some application programs may wind up still finding and using the
  old (and buggy) terminfo data that sill exists in a "usual" location.


  15.2.2.2.  Source-code database locations

  The source code you use may reside in /etc/termcap and/or in
  terminfo.src (or another name).  See the man pages: terminfo(5) or
  termcap(5) for the format required to create (or modify) these source
  files.  The file terminfo.src may be in various locations on your
  computer or it may not be included with your linux distribution.  Use
  the locate command to try to find it.  It is available for downloading
  (under the name termtypes.ti) from
  <http://sagan.earthspace.net/terminfo>





  15.2.3.  Terminfo Compiler (tic)

  The data in the source files is compiled with the "tic" program which
  is capable of converting between termcap format and terminfo format.
  Thus you can create a compiled terminfo data base from termcap source.
  The installation program which was used to install Linux probably
  installed the compiled files on your hard disk so you don't need to
  compile anything unless you modify /etc/termcap (or terminfo.src ).
  "tic" will automatically install the resulting compiled files into a
  terminfo directory ready to be used by application programs.


  15.2.4.  Look at Your Terminfo

  It's a good idea to take a look at the terminfo entry for the terminal
  you are using (source code of course) and read the comments.  A quick
  way to inspect it without comments is to just type "infocmp".  But the
  comments may tell you something special about the terminal such as how
  you need to set it up so that it will work correctly with the terminfo
  database.


  15.2.5.  Deleting Data Not Needed

  In order to save disk space, one may delete all of the database except
  for the terminals types that you have (or might need in the future).
  Don't delete any of the termcaps for a "Linux terminal" (the console)
  or the xterm ones if you use X-Windows.  The terminal type "dumb" may
  be needed when an application program can't figure out what type of
  terminal you are using.  It would save disk space if install programs
  only installed the terminfo for the terminals that you have and if you
  could get a termcap for a newly installed terminal over the Internet
  in a few seconds.


  15.3.  Bugs in Existing Terminfo Files (and Hardware)

  Unfortunately, there are a number of bugs in the terminfo and termcap
  files.  In addition, many of these definitions are incomplete and do
  not define certain features available on the terminals.  Sometimes you
  can get by without modifying the terminfo but in other cases you need
  to modify it or possibly use another emulation that has a good
  terminfo.

  The sad state of the supplied terminfo files is due to a number of
  reasons.  One is that during the 1980's when many of them were written
  (often in termcap format), application programs did not utilize more
  advanced terminal features.  Thus if such feature were not in the
  termcap (or terminfo) file, no one complained.  Today, programs such
  as vim use "context highlighting" and minicom uses the terminal's
  graphics character set.  These often need more definitions to be added
  to the old termcap.  This may (or may not) have already been done.

  Most terminals had hardware bugs (in their firmware) and sometimes
  they were "fixed" by modifying the termcap.  Then the manufacturer
  might send out replacement chips which would fix the bug.  Not all
  owners would bother to get the replacement chips.  Thus there may be 2
  or more terminfos for your terminal, depending on what firmware chips
  it has in it.  This situation was often not noted in the termcap and
  only one termcap may be supplied with Linux.  Some hardware bugs which
  existed for features that were almost never used in the past likely
  never did get fixed.  Also, some reported hardware bugs may never have
  been fixed since they were not of much significance at the time or
  because the company went out of business, etc.


  15.4.  Modifying Terminfo Files

  To do this you need a manual for your terminal showing what escape
  sequences it uses.  Newer manuals from the 1990's often don't show
  this.  You also need a terminfo manual (or the like).  For example, in
  order to add graphic capability you must assign values to the terminfo
  variables: enacs, rmacs, and smacs by editing a source file.  Then by
  using "tic" you may compile it.  "tic" should automatically put the
  compiled terminfo file in the correct directory reserved for it.

  If you would like to find a better terminfo entry for a certain
  terminal than the one supplied, you might try searching the Internet
  (but what you find may be worse).  If your new terminfo entry is
  better than the old one and it seems stable (you've used it for a
  while with no problems) then you should send a copy to the maintainer
  of terminfo as noted at the start of the source file for terminfo (or
  termcap).


  15.5.  Init String

  Included in the terminfo are often a couple of initialization strings
  which may be sent to the terminal to initialize it.  This may change
  the appearance of the screen, change what mode the terminal is in,
  and/or make the terminal emulate another terminal.  An initialization
  string is not automatically sent to the terminal to initialize it.
  One might expect that the getty program should do this but if it did,
  one could make a change to the set-up at the terminal and this change
  wouldn't be implemented because the init string would automatically
  cancel it.  You must use a command given on the command line (or in a
  shell script) to send the init string such.  Such commands are:
  "tset", "tput init", or "setterm -initialize".  Sometimes there is no
  need to send the init string since the terminal may set itself up
  correctly when it is powered on (using options/preferences one has set
  up and saved in non-volatile memory of the terminal).


  15.6.  TERM Variable

  The Environment variable TERM should be set to the name of terminal
  which you are using.  If TERM hasn't been set yet and you don't know
  the name of your terminal see ``What is the terminfo name of my
  terminal ?''.  It is normally set by the terminal_type parameter
  passed to the getty program (look at it in the /etc/inittab file).
  This name must be in the Terminfo data base.  Just type "set" at the
  command line to see what TERM is set to (or type: tset -q).  At a
  console (monitor) TERM is set to "linux" which is the PC monitor
  emulating a fictitious terminal model named "linux".  Since "linux" is
  close to a vt100 terminal and many text terminals are also, the
  "linux" designation will sometimes work as a temporary expedient with
  a text terminal.

  If more than one type of terminal may be connected to the same port
  (/dev/tty...) (for example, if there is a switch to permit different
  terminal types to use the same serial port, or if the port is
  connected to a modem to which people call in from different types of
  terminals) then TERM needs to be set each time someone connects to the
  serial port.  There is often a query escape sequence so that the
  computer may ask the terminal what type it is.  Another way is to ask
  the user to type in (or select) the type of terminal s/he is using.
  You may need to use tset for this or write a short shell script to
  handle this.


  One way to do this is to use "tset" (see the manual page).  tset tries
  to determine the terminal name of the terminal you are using.  Then it
  looks up the data in terminfo and sends your terminal an init string.
  It can also set the value of TERM.  For example, a user dials in and
  logs in.  The .profile login script is executed which contains within
  it the following statement: eval `tset -s ?vt100`.  This results in:
  The user is asked if s/he is using a vt100.  The user either responds
  yes or types in the actual terminal type s/he is using.  Then tset
  sends the init string and sets TERM to this terminal name (type).


  15.7.  Terminfo/Termcap Documents



  o

  o  manual pages for terminfo(5) (best) and/or termcap(5).  The Termcap
     Manual <http://www.delorie.com/gnu/docs/termcap/termcap_toc.html>
     (2nd ed.) by Richard M. Stallman is a GNU manual which is somewhat
     obsolete since it doesn't include terminfo.

  o  the files: terminfo.src and /etc/termcap have info about various
     versions of termcap files, naming conventions for terminals, and
     special capabilities code named u6-u9. If you don't have one, go to
     <http://sagan.earthspace.net/terminfo>

  o  "Termcap and Terminfo" is a book published by O'Reilly in 1988.


  16.  Using the Terminal

  16.1.  Intro to Using Terminal

  This section is about controlling the terminal-computer interface
  and/or changing the terminal set-up while using the terminal.  It
  explains (or points to explanations of) how the user of a terminal can
  control and inspect the interface and how to use various commands
  provided by the device driver.  It does not explain how to use the
  many application programs, shells or most Linux utilities.  Two
  commands commonly used at the terminal are:


  o  clear (to clear the screen)

  o  reset (to reset the terminal)


  16.2.  Starting Up the Terminal

  Of course the power must be on for the terminal to work.  If you don't
  see a login prompt hit the "return" (or "enter") key a few times.
  Then type your account name (followed by a return/enter) and your
  password when prompted for it (also followed by return/enter).  Make
  sure not to type all capital letters.  If you do, the computer may
  think that you have an old terminal that can't send lowercase letters
  and the serial driver may set itself up to send only capital letters
  to the terminal.

  If nothing happens, make sure that both the host computer and the
  terminal are OK.  If the host computer is shut down (no power) what
  you type at the terminal keyboard may appear on the screen since the
  transmit and receive pins at the computer may be connected together
  resulting in echoing of characters by an "off" computer.  If you can't
  log in when the host computer is running, see ``Trouble-Shooting''.



  16.3.  Terminal (Serial) Device Driver

  When typing at the command line, the shell (such as the Bash shell) is
  reading what you type and reacting to it.  What you type first passes
  thru the terminal driver part of your operating system.  This driver
  may translate certain characters (such as changing the "return"
  character generated by the "return" key into a "new-line" character
  for Linux files).  It also recognizes certain control codes which you
  may type at the keyboard such as ^C to interrupt the execution of a
  program.  It also normally echoes what you type back to the display.
  ``Stty'' may be used to configure how this terminal driver behaves,
  including disabling some (or all) of its functionality.


  16.4.  Problems with Editors

  There may be some problems with using both emacs and vi on some
  terminals.


  16.4.1.  emacs and ^S, ^Q

  If software flow control exists, then the ^S command in emacs will
  freeze the display.  The ^Q command will unfreeze the display.  The
  fix is to map this to another key-press by configuring emacs that way.


  16.4.2.  vi and Cursor-Keys

  Vi uses the esc-key as a command to exit insert mode.  If one hits an
  arrow-key (cursor-key) an escape sequence (starting with the ESC
  character) is sent to the host.  Vi must distinguish between these two
  meanings of ESC.  A smart vi (such as vim if configured for it) is
  able to detect the difference by noting the time between the ESC and
  the next key.  If it's a short time, then it's likely that a cursor
  key was pressed.  Use "help cursor-keys" in vim to find out more.

  Here's another way to fix this.  On VT terminals the left-arrow-key
  may be either set to send ESC [ D or ESC O D.  The other arrow keys
  are similar but use A, B, and C instead of D.  If you're having
  problems, choose ESC [ D since the "O" in the other alternative could
  be interpreted by vi as a command to "Open a line".  The "[" should be
  interpreted by vi to mean that an arrow-key has been pressed.  ESC [ D
  will be sent provided "Cursor Key Application Mode" has not been set.
  ESC [ D is normally the default so everything is seemingly OK.  Except
  that many termcaps contain a string (not the init string) which sets
  what you want to avoid: "Application Mode".  Editors may send this
  string to the terminal when the editor starts up.  Now you are in
  trouble.

  This string has the termcap code "ks" (smkx in terminfo) meaning
  enable the function (and related) keys (including the arrow keys).  An
  application enables these keys by sending the "ks" string to the
  terminal.  Whoever wrote the termcap reasoned that if an application
  wants to enable these keys, then they should be put into "Application
  Mode" since this is an "application", but you don't want this.

  The Linux console has no "ks" string so you can't fall into this trap
  at the console.  For other terminals you may need to edit the termcap
  (or terminfo) or use another termcap entry.  You need to change not
  only the "ks" string but also the termcap definitions of what they
  send: kd, kl, kr, ku.  Then run tic to install it.

  For vim (vi iMproved) there is a way to set it up to work OK with ESC
  O D (so you don't need to edit termcap): See vim help for
  "vt100-cursor-keys".  You may run "gitkeys" and then press your cursor
  keys to see what they send but they may be set to send something
  different when you're in an editor.


  16.5.  Color ls Corruption

  If ls is corrupting your terminal emulation with the color feature,
  turn it off.  ls --color, and ls --colour all use the color feature.
  Some installations have ls set to use color by default.  Check
  /etc/profile, etc.  for ls aliases.  See ``Example for ls Function''
  for how to have ls do color for the console and do monochrome for
  terminals.


  16.6.  Display Freezes (hung terminal)

  The symptom of a hung terminal is where what you type doesn't display
  on the terminal (or in some cases displays but doesn't do anything).
  If what you type is invisible (or does nothing) type ^Q to restart
  flow (if flow control stopped it).  Hanging may also be due to:
  ``Sent Terminal Binary'' or ``Abnormally Exited a Program''
  If you didn't do any of these two, then your program could by buggy or
  you interaction with it fatally illegal.

  If you want to quit the program you were running and you can't do it
  by the usual methods (some programs have special keys you must hit to
  exit) then try killing it from another terminal using "top" or "kill".
  If the process refuses to die, you may try sending it signal 9 from
  top which should force it to exit.  The "9" type of forced exit may
  leave some temporary files lying around as well as a corrupted
  interface.  Killing the login shell should result in a startup of
  getty with a new login prompt.


  People new to Linux may unintentionally press Ctrl-S (^S) (or the "No
  Scroll" key) which mysteriously freezes the screen (although that is
  what this key is supposed to do if you use software flow-control).  To
  restore normal screen interaction, press Ctrl-Q (^Q).  Note that
  everything typed during the "freeze" gets executed but you don't see
  any report of this until you hit ^Q.  Thus when it's frozen, don't
  type anything drastic that might destroy files, etc.  One argument for
  using hardware flow-control is to prevent such freezes.


  16.7.  Corrupted Terminal Interface

  This includes the case of a "frozen display" = "hung terminal" of the
  previous section.


  16.7.1.  Symptoms

  When the display doesn't look right, or when what you type doesn't
  display correctly (if at all), or nothing happens when you type a
  command, you may have a corrupted terminal interface.  In rare cases
  when the serial port hardware gets itself corrupted, the only fix may
  be to cycle power (turn off the PC and reboot).  The corruption may be
  due to things such as bug in the program you're using, a hardware
  failure (including an obscure hardware defect that you can normally
  live with), or possibly an incorrect configuration.  If everything was
  working normally but it suddenly goes bad, it may be that the
  interface got corrupted by something you did.  Three mistakes you
  might have made to corrupt the interface are:



  o  ``Sent Terminal Binary''

  o  ``Abnormally Exited a Program''

  o  ``Typed ctrl-S by mistake''


  16.7.2.  Sent Terminal Binary Characters

  Your terminal will change its characteristics if sent certain escape
  sequences or control characters.  It you inadvertently try to display
  a binary file, it might by chance contain such sequences which may put
  your terminal into some strange mode of operation or even make it
  unusable.  Always view or edit a binary file with programs designed
  for that purpose so that this doesn't happen.  Most editors and pagers
  will handle binary OK so as not to corrupt the interface.  Some may
  display a message telling you that they can't edit binary.  But using
  "cat ...." or "cp .... /dev/tty.." where .... is a binary file, will
  send the binary to the terminal and likely corrupt things.

  Corruption it can also happen when using a communications program
  where a remote computer may send binary to your screen.  There are
  numerous other ways it can happen so be prepared for it.  Even a
  supposed ASCII file may contain unwanted control codes.

  To fix this problem reset the terminal.  You may try typing either
  "reset" or "setterm -reset" (although you may not be able to see what
  you're typing).  This will send the reset string from the terminfo
  entry to the terminal.  If the correct set-up has been saved inside
  the terminal, then pressing a special key(s) (perhaps in setup mode)
  may restore this setting.  Then you might still need to use "reset" to
  send the init string if you use it to set up your terminal.


  16.7.3.  Abnormally Exited a Program

  Large application programs (such as editors) often use the stty
  command (or the like) in their code to temporarily change the stty
  configuration when you are running the program.  This may put the
  device driver into "raw" mode so that every character you type goes
  directly thru to the application program.  Echoing by the driver is
  disabled so that everything you see on the screen comes directly from
  the application program.  Thus many control commands (such as ^C) may
  not work within such applications.

  When you tell such an application to quit, the application program
  first restores the stty settings to what they were before the
  application program started.  If you abnormally exit the program (you
  may guess this has happened when what you type no longer displays on
  the screen) then you may still be in "raw mode" on the command line.

  To get out of raw mode and restore the normal stty settings type "stty
  sane".  However, you must type this just after a "return" and end it
  with a "return".   But hitting the "return" key doesn't do the job
  since the "return" code no longer gets translated to the new-line
  characters that the shell is waiting for.  So just type new-line (^J)
  instead of "return".  The "sane" terminal interface may not be exactly
  the same as the normal one but it usually works.  "stty sane" may also
  be useful to get out of a corrupted interface due to other causes.


  16.8.  Special (Control) Characters

  A number of control characters which you may type at the keyboard are
  "caught" by the terminal driver and perform various tasks.  To see
  these control commands type: stty -a and look at lines 2-4.  They are
  tersely explained in the stty manual pages.  They may be changed to
  different control characters or disabled using the stty command.  Thus
  your control characters might be different than those described below.
  They are used for command-line editing, interrupting, scrolling, and
  to pass the next character thru transparently.


  16.8.1.  Command-Line Editing

  While the terminal driver has a few commands for command-line editing,
  some shells have a built-in real editor (such as "readline" in the
  Bash shell).  Such an editor is normally on by default so you don't
  need to do anything to enable it.  If it's available you don't need to
  learn many of the following commands although they often still work
  along with the command-line editor.  The most important to learn are
  ^C (interrupt), ^D, and how to stop scrolling.


  o  Delete-key (shown by stty as ^?) erases the last character

  o  ^U kills (deletes) the line

  o  ^W deletes a word backwards

  o  ^R reprints the line.  Useful mainly on hard copy terminals ??


  16.8.2.  Interrupting (& Quit, Suspend, EOF, Flush)



  o  ^C interrupts.  Exits the program and returns you to the command-
     line prompt.

  o  ^/ quits.  Same as interrupt ^C but weaker.  Also dumps a "core"
     file (which you likely have no use for) into your working
     directory.

  o  ^Z suspends.  Stops the program and puts it in the background.
     Type fg to restart it.

  o  ^D end of file.  If typed at the command-line prompt, exits the
     shell and goes to where you were before the shell started.

  o  ^O flush.  Not implemented in Linux.  Sends output to /dev/null.


  16.8.3.  Stop/Start Scrolling

  If what you want to see scrolls off the bottom of the screen, you may
  prevent this by sending a "stop" signal (^S or Xoff) to the host
  (provided Xon-Xoff ``Flow Control'' is enabled).  Send a "start signal
  to resume (^Q or Xon).  Some terminals have a "No Scroll" key which
  will alternately send Xoff and Xon or possibly send the hardware flow
  control signals ??   Here's what ctrl-S (^S) and ctrl-Q (^Q) do:


  o  ^S stops scrolling (Xoff)

  o  ^Q resume scrolling (Xon)

  If you want to both stop scrolling and quit, use ^C.  If you want to
  stop scrolling to do something else but want to keep the program that
  was generating the output in memory so you can resume scrolling later,
  use ^Z suspend.

  An alternative scrolling method is to pipe the output thru a pager
  such as more, less, or most.  However, the output might not be
  standard output but could be error output which the pager doesn't
  recognize.  To fix this you may need to use redirection "2>&1" to get
  the pager to work OK.  It is often simpler to just use ^S and ^Q
  unless you need to scroll backwards.

  At a PC console (emulating a terminal) you may scroll backwards by
  using Shift-PageUp.  This is frequently needed since the scrolling is
  often too fast to stop using ^S.  Once you've scrolled backwards
  Shift-PageDown will scroll forward again.


  16.8.4.  Take Next Character Literally

  ^V sends the next character typed (usually a control character)
  directly thru the device driver with no action or interpretation.
  Echoed back are two ASCII characters such as ^C.


  16.9.  Viewing Latin-1 Files on a 7-bit Terminal

  Some "text" files are 8-bit Latin1 (see ``Character-Sets'').  If you
  have a terminal that will not display Latin1 (or don't have the Latin1
  character set selected), then a bullet symbol will display as a 7,
  etc.  When viewing manual pages (they are Latin1) you may give the
  option -7 to man so as to translate the 7's, etc. to something close
  to a bullet (in ASCII).  Are there some pagers that make these
  translations ??


  16.10.  Inspecting the Interface

  These utility programs will provide information about the terminal
  interface:

  o  gitkeys: shows what byte(s) each key sends to the host.

  o  tty: shows what tty port you are connected to.

  o  set: shows the value of TERM (the terminfo entry name)

  o  stty -a: shows all stty settings.

  o  setserial -g /dev/tty?? (you fill in ??) shows UART type, port
     address and IRQ number.

  o  infocmp: shows the current terminfo entry (less comments)


  16.11.  Changing the Terminal Settings

  The terminal settings are normally set once when the terminal is
  installed using the setup procedures in the terminal manual.  However,
  some settings may be changed when the terminal is in use.  You
  normally would not give any "stty" of "setserial" commands when the
  terminal is in use as they are likely to corrupt the terminal
  interface.  However, there are changes you may make to the appearance
  of the terminal screen or to its behavior without destroying the
  integrity of the interface.  Sometimes these changes are made
  automatically by application programs so you may not need to deal with
  them.

  One direct method of making such changes is to use the setup key (or
  the like) at the terminal and then use menus (or the like) to make the
  changes.  To do this you may need to be familiar with the terminal.
  The other 3 methods send an escape sequence from the computer to the
  terminal to make the changes.  These 3 examples show different methods
  of doing this to set reverse video:


  1. setterm -reverse

  2. tput -rev

  3. echo ^[[7m


  16.11.1.  setterm

  This is the easiest command to use.  It uses long options (but doesn't
  use the normal -- before them).  It consults the terminfo database to
  determine what code to send.  You may change the color, brightness,
  linewrap, keyboard repeat, cursor appearance, etc.


  16.11.2.  tput

  The "tput" command is similar to "setterm" but instead of using
  ordinary words as arguments, you must use the abbreviations used by
  terminfo.  Many of the abbreviations are quite terse and hard to
  remember.


  16.11.3.  echo

  In the example "echo ^[[7m" to set reverse video, the ^[ is the escape
  character.  To type it type ^V^[ (or ^V followed by the escape key).
  To use this "echo" method you must find out what code to use from a
  terminal manual or from terminfo or termcap.  It's simpler to use
  setterm or tput if you are typing on the command line.  Since "echo
  ..." will execute faster (since it doesn't do any lookups) it's good
  for using in shell scripts which run at start-up, etc.


  16.11.4.  Saving Changes

  When you turn off the terminal the changes you made will be lost
  (unless you saved them in non-volatile terminal memory by going into
  set-up mode and saving it).  If you want to use them again without
  having to retype them, put the commands in a shell script or make it a
  shell function.  Then run it when you want to make the changes.  One
  way to make the changes semi-permanent is to put the commands in a
  file that runs each time you login or start up the computer.


  16.12.  Make a Terminal the Console

  This is also called a "serial console".  Many messages from the system
  are normally only sent to the console (the PC monitor).  Some of the
  messages sent to the console at boot-time may also be seen on any
  terminal after the boot succeeds by typing the command: dmesg.  If the
  boot fails this will not be of any use.  It's possible to modify the
  Linux kernel so as to make a terminal serve as the console and receive
  all the messages from Linux intended for the console.  Unfortunately,
  the messages from the BIOS (which display on the monitor when a PC is
  first started) will not display on this terminal.

  Creating a "serial console" means that the console (in the form of a
  terminal) is now on the serial port.  Of course it will not have the
  colors, fonts, graphical capabilities, and keyboard remapping
  possibilities of a real PC console-monitor.  Prior to kernel 2.2 you
  had to manually patch the kernel to do this.  Starting with kernel
  2.2, support is built into the kernel provided the kernel has been
  compiled with the option "console on the serial port" (or the like).
  These two cases are covered in the next two sub-sections:


  16.12.1.  For Kernels 2.2 or higher

  The instructions for creating a serial-console are included with
  source code documentation in the file: serial-console.txt.  Normally,
  the device /dev/console is linked to tty0 (the PC console).  For a
  serial-console you create a new /dev/console which is a true device
  (and not linked to something else).  You must also put a statement
  regarding the serial-console into /etc/lilo.conf and then run lilo.
  This is because the equivalent of "setserial" needs to be run to set
  up your serial-console before the kernel is loaded.  See the above
  mentioned documentation for details.


  16.12.2.  For Kernels before 2.2

  The Linux Journal in April 1997 had an article on patching the Linux
  kernel.  You add a couple of #defines at the start of
  src/linux/drivers/char/console.c:


  #define CONFIG_SERIAL_ECHO
  #define SERIAL_ECHO_PORT 0x2f8  /* Serial port address  */

  The following was not in the Linux Journal article.
  In kernel 2.+ (and earlier ??) you need to also set the baud
  rate (unless 9600 is OK).  Find these 2 lines:

  serial_echo_outb(0x00, UART_DLM); /* 9600 baud */
  serial_echo_outb(0x0c, UART_DLL);

  Change 0x0c in the line above (depending on the baud rate you want):

  115200 baud: 0x01       19200 baud: 0x06        2400 baud: 0x30
   57600 baud: 0x02        9600 baud: 0x0c        1200 baud: 0x60
   38400 baud: 0x03        4800 baud: 0x18



  If you currently use the console to select which operating system to
  boot (using LILO), but would like to do this from a terminal, then you
  need to add a line to the /etc/lilo.conf file.   See the manual page
  for lilo.conf and search for "serial=".


  16.12.3.  Can I Run Linux without a Monitor (PC Console) ?

  Yes, you use a terminal and make it behave like the console per above.
  You will likely still need a video card since most BIOSs require one
  to get the PC started.  Your BIOS may also require a keyboard to get
  started or it may have an option where you can set the BIOS not to
  require a keyboard.


  16.13.  Multiple Sessions

  The "screen" package runs multiple sessions something like virtual
  terminals on the console: See ``The Console: /dev/tty?''.  However,
  this is not like "pages" (``Pages'') since the image of the pages are
  stored in the host computer and not inside the terminal as they are
  with "pages".
  16.14.  Logging Out

  To log out type either "logout" or "exit".   Under some circumstances
  your request will be refused, but you should be told why.  One reason
  for refusal is if you are not in the same shell into which you logged
  into.  Another way to log out is to press ^D.  Since ^D is also used
  for other purposes, you may not want it to log you out.  If you set
  IGNOREEOF in the Bash shell then ^D will no longer log you out.


  16.15.  Chatting between Terminals, Spying

  If two persons logged into terminals on the same host computer want to
  chat with each other they may use the "write" or the "talk" program.
  On the Internet, chatting may be done using the "lynx" browser.

  For spying on what someone else is doing at their terminal use the
  "ttysnoop" program.  In "ttysnoop" the two terminals have the same
  status and anything typed on either keyboard appears on both screens
  (in the same location).  So if you're really spying and don't want to
  be detected, you shouldn't type anything.

  "ttysnoop" can be used for training with instructor and trainee
  sitting side-by-side, each at their own terminal.  The instructor may
  watch what the trainee is typing and can correct any mistakes by
  typing it correctly.  The trainee can watch what the instructor types
  and then try typing it.  It's just like they used one terminal with
  both people having their hands on the keyboard at the same time.

  There's one shortcoming to "ttysnoop" and that is that the terminals
  involved should all be (or emulate) the same type of terminal (such as
  vt100).  Since the "Linux" emulation done by a console (monitor) and
  the "minicom" emulation are close to vt100 one may use ttysnoop using
  two PCs, one running "minicom" which emulates a terminal.

  There's a non-free program called "DoubleVision" that is something
  like ttysnoop but does much more.  Terminals may be of different types
  and it can remember and playback sessions on terminals so you can
  observe what happened in the past.  The webpage is at
  <http://www.tridia.com>.


  17.  Trouble-Shooting (software)

  If you suspect that the problem is a hardware problem, see the
  ``Repair and Diagnose'' section.  If the problem involves the serial
  port itself see the Serial-HOWTO.

  Here is a list of possible problems:

  o  ``Is the Terminal OK ?''  Suspect the terminal is defective.

  o  ``Missing Text'' Either skips over some text or displays some text
     OK and hangs.

  o  ``Getty Respawning Too Rapidly'' (console error message)

  o  ``Fails Just After Login''

  o  ``Can't Login'' but login prompt is OK.

  o  ``Garbled Login Prompt''

  o  ``No Sign of any Login Prompt''


  There are two cases where the terminal goes bad.  One is when it's
  been recently working OK and suddenly goes bad.  This is discussed in
  the next sub-section.  The other case is where things don't work right
  just after you install a terminal.  For this case you may skip over
  the next section.


  17.1.  Terminal Was Working OK

  When a formerly working terminal suddenly goes bad it is often easy to
  find the problem.  That's because if you think about what recently
  happened it will likely give a clue to the cause of the problem.

  The problem may be obvious such as an error message when the terminal
  is first turned on.  If it makes a noise it likely needs repair.  See
  ``Repair & Diagnose''.  First, think about what has been done or
  changed recently as it's likely the cause of the problem.  Did the
  problem happen just after new system software was installed or after a
  change in the configuration?

  If the terminal isn't responding correctly (if at all) to what you
  type to it, you may have a ``Corrupted Terminal Interface''.


  17.2.  Terminal Newly Installed

  If you've just connected up a terminal to a computer per instructions
  and it doesn't work this section is for you.  If a terminal that
  formerly worked OK doesn't work now then see ``Terminal Was Working
  OK'' If you suspect that the serial port on your computer may be
  defective you might try running a diagnostic test program on it.  At
  present (June 1998) it seems that Linux doesn't yet have such a
  diagnostic program so you may need to run diagnostics under MS
  DOS/Windows.  There are some programs to monitor the various serial
  lines such at DTR, CTS, etc. and this may help.  See ``Serial
  Monitoring/Diagnostics''

  One approach is to first see if the the terminal will work by trying
  to copy a file to the terminal (cp my_file /dev/ttyS?) under the most
  simple situation.  This means with the modem control lines disabled
  and at a show speed that doesn't need flow control (make sure that any
  hardware flow control is disabled).  If this copy works, then make the
  situation a little more complicated and see if it still works, etc.,
  etc.  When the trouble appears just after you made a change, then that
  change is likely the source of the trouble.  Actually, its more
  efficient (but more complex) to jump from the simple situation to
  about half way to the final configuration so that the test eliminates
  about half of the remaining possible causes of the problem.  Then
  repeat this methodology for the next test.  This way it would only
  take about 10 tests to find the cause out of a thousand possible
  causes.  You should deviate a little from this method based on hunches
  and clues.


  17.3.  Is the Terminal OK ?

  A good terminal will usually start up with some words on the screen.
  If these words convey no error message, its probably OK.  If there is
  no sign of power (no lights on, etc.) re-plug in the computer power
  cord at both ends.  Make sure there is power at the wall jack (or at
  the extension cord end).  Try another power cord if you have one.
  Make sure the terminal is turned on and that its fuse is not blown.  A
  blank (or dim) screen may sometimes be fixed by just turning up the
  brightness and contrast using knobs or a keyboard key in set-up mode.
  If it still won't work See ``Repair & Diagnose'' for tips on repairing
  it.
  If the terminal starts up OK, but you suspect that something may be
  wrong with it, go into "local mode" where is works like a typewriter
  and try typing on it.  See ``Local Mode''.


  17.4.  Missing Text

  If some text displays on the terminal OK and then it stops without
  finishing (in the middle of a word, etc.) or if chunks of text are
  missing, you likely have a problem with flow control.  If you can't
  figure out right away what's causing it, decrease the speed.  If that
  fixes it, it's likely a flow control problem.  It may be that flow
  control is not working at all due to failure to configure it
  correctly, or due to incorrect cable wiring (for hardware flow
  control).  See ``Flow Control''

  If single characters are missing, perhaps the serial port is being
  overrun by too fast a speed.  Try a slower baud rate.

  If you are using a baud rate under 1200 (very slow, mostly used for
  old hard-copy terminals and printers) and the text gets truncated,
  then the problem may be in the serial device driver.  See Printing-
  HOWTO under "Serial devices" on how to fix this.


  17.5.  Getty Respawning Too Rapidly

  17.5.1.  Serial Module Not Loaded

  Use the "lsmod" command to see if the serial module is loaded.


  17.5.2.  No Modem Control Voltage

  If getty can't open and/or use a port because of the lack of a
  positive modem control voltage on one of the pins, then getty might be
  killed.  Then, per the instructions in inittab, getty respawns and
  tries again, only to be killed again, etc., etc.  You may see an error
  message indicating that due to getty respawning too rapidly it has
  been temporarily disabled.  Try using the "local" option with getty
  and/or check the modem control settings and voltages.


  17.5.3.  Key Shorted

  Another possible cause of getty respawning is if a keyboard key is
  shorted, giving the same result as if the key was continuously held
  down.  With auto-repeat enabled, this "types" thousands characters to
  the login prompt.  Look for a screen filled with all the same
  character (in some cases with 2 or more different characters).


  17.6.  Fails Just After Login

  If you can login OK but then you can't use the terminal it may be
  because the starting of the login shell has reconfigured the terminal
  (to an incorrect setting) by a command which someone put into one of
  the files that are run when you login and a shell starts.  These files
  include /etc/profile and  /.bashrc.  Look for a command starting with
  "stty" or "setserial" and make sure that it's correct.  Even if it's
  done OK in one initialization file, it may be reset incorrectly in
  another initialization file that you are not aware of.  Ways to get
  into the systems to fix it are to use another terminal or console, use
  a rescue diskette, or type: "linux single" at the lilo prompt which
  puts you into single user mode without running startup files.

  17.7.  Can't Login

  If you get a login prompt but get no response (or perhaps a garbled
  response) to your login attempts a possible cause is that the
  communication is bad one-way from the terminal to the computer.  It
  could be a bad or mis-wired cable/connector.  If you're not already
  using the "local" option with getty, do so to disable the modem
  control lines.  See ``Getty (in /etc/inittab)''.  You might also
  disable hardware flow control (stty -crtscts) if it was enabled.  If
  it now works OK then your modem control lines are likely either not
  wired correctly or there's a mistake in your set-up.  Some terminals
  allow setting different values (such as baud rate) for send and
  receive so the receive could be OK but the send bad.

  You should also (at the console) try "stty < /dev/ttyS1" (if you use
  ttyS1) to see that it's set up correctly.  It will often be in raw
  mode (and this is probably OK) with -icanon and -echo etc.  If the
  terminal incorrectly set at half-duplex (HDX), then one set of the
  characters you see when you type are coming from the terminal itself.
  If the characters are doubled, then the echos from the computer are OK
  and you may switch to full-duplex to fix this.  But if half-duplex is
  set and you only see what looks like normal "echos", then they are not
  coming from the computer as they should be.

  If you get a message saying something like "login failed" then if
  there is no error in typing or in the password, there may be some
  restrictions on logins which will not allow you to log in.
  Unfortunately, this message, may not tell you why it failed.  See
  ``Login Restrictions''


  17.8.  Garbled Login Prompt

  This may be due to using the wrong character set, transmission errors
  due to too high of a baud rate, incompatible baud rates, incompatible
  parity, or the wrong number of bits per byte.  If it's a variety of
  strange characters you have the wrong character set or a high order
  bit is being set by mistake.  If words are misspelled, try a lower
  baud rate.  For baud, parity, or bits/character incompatibilities you
  see a lot of the same "error character" which represents a real
  character that can't be displayed correctly due to an error in parity
  or baud rate.

  If you are using agetty (often just named getty), the agetty program
  will detect and set parity and/or bits/character if you type
  something.  Try it with a return to see if it fixes it.


  17.9.  No Sign of any Login Prompt

  This is when nothing at all happens at the terminal, but the terminal
  seems to be working OK.  One of the first things to do is to make sure
  that all cable connections are tight and connected to the correct
  connector on both the computer and terminal.  Other causes include
  defective hardware or cables (must be null modem), getty not running,
  terminal setup at wrong baud-rate, terminal in local mode, etc..  At
  this point two different avenues of approach are (you may pursue more
  than one at a time).


  o  ``Diagnose Problem from the Console''

  o  Measure Voltages ``Measure  Voltages''



  17.9.1.  Diagnose Problem from the Console

  At the console (or another working terminal), use "top" or "ps -al" to
  see if getty is running on the port.  Don't confuse it with getty
  programs running on other ports or on the virtual consoles.  You will
  not get a login prompt unless getty runs.

  One test is to try to copy a short file to the terminal (It might be a
  good idea to try this near the start of the installation process
  before setting up getty).  Use the Linux copy command such as: cp
  file_name /dev/ttyS1.  If it doesn't work, use stty to make the
  interface as simple as possible with everything disabled (such as
  hardware flow control: -crtscts; parity, and modem control signals:
  clocal).  Be sure the baud rates and the bits/byte are the same.  If
  nothing happens verify that the port is alive with a voltmeter per the
  next section.


  17.9.2.  Measure Voltages

  If you have a voltmeter handy check for a negative voltage (-4v to
  -15v) at pin 3 (receive data) at the terminal side of the null modem
  cable.  The positive lead of the meter should be connected to a good
  ground (the metal connectors on the ends of cables are often not
  grounded).  If there is no such negative voltage then check for it at
  the transmit pin (TxD) on the computer (see ``DB9-DB25'' for the pin-
  out).  If it's present there but not at the receive pin (RxD) at the
  terminal, then the cable is bad (loose connection, broken wire, or not
  a null modem).  If voltage is absent, then the serial port on the
  computer is dead.  Test it with a software diagnostic test or replace
  it.

  If the serial port is alive, you may want to send a file to it (with
  modem controls disabled) and see if anything gets to it.  To check for
  a transmitted signal with an analog voltmeter, look at the needle at
  -12 V when the line is idle.  Then start sending a file (or start
  getty).  You should see the needle dropping to 0 and fluttering about
  0 as it measures short-run averages of the bit stream.  You can see
  this also on the AC scale provided that your meter has a capacitor to
  block out DC voltages when on the AC scale.  If it doesn't, then the
  idle DC of -12 V will cause a high false AC reading.  Without a meter,
  you could connect a known good device (such as another terminal or an
  external  modem) to the serial port and see if it works OK.


  17.10.  Slow: pauses of several seconds between bursts of characters

  You likely have mis-set interrupts>  See the Serial-HOWTO section
  starting with "Slow:"


  17.11.  Serial Monitoring/Diagnostics

  A few Linux programs will monitor the modem control lines and indicate
  if they are positive (1) or negative (0).

  o  statserial (in Debian distribution)

  o  serialmon (doesn't monitor RTS, CTS, DSR but logs other functions)

  o  modemstat (only works on Linux PC consoles.  Will coexist with the
     command line)

     You may already have them.  If not, go to Serial Software
     <http://sunsite.unc.edu/pub/Linux/system/serial/>.  When using
     these, bear in mind that what you see is the state of the lines at
     the host computer.  The situation at the terminal will be different
     since some wires are often missing from cables while other wires
     cross over.  As of June 1998, I know of no diagnostic program in
     Linux for the serial port.


  17.12.  Local Mode

  In local mode, the terminal disconnects from the computer and behaves
  like a typewriter (only it doesn't type on paper but on the screen).
  Going back into on-line reconnects to the computer allowing you to
  resume activities at the same point where you left off when you went
  into "local".  This is useful both for testing the terminal and for
  educational purposes.  When in local mode you may type escape
  sequences (starting with the ESC key) and observe what they do.  If
  the terminal doesn't work correctly in local mode, it's unlikely that
  it will work correctly when connected to the computer.  If you're not
  exactly sure what an escape sequence does, you can try it out in local
  mode.  You might also use it for trying out a terminal that is for
  sale.  To get into local mode you first enter set-up mode and then
  select "local" from a menu (or press a certain key).  See ``Getting
  Into Set-Up (Configuration) Mode''.


  17.13.  Serial Electrical Test Equipment

  17.13.1.  Breakout Gadgets, etc.

  While a multimeter (used as a voltmeter) may be all that you need for
  just a few terminals, simple special test equipment has been made for
  testing serial port lines.  Some are called "breakout ... " where
  breakout means to break out conductors from a cable.  These gadgets
  have a couple of connectors on them and insert into the serial cable.
  Some have test points for connecting a voltmeter.  Others have LED
  lamps which light when certain modem control lines are asserted
  (turned on).  Still others have jumpers so that you can connect any
  wire to any wire.  Some have switches.

  Radio Shack sells (in 1998) a "RS-232 Troubleshooter" or "RS-232 Line
  Tester" which checks TD, RD, CD, RTS, CTS, DTR, and DSR.  A green
  light means on (+12 v) while red means off (-12 v).  They also sell a
  "RS-232 Serial Jumper Box" which permits connecting the pins anyway
  you choose.


  17.13.2.  Measuring Voltages

  Any voltmeter or multimeter, even the cheapest that sells for about
  $10, should work fine.  Trying to use other methods for checking
  voltage is tricky.  Don't use a LED unless it has a series resistor to
  reduce the voltage across the LED.  A 470 ohm resistor is used for a
  20 ma LED (but not all LED's are 20 ma).  The LED will only light for
  a certain polarity so you may test for + or - voltages.  Does anyone
  make such a gadget for automotive circuit testing??  Logic probes may
  be damaged if you try to use them since the TTL voltages for which
  they are designed are only 5 volts.  Trying to use a 12 V incandescent
  light bulb is not a good idea.  It won't show polarity and due to
  limited output current of the UART it probably will not even light up.

  To measure voltage on a female connector you may plug in a bent paper
  clip into the desired opening.  The paper clip's diameter should be no
  larger than the pins so that it doesn't damage the contact.  Clip an
  alligator clip (or the like) to the paper clip to connect up.



  17.13.3.  Taste Voltage

  As a last resort, if you have no test equipment and are willing to
  risk getting shocked (or even electrocuted) you can always taste the
  voltage.  Before touching one of the test leads with your tongue, test
  them to make sure that there is no high voltage on them.  Touch both
  leads (at the same time) to one hand to see if they shock you.  Then
  if no shock, wet the skin contact points by licking and repeat.  If
  this test gives you a shock, you certainly don't want to use your
  tongue.

  For the test for 12 V, Lick a finger and hold one test lead in it.
  Put the other test lead on your tongue.  If the lead on your tongue is
  positive, there will be a noticeable taste.  You might try this with
  flashlight batteries first so you will know what taste to expect.


  18.  Repair & Diagnose

  Repairing a terminal has much in common with repairing a monitor
  and/or keyboard.  Sometimes the built-in diagnostics of the terminal
  will tell you what is wrong on the screen.  If not, then by the
  symptoms, one may often isolate the trouble to one of the following:
  bad keyboard, CRT dead, terminal digital electronics failure.  It's
  best to have a service manual, but even if you don't have one, many
  terminals may still be repaired.


  18.1.  Repair Books & Websites

  18.1.1.  Books

  Bigelow, Stephen J.: Troubleshooting & Repairing Computer Monitors,
  2nd edition, McGraw-Hill, 1997.  Doesn't cover the character
  generation electronics nor the keyboard.


  18.1.2.  Websites

  The FAQ  <http://www.repairfaq.org> for the newsgroup:
  sci.electronics.repair is long and comprehensive, although it doesn't
  cover terminals per se.  See the section "Computer and Video
  Monitors".  Much of this information is applicable to terminals as are
  the sections: "Testing Capacitors", "Testing Flyback Transformers",
  etc.  Perhaps in the future, the "info" on repair in this HOWTO will
  consist mainly of links to the above FAQ (or the like).  Shuford's
  repair archive
  <http://www.cs.utk.edu/~shuford/terminal/repair_hints_news.txt> of
  newsgroup postings on terminal repair is another source of info.


  18.2.  Safety

  CRT's use high voltage of up to 30,000 volts for color (less for
  monochrome).  Be careful not to touch this voltage if the set is on
  and the cover off.  It probably won't kill you even if you do since
  the amount of current it can supply is limited.  But it is likely to
  badly burn and shock you, etc.  High voltage can jump across air gaps
  and go thru cracked insulation so keep your hands a safe distance from
  it.  You should notice the well-insulated high voltage cable connected
  to one side of the picture tube.  Even when the set is off, there is
  still enough residual voltage on the picture tube cable connection to
  give you quite a shock.  To discharge this voltage when the set is
  unplugged use a screwdriver (insulated handle) with the metal blade
  grounded to the picture tube ground cable with a jumper wire.  Don't
  use chassis ground.
  The lower voltages (of hundreds of volts) can be even more dangerous
  since they are not current limited.  It is even more dangerous if your
  hands are wet or if you are wearing a metal watchband, ring or the
  like.  In rare cases people have been killed by it so be careful.  The
  lowest voltages of only several volts on digital circuitry are fairly
  safe but don't touch anything (except with a well insulated tool)
  unless you know for sure.


  18.3.  Appearance of Display

  If the display is too dim, turn up the brightness and/or contrast.
  using knobs on the exterior of the unit (if they exist).  If the
  width, height or centering is incorrect, there are often control knobs
  for these.  For some older terminals one must press an arrow key (or
  the like) in set-up mode.

  You may need to remove the cover to make adjustments, especially on
  older models.  You could arrange things so that a large mirror is in
  front of the terminal so as to view the display in the mirror while
  making adjustments.  The adjustments to turn may be on a printed
  circuit board.  While a screwdriver (possibly Phillips-head) may be
  all that's needed, inductors may require special TV alignment tools
  (plastic hex wrenches, etc.).  The abbreviated name of the adjustment
  should be printed on the circuit board.  For example, here are some
  such names:


  o  V-Size adjusts the Vertical height (Size)

  o  H-Size adjusts the Horizontal width (Size).  It may be an inductor.

  o  V-Pos adjusts the Vertical Position

  o  H-Pos adjusts the Horizontal Position

  o  V-Lin adjusts Vertical Linearity (Use if width of scan lines
     differs at the top and bottom of the screen)

  o  V-Hold adjusts Vertical Hold (Use if screen is uncontrollable
     scrolling)

  o  Bright adjusts brightness (an external knob may also exist)

  o  Sub-Bright adjusts brightness of subdued intensity mode (often the
     normal mode: dimmer than bold or bright mode).

  Changing linearity may change the size so that it will need to be
  readjusted.  A terminal that has been stored for some time may have a
  small display rectangle on the screen surrounded by a large black
  border.  If it's difficult to adjust, wait a while before adjusting it
  since it will likely recover some with use (the black borders will
  shrink).


  18.4.  Diagnose

  18.4.1.  Terminal Made a Noise

  If the terminal made some noise just before it failed (or when you
  turn it on after it failed) that noise is a clue to what is wrong.  If
  you hear a sparking noise or see/smell smoke, immediately turn the
  terminal off to prevent further damage.  The problem is likely in the
  high voltage power supply of several thousand volts.  Remove the cover
  and if the bad spot is not evident, turn it on again for a short time
  in a dimly lit room to look for arcing.  The high voltage cable (runs
  between the flyback transformer and the side of the picture tube) may
  have broken insulation that arcs to ground.  Fix it with high-voltage
  insulating dope, or special electrical tape designed say for 10,000
  volts.

  The flyback transformer (high voltage) may make only a faint clicking
  or sparking noise if it fails.  You may not hear it until you turn the
  terminal off for a while to rest and then turn it back on again.  To
  track down the noise you may use a piece of small rubber tubing (such
  as used in automobiles) as a stethoscope  to listen to it.  But while
  you are listening for the noise, the terminal is suffering more damage
  so try find it fast (but not so fast as to risk getting shocked).

  A short in the power supply may cause a fuse to blow with a pop.
  Replacing a blown fuse may not solve the problem as the same short may
  blow the fuse again.  Inspect for any darkened spots due to high heat
  and test those components.  Shorted power transistors may cause the
  fuse to blow.  They may be tested with a transistor checker or even
  with an ohm-meter.  Use the low ohm scale on an ohm-meter so that the
  voltage applied by the meter is low.  This will reduce the possible
  damage to good components caused by this test voltage.

  If the terminal has been exposed to dampness such as being stored in a
  damp place or near a kitchen with steam from cooking, a fix may be to
  dry out the unit.  Heating a "failed" flyback transformer with a blow
  dryer for several minutes may restore it.


  18.4.2.  Terminal Made No Noise

  A blank screen may be due to someone turning the brightness control to
  the lowest level or to aging.  The next thing to do is to check the
  cables for loose or broken connections.  If there is no sign of power,
  substitute a new power cord after making sure that the power outlet on
  the wall is "hot".

  If the keyboard is suspected, try it on another terminal of the same
  type or substitute a good keyboard.  Wiggle the keyboard cable ends
  and the plug.  Wires inside cables may break, especially near their
  ends.  If the break is verified by wiggling it (having the problem go
  on and off in synchronization with the wiggles), then one may either
  get a new cable or cut into the cable and re-solder the breaks, etc.

  One of the first things to do if the keyboard works is to put the
  terminal into ``Local Mode''.  If it works OK in local, then the
  problem is likely in the connection to the host computer (or incorrect
  interface) or in the UART chips of the terminal.

  By carefully inspecting the circuitry, one may often find the cause of
  the problem.  Look for discoloration, cracks, etc.  An intermittent
  problem may sometimes be found by tapping on components with a ball-
  point pen (not the metal tip of course).  A break in the conductor of
  a printed circuit board may sometimes be revealed by flexing the
  board.  Solder that looks like it formed a drop or a solder joint with
  little solder may need re-soldering.  Soldering may heat up
  transistors (and other components) and damage them so use a heat sink
  if feasible.

  If you have a common brand of terminal, you may be able to search
  newsgroup postings on the Internet to find out what the most frequent
  types of problems are for your terminal and perhaps information on how
  to fix them.

  To see if the digital electronics work, try (using a good keyboard)
  typing at the bad terminal.   Try to read this typing at a good
  terminal (or the console) using the copy command or with a terminal
  communication program such as minicom.  You may need to hit the return
  key at the terminal in order to send a line.  One may ask the bad
  terminal for its identity etc. from another terminal.  This will show
  if two-way communication works.


  18.5.  Error Messages on the Screen

  You are in luck if you see an error message on the screen.  This
  usually happens when you first turn the terminal on.


  18.5.1.  Keyboard Error

  This usually means that the keyboard is not plugged in, or that the
  connection is loose.  For more serious problems see ``Keyboards''


  18.5.2.  Checksum Error in NVR

  NVR is "Non-Volatile RAM".  This means that the NVR where the set-up
  information is stored has become corrupted.  The terminal will likely
  still work but the configuration that was last saved when someone last
  configured the terminal has likely been lost.  Try configuring again
  and then save it.  It might work.  On very old terminals (early
  1980's) there was a battery-powered CMOS to save the configuration so
  in this case the problem could be just a dead battery.  Sometimes the
  EEPROM chip (no battery needed) goes bad after too many saves.  It may
  be hard to find.  If you can't fix it you are either stuck with the
  default configuration or you may have escape sequences sent to the
  terminal when you start it up to try to configure it.


  18.6.  Capacitors

  Electrolytic capacitors have a metal shell and are may become weak or
  fail if they set for years without being used.  Sometimes just leaving
  the terminal on for a while will help partially restore them.  If you
  can, exercise any terminals you have in storage by turning them on for
  a while every year or two.


  18.7.  Keyboards

  18.7.1.  Interchangeability

  The keyboards for terminals are not the same as keyboards for PC's.
  The difference is not only in the key layout but in the codes
  generated when a key is pressed.  Also, keyboards for various brands
  and models of terminals are not always interchangeable with each
  other.  Sometimes one get an "incompatible" keyboard to partially work
  on a terminal.  All the ASCII keys will work OK, but special keys such
  as set-up and break will not work correctly.


  18.7.2.  How They Work

  Most keyboards just make a simple contact between two conductors when
  you press a key.  Electronics inside a chip in the keyboard converts
  this contact closure into a code sent over the keyboard's external
  cable.  Instead of having a separate wire (or conductor) going from
  each key to the chip, the following type scheme is used.  Number the
  conductors say from 1-10 and A-J.  For example: conductor 3 goes to
  several keys and conductor B goes to several keys, but only one key
  has both conductors 3 and B going to it.  When that key is pressed, a
  short circuit is established between 3 and B.  The chip senses this
  short and knows what key has been pressed.  Such a scheme reduces the
  number of conductors needed (and  reduces the number of pins needed on
  the chip).  It's a similar scheme to what is called a "crossbar
  switch".


  18.7.3.  One Press Types 2 Different Characters

  If, due to a defect, conductors 3 and 4 become shorted together then
  pressing the 3-B key will also short 4 and B and the chip will think
  that both keys 3-B and 4-B have been pressed.  This is likely to type
  2 different characters when all you wanted was one character.


  18.7.4.  Modern vs Old Keyboards

  While the modern keyboard and the old fashioned type look about the
  same, the mechanics of operation are different.  The old ones have
  individual key switches under the key-caps with each switch enclosed
  in a hard plastic case.  The modern ones use large flexible plastic
  sheets (membrane) the size of the keyboard.  A plastic sheet with
  holes in it is sandwiched between two other plastic sheets containing
  printed circuits (including contact points).  When you press a key,
  the two "printed" sheets are pressed together at a certain point,
  closing the contacts printed on the sheets at that point.


  18.7.5.  Keyboard Doesn't Work At All

  If none of the keys work try another keyboard (if you have one) to
  verify that the keyboard is the problem.  The most likely cause is a
  broken wire inside the cord (cable) that connects it to the terminal.
  The most likely location of the break is at either end of the cord.
  Try wigging the ends of the cord while tapping on a key to see if it
  works intermittently.  If you find a bad spot, you may carefully cut
  into the cord with a knife at the bad spot and splice the broken
  conductor.  Sometimes just a drop of solder will splice it.  Seal up
  the cord with electrical tape, glue, or caulk.


  18.7.6.  Typing b Displays bb, etc. (doubled)

  If all characters appear double there is likely nothing wrong with the
  keyboard.  Instead, your terminal has likely been incorrectly set up
  for half-duplex (HDX or local echo=on) and every character you type is
  echoed back both from the electronics inside your terminal and from
  your host computer.  If the two characters are not the same, there may
  be a short circuit inside your keyboard.  See ``One Press Types 2
  Different Characters''


  18.7.7.  The Keyboard Types By Itself

  If a key is shorted out it is likely to type out a large number of the
  same character if auto-repeat is enabled.  If more than one key is
  shorted out then repeating sequences of a few characters will be
  typed.  This may cause getty to respawn too fast if it happens at the
  login prompt.  See ``Key Shorted''.  The fix is to clean the contacts
  per ``Cleaning Keyboard Contacts''.


  18.7.8.  Liquid Spilled on the Keyboard

  If water or watery liquid has been spilled on the keyboard (or if it
  was exposed to rain, heavy dew, or dampness) some keys may not work
  right.  The dampness may cause a key to short out (like it was pressed
  down all the time) and you may see the screen fill up with that letter
  if auto-repeat is enabled.  If it's gotten wet and then partially (or
  fully) dried out, certain keys may not work due to deposits on the
  contact surfaces.  For the modern type of keyboard, one may readily
  take apart the plastic sheets inside and dry/clean them.  For the old
  type one may let it dry out in the sun or oven (low temp.).  When it's
  dry it may still need contact cleaner on some keys as explained below.


  18.7.9.  Cleaning Keyboard Contacts

  18.7.9.1.  Keyboards with Membranes

  On some newer keyboards, the plastic sheets (membranes) are easy to
  remove for inspection and cleaning if needed.  You only need to remove
  several screws to take apart the keyboard and get to the sheets.  On
  some old IBM keyboards the sheets can't be removed without breaking
  off many plastic tabs which will need to be repaired with glue to put
  back (probably not worthwhile to repair).  Such a keyboard may
  sometimes be made to work by flexing, twisting, and/or pounding the
  assembly containing the plastic sheets.


  18.7.9.2.  Keyboards with Individual Switches

  What follows is for older keyboards that have separate hard plastic
  switches for each key.  Before going to all the work of cleaning
  electrical contacts first try turning the keyboard upside-down and
  working the bad keys.  This may help dislodge dirt, especially if you
  press the key hard and fast to set up vibration.  Pressing the key
  down and wiggling it from side to side, etc. often helps.

  Often the key-caps may be removed by prying them upward using a small
  screwdriver as a lever while preventing excessive tilting with a
  finger.  There exists a special tool known as keycap puller but you
  can get by without it.  (Warning: Key-caps on modern keyboards don't
  pry up.)  The key-cap may tilt a bit and wobble as it comes loose.  It
  may even fly up and onto the floor.  Then you have two choices on how
  to clean the contacts: Use contact cleaner spray directly on top of
  the key switch, or take the key switch apart and clean it.  Still
  another choice is to replace the key switch with a new or used one.

  Directly spraying contact cleaner or the like (obtained at an
  electronics store) into the top of the key switch is the fastest
  method but may not work and it also might damage the plastic.  Before
  spraying, clean the area next to the switch rods.  With the keyboard
  live (or with the key contacts connected to an ohm-meter) use the tube
  which came with the spray to squirt cleaner so it will get inside the
  key switch.  Don't let the cleaning liquid get under nearby keys where
  it may pick up dust and then seep (with the dust) into adjacent key
  switches.  If you make this mistake you may fix one key but damage
  nearby keys.  If this should happen, immediately work the affected
  nearby keys until they work OK.

  You might tilt the keyboard so that the cleaner flows into the
  contacts.  For the CIT101e terminal with an Alps keyboard, this means
  tilting the digit row up toward the ceiling.  Work the key switch up
  and down with a pen or small screwdriver handle to avoid getting the
  toxic cleaner liquid on your skin (or wear gloves).  Finally turn the
  keyboard upside-down while working the key to drain off remaining
  cleaner.  The more cleaner you squirt in the more likely it will fix
  it but it is also more likely to do more damage to the plastic or
  contaminate adjacent keys, so use what you think is just enough to do
  the job.  Once the key works OK, work it up and down a little more and
  test it a half minute later, etc. to make sure it will still work OK.

  Sometimes a key works fine when the contacts inside are saturated with
  contact cleaner liquid, but when the liquid dries a few minutes later
  then the resulting scale on the contacts prevents good contact and the
  key works erratically (if at all).  Operating the key when the liquid
  is drying inside may help.  Some switches have the contacts nearly
  sealed inside so little if any contact cleaner reaches the contacts.
  The cleaner that does get to the contacts may carry contamination with
  it (cleaning around the tops before spraying helps minimize this).

  If you need to disassemble the key switch, first inspect it to see how
  it is installed and comes apart.  Sometimes one may remove the cover
  of the switch without removing the switch from the keyboard.  To do
  this pry up (or pull up) the top of the key switch after prying apart
  thin plastic tabs that retain it.  Don't pry too hard or you may break
  the thin plastic.  If this can't be done, you may have to unsolder the
  switch and remove it in order to take it apart (or replace it).  Once
  the switch has been taken apart you still may not be able to see the
  contacts if the contact surfaces are sandwiched together (nearly
  touching).  You may get contact cleaner on the contacts by slightly
  prying apart the conducting surfaces and squirting cleaner between
  them.  There may be some kind of clip holding the contact surfaces
  together which needs to be removed before prying these surfaces apart.
  With cleaner on the contacts, work them.  Tilting the keyboard or
  inverting it may help.  Take care not to loose small parts as they may
  fly up into the air when taking apart a key switch.


  19.  Appendix A: General

  19.1.  List of Linux Terminal Commands

  19.1.1.  Sending a Command to the Terminal


  o  ``setterm'': long options

  o  ``tput'': terse options

  o  tset: initializes only

  o  clear: clears screen

  o  reset: sends reset string


  19.1.2.  Configuring the Terminal Device Driver


  o  ``Setserial'':

  o  ``Stty''


  19.1.3.  Terminfo


  o  ``Terminfo Compiler (tic)'' terminfo compiler & translator

  o  toe: shows list of terminals for which you have terminfo files

  o  ``infocmp'' compares or displays terminfo entries





  19.1.4.  Other


  o  gitkeys: shows what bytes each key sends to the host.

  o  tty: shows what tty port you are connected to.

  o  set (or tset -q): shows the value of TERM, the terminfo entry name

  o  ``tset'': sets TERM interactively and initializes


  19.2.  The Internet and Books

  19.2.1.  Terminal Info on the Internet



  o  Shuford's Website
     <http://www.cs.utk.edu/~shuford/terminal_index.html> at the
     University of Tennessee has a great deal of useful information
     about text terminals.

  o  Boundless <http://www.boundless.com/textterm/> purchased the VT and
     Dorio terminal business from DEC.  To get Specs select either ADDS,
     VT, or DORIO links.  Then select a "data sheet" link.  Then on the
     data sheet select the "Go to Specs" link.

  o  Wyse text-terminals kbase
     <http://www.wyse.com/service/support/kbase/wyseterm.asp> is a major
     manufacturer of terminals.  For new models see Wyse terminal
     <http://www.wyse.com/terminal/>  See also Old  Wyse terminal Specs
     <http://www.wyse.com/service/faq/wysetter.htm>

  o  Escape Seqs.; N. America <http://www.pericom-
     usa.com/twdocs/doc/twproae.htm> or Escape Seqs.; Europe
     <http://www.pericom.co.uk/teemworld/doc/twproae.htm> is a list of
     escape sequences (and control codes) for some terminal emulations
     (including VT 100, 300, 420, and Wyse).

  o  comp.terminals is the newsgroup for terminals


  19.2.2.  Books Related to Terminals



  o  EIA-232 serial port see ``EIA-232 (RS-232) Books''.

  o  Repair see ``Repair Books &  Websites''.

  o  Terminfo database see ``Termcap Documents''


  19.2.3.  Entire Books on Terminals

  As far as I know, there is no satisfactory book on text terminals
  (unless you are interested in antique terminals of the 1970's).

  o  Handbook of Interactive Computer Terminals by Duane E. Sharp;
     Reston Publishing Co. 1977. (mostly obsolete)

  o  Communicating with Display Terminals by Roger K. deBry; McGraw-Hill
     1985.  (mostly on IBM synchronous terminals)


  The "HANDBOOK ... " presents brief specifications of over 100
  different models of antique terminals made in the early 1970's by over
  60 different companies.  It also explains how they work physically but
  incorrectly shows a diagram for a CRT which uses electrostatic
  deflection of the electron beam (p. 36).  Terminals actually used
  magnetic deflection (even in the 1970's).  This book explains a number
  of advanced technical concepts such as "random scan" and "color
  penetration principle".

  The "COMMUNICATING ... " book in contrast to the "Handbook ... "
  ignores the physical and electronic details of terminals.  It has an
  entire chapter explaining binary numbers (which is not needed in a
  book on terminals since this information is widely available
  elsewhere).  It seems to mostly cover old IBM terminals (mainly the
  3270) in block and synchronous modes of operation.  It's of little use
  for the commonly used ANSI terminals used today on Unix-like systems.
  Although it does discuss them a little it doesn't show the various
  wiring schemes used to connect them to serial ports.


  19.2.4.  Books with Chapters on Terminals

  These chapters cover almost nothing about the terminals themselves and
  their capabilities.  Rather, these chapters are mostly about how to
  set up the computer (and its terminal driver) to work with terminals.
  Due to the differences of different Unix-like systems, much of the
  information does not not apply to Linux.


  o  Unix Power Tools by Jerry Peck et. al.  O'Reilly 1998.  Ch. 5
     Setting Up Your Terminal, Ch. 41: Terminal and Serial Line
     Settings, Ch. 42: Problems With Terminals

  o  Advanced Programming in the Unix Environment by W. Richard Stevens
     Addison-Wesley, 1993.  Ch. 11: Terminal I/O, Ch. 19: Pseudo
     Terminals

  o  Essential System Administration by Aleen Frisch, 2nd ed.  O'Reilly,
     1998.  Ch. 11: Terminals and Modems.

  The "UNIX POWER TOOLS" book has 3 short chapters on text terminals.
  It covers less ground than this HOWTO but gives more examples to help
  you.

  The "ADVANCED PROGRAMMING ... " Chapter 11 covers only the device
  driver included in the operating system to deal with terminals.  It
  explains the parameters one gives to the stty command to configure the
  terminal.

  The "ESSENTIAL SYSTEM ..." book's chapter has more about terminals
  than modems.  It seems well written.


  19.3.  Non-Linux OS's

  The configuration of the host computer for terminals for non-Linux
  operating systems is usually significantly different than for Linux.
  Here are some links to on-line manuals for Unix-like systems.


  o  SCO's OpenServer Adding Serial Terminals
     <http://www2.sco.com:1996/HANDBOOK/serial_terminal_adding.html> in
     SCO OpenServer Handbook.

  o  Hewlett-Packard's HP-UX Configuring Terminals and Modems
     <http://www.software.hp.com/OS_transition/DOCS/PERIPH/TERMS3.HTM>
  20.  Appendix B: Escape Sequence Commands Terminology

  These are sometimes called "control sequences".  This section of Text-
  Terminal-HOWTO is incomplete (and may never be complete as there are
  such a huge number of control sequences).  This section is for
  reference and perhaps really belongs in something that would be called
  "Text-Terminal-Programming-HOWTO".

  An example of an ANSI standard escape sequence is ESC[5B which moves
  the cursor down 5 lines.  ESC is the Escape character.  The parameter
  5 is included in the sequence.  If it were 7 the cursor would move
  down 7 lines, etc.  A listing for this sequence as "move cursor down x
  lines: ESC[xB" is easy to to understand.  But command jargon such as:
  "tertiary device attribute request" is less comprehensible.  This
  section will try to explain some of the more arcane jargon used for
  escape sequence commands.  A full listing (including the escape
  sequence codes for the ANSI standard) is a "wish list" project.  Since
  many escape sequences do the same thing as is done when setting up the
  terminal with ``Set-Up Options'', such escape sequences options will
  not be repeated here.


  20.1.  Esc Sequence List

  For a list of many (but not all) escape sequences for various
  terminals see Escape Seqs.; N. America <http://www.pericom-
  usa.com/twdocs/doc/twproae.htm> or Escape Seqs.; Europe
  <http://www.pericom.co.uk/teemworld/doc/twproae.htm>.  These are  used
  for terminal emulation and are not always the same as on the
  corresponding real terminal..  A list for VT (not maintained) may be
  found at Emulators FAQ <http://www.cs.ruu.nl/wais/html/na-
  dir/emulators-faq/part3.html>.  Search for "VT".


  20.2.  8-bit Control Codes

  Table of 8-bit DEC control codes (in hexadecimal).  Work on VT2xx or
  later.  CSI is the most common.

  ACRONYM  FULL_NAME                      HEX     REPLACES
  IND Index (down one line)               84      ESC D
  NEL Next Line                           85      ESC E
  RI  Reverse Index (one line up)         8D      ESC M
  SS2 Single Shift 2                      8E      ESC N
  SS3 Single Shift 3                      8F      ESC O
  DCS Device Control String               90      ESC P
  CSI Control Sequence Introducer)        9B      ESC [
  ST  String Terminator                   9C      ESC \




  20.3.  Printer Esc



  o  Auto Print on/off: When on, data from the host is also teed (sent)
     to the printer port of the terminal (and also shows on the terminal
     screen).

  o  Print Controller on/off: When on, data from the host is sent only
     to the printer (nothing shows on the terminal screen).




  20.4.  Reports

  These sequences are usually a request sent from the host to request a
  report from the terminal.  The terminal responds by sending a report
  (actually another escape sequence) to the host which has embedded in
  it certain values telling the host about the current state of the
  terminal.  In some cases a report may be sent to the host even if it
  wasn't asked for.  This sometimes happens when set-up is exited.  By
  default no unsolicited reports should be sent.


  o  Request for Status (Report Operating Status):  Meaning of replies
     for VT100 is either "I'm OK" or "I'm not OK"

  o  Request for Device Attributes:  The "device" is usually the
     printer.  Is there a printer?  Is it ready?

  o  Reqest for Tertiary Device Attributes (VT):  Reply is report that
     was entered during set-up.  The tertiary device is the 3rd device
     (the printer or auxiliary port device ??).  The 1st device may be
     the host computer and the 2nd device the terminal.

  o  Request for Terminal Parameters:  What is the parity, baud rate,
     byte width, etc.  This request doesn't seem to make much sense,
     since if the host didn't already know this it couldn't communicate
     with the terminal or send a reply.



  20.5.  Cursor Movements

  The cursor is where the next character received from the host will be
  displayed.  Most of the cursor movements are self-explanatory.  "index
  cursor" means to move the cursor down one line.  Cursor movements may
  be relative to the current position such as "move 4 spaces left" or
  absolute such as "move to row 3, column 39".  Absolute is called
  "Direct Cursor Positioning" or "Direct Cursor Addressing".

  The home position is row 1 col. 1 (index origin is 1).  But where this
  home position is on the physical screen is not completely clear.  If
  "Cursor Origin Mode" = "Relative Origin Mode" is set, then home is at
  the top of the scrolling region (not necessarily the top of the
  screen) at the left edge of the screen.  If "Absolute Origin Mode" is
  set (the same as unsetting any of the two modes in the previous
  sentence) then home is at the upper left corner of the screen.  On
  some old terminals if "Cursor Origin Mode" is set it means that it's
  relative.


  20.6.  Pages (definition)

  See ``Pages'' for an explanation of pages.  There are a number of
  escape sequences to deal with pages.  Text may be copied from one page
  to another and one may move the cursor from page to page.  Switching
  pages may or may not be automatic: when the screen becomes full (page
  1) then more data from the host goes to page 2.  The cursor may only
  be on one page at a time and characters which are sent to the terminal
  go there.  If that page is not being displayed, new text will be
  received by the terminal and go into display memory, but you will not
  see it (until the terminal is switched to that page).


  21.  Appendix C: Serial Communications on EIA-232 (RS-232)



  21.1.  Intro to Serial Communication

  (Much of this section is now found in Serial-HOWTO.)  Text terminals
  on Unix-like systems (and on PC's) are usually connected to an
  asynchronous 232 serial port of a computer.  It's usually a RS-232-C,
  EIA-232-D, or EIA-232-E.  These three are almost the same thing.  The
  original RS prefix became EIA (Electronics Industries Association) and
  later EIA/TIA after EIA merged with TIA (Telecommunications Industries
  Association).  The EIA-232 spec provides also for synchronous (sync)
  communication but the hardware to support sync is almost always
  missing on PC's.  The RS designation is obsolete but is still in use.
  EIA will be used in this article.

  The serial port is more than just a physical connector on the back of
  a computer or terminal.  It includes the associated electronics which
  must produce signals conforming to the EIA-232 specification.  The
  standard connector has 25 pins, most of which are unused.  An
  alternative connector has only 9 pins.  One pin is used to send out
  data bytes and another to receive data bytes.  Another pin is a common
  signal ground.  The other "useful" pins are used mainly for signalling
  purposes with a steady negative voltage meaning "off" and a steady
  positive voltage meaning "on".

  The UART (Universal Asynchronous Receiver-Transmitter) chip does most
  of the work.  Today, the functionality of this chip is usually built
  into another chip.


  21.2.  Voltages

  21.2.1.  Voltage for a Bit

  At the EIA-232 serial port, voltages are bipolar (positive or negative
  with respect to ground) and should be about 12 volts in magnitude
  (some are 5 or 10 volts).   For the transmit and receive pins +12
  volts is a 0-bit (sometimes called "space") and -12 volts is a 1-bit
  (sometimes called "mark").  This is known as inverted logic since
  normally a 0-bit is both false and negative while a one is normally
  both true and positive.  Although the receive and transmit pins are
  inverted logic, other pins (modem control lines) are normal logic with
  a positive voltage being true (or "on" or "asserted") and a negative
  voltage being false (or "off" or "negated").  Zero voltage has no
  meaning (except it usually means that the unit is powered off).

  A range of voltages is allowed.  The specs say the magnitude of a
  transmitted signal should be between 5 and 15 volts but must never
  exceed 25 V.  Any voltage received under 3 V is undefined (but some
  terminals will accept a lower voltage as valid).  One sometimes sees
  erroneous claims that the voltage is commonly 5 volts (or even 3
  volts) but it's usually 11-12 volts.  If you are using a EIA-422 port
  on a Mac computer as an EIA-232 (requires a special cable) or EIA-423
  then the voltage will actually be only 5 V.  The discussion here
  assumes 12 V.  There is much confusion about voltages on the Internet.

  Note that normal computer logic normally is just a few volts (5 volts
  was once the standard) so that if you try to use test equipment
  designed for testing 3-5 volt computer logic (TTL) on the 12 volts of
  a serial port, it may damage the test equipment.


  21.2.2.  Voltage Sequence for a Byte

  The transmit pin (TxD) is held at -12 V (mark) at idle when nothing is
  being sent.  To start a byte it jumps to +12 V (space) for the start
  bit and remains at +12 V for the duration (period) of the start bit.
  Next comes the low-order bit of the data byte.  If it's a 0-bit
  nothing changes and the line remains at +12 V for another bit-period.
  Then comes the next bit, etc.  Finally, a parity bit may be sent and
  then a -12 V (mark) stop bit.  The line remains at -12 V (idle) until
  the next start bit.  Note that there is no return to 0 volts and thus
  there is no simple way (except by a synchronizing signal) to tell
  where one bit ends and the next one begins for the case where 2
  consecutive bits are the same polarity (both zero or both one).

  A 2nd stop bit would also be -12 V, just the same as the first stop
  bit.  Since there is no signal to mark the boundaries between these
  bits, the only effect of the 2nd stop bit is that the line must remain
  at -12 V idle twice as long.  The receiver has no way of detecting the
  difference between a 2nd stop bit and a longer idle time between
  bytes.  Thus communications works OK if one end uses one stop bit and
  the other end uses 2 stop bits, but using only one stop bit is
  obviously faster.  In rare cases 1 1/2 stop bits are used.  This means
  that the line is kept at -12 V for 1 1/2 time periods (like a stop bit
  50% wider than normal).


  21.3.  Parity Explained

  Characters are normally transmitted with either 7 or 8 bits (of data).
  An additional parity bit may (or may not) be appended to this
  resulting in a byte length of 7, 8 or 9 bits.  Some terminal emulators
  and older terminals do not allow 9 bits.  Some prohibit 9 bits if 2
  stop bits are used (since this would make the total number of bits too
  large: 12 bits total).

  The parity may be set to odd, even or none (mark and space parity may
  be options on some terminals).  With odd parity, the parity bit is
  selected so that the number of 1-bits in a byte, including the parity
  bit, is odd.  If a such a byte gets corrupted by a bit being flipped,
  the result is an illegal byte of even parity.  This error will be
  detected and if it's an incoming byte to the terminal an error-
  character symbol will appear on the screen.  Even parity works in a
  similar manner with all legal bytes (including the parity bit) having
  an even number of 1-bits.  During set-up, the number of bits per
  character usually means only the number of data bits per byte (7 for
  true ASCII and 8 for various ISO character sets).

  A "mark" is a 1-bit (or logic 1) and a "space" is a 0-bit (or logic
  0).  For mark parity, the parity bit is always a one-bit.  For space
  parity it's always a zero-bit.  Mark or space parity only wastes
  bandwidth and should be avoided when feasible.  "No parity" means that
  no parity bit is added.   For terminals that don't permit 9 bit bytes,
  "no parity" must be selected when using 8 bit character sets since
  there is no room for a parity bit.


  21.4.  Forming a Byte (Framing)

  In serial transmission of bytes via EIA-232 ports, the low-order bit
  is always sent first.  Serial ports on PC's use asynchronous
  communication where there is a start bit and a stop bit to mark the
  beginning and end of a byte.  This is called framing and the framed
  byte is sometimes called a frame.  As a result a total of 9, 10, or 11
  bits are sent per byte with 10 being the most common.   8-N-1 means 8
  data bits, No parity, 1 stop bit.  This adds up to 10 bits total when
  one counts the start bit.  One stop bit is almost universally used.
  At 110 bits/sec (and sometimes at 300 bits/sec) 2 stop bits were once
  used but today the 2nd stop bit is used only in very unusual
  situations (or by mistake since it seemingly still works OK that way).



  21.5.  Limitations of EIA-232

  21.5.1.  Low Speed & Short Distance

  The conventional EIA-232 serial port is inherently low speed and is
  severely limited in distance.  Ads often read "high speed" but it can
  only work at high speed over very short distances such as to a modem
  located right next to the computer.  All of the wires use a common
  ground return so that twisted-pair technology (needed for high speeds)
  can't be used without additional hardware.  However some computers
  have more modern interfaces.  See ``Successors to EIA-232''.

  It is somewhat tragic that the RS-232 standard from 1969 did not use
  twisted pair technology which could operate about a hundred times
  faster.  Twisted pairs have been used in telephone cables since the
  late 1800's.  In 1888 (over 110 years ago) the "Cable Conference"
  reported its support of twisted-pair (for telephone systems) and
  pointed out its advantages.  But over 80 years after this approval by
  the "Cable Conference", RS-232 failed to utilize it.   Since RS-232
  was originally designed for connecting a terminal to a low speed modem
  located nearby, the need for high speed and longer distance
  transmission was apparently not recognized.


  21.5.2.  Successors to EIA-232

  A number of EIA standards have been established for higher speeds and
  longer distances using twisted-pair (balanced) technology.  Balanced
  transmission can sometimes be a hundred times faster than unbalanced
  EIA-232.  For a given speed, the distance (maximum cable length) may
  be many times longer with twisted pair.  But PC-s keep being made with
  the "obsolete" EIA-232 since it works OK with modems connected to slow
  telephone lines, and it works OK with mice.

  One exception is Apple's Mac computer with its EIA-232/EIA-422 GeoPort
  which provides twisted-pairs (balanced) for transmit and receive.  It
  uses a small round "mini-DIN" connector.  It also provides
  conventional EIA-232 but only at 5 volts (which is still legal
  EIA-232).  However, due to the fact that Macs cost more than PC's,
  they are seldom used as a host computer for terminals.  Some newer
  terminals use EIA-423 but this is just like the unbalanced EIA-232 and
  can be connected to a EIA-232 port.  This EIA-423 is only 5 volts, but
  the specs call for higher speeds than the EIA-232 (which will be of no
  help on a long run where it's the unbalance that causes interference).

  EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s
  (balanced) was intended to be a replacement for EIA-232 but few have
  been installed.  It uses the same 25-pin connector as EIA-232.  The
  High Speed Serial Interface ( HSSI = EIA-612/613) uses a 50-pin
  connector and goes up to about 50 Mbits/s but the distance is limited
  to only several meters.  The Universal Serial Bus (USB) is being built
  into PCI chips.  It is 12 Mbits/s over a twisted pair with a 4-pin
  connector (2 wires are power supply) but it also is limited to short
  distances of at most 5 meters (depends on configuration).


  21.5.3.  Line Drivers

  For a text terminal, the EIA-232 speeds are fast enough but the usable
  cable length is often too short.  Balanced technology could fix this.
  The common method of obtaining balanced communication with a text
  terminal is to install 2@ line drivers in the serial line to convert
  unbalanced to balanced (and conversely).  They are a specialty item
  and are expensive if purchased new.


  21.6.  Synchronization & Synchronous

  21.6.1.  How "Asynchronous" is Synchronized

  Per EIA-232 there are only two states of the transmit (or receive)
  wire: mark (-12 V) or space (+12 V).  There is no state of 0 V.  Thus
  a sequence of 1-bits is transmitted by just a steady -12 V with no
  markers of any kind between bits.  For the receiver to detect
  individual bits it must always have a clock signal which is in
  synchronization with the transmitter clock.  Such clocks generate a
  "tick" in synchronization with each transmitted (or received) bit.

  For asynchronous transmission, synchronization is achieved by framing
  each byte with a start bit and a stop bit (done by hardware).  The
  receiver listens on the line for a start bit and when it detects one
  it starts its clock ticking.  It uses this clock tick to time the
  reading of the next 7, 8 or 9 bits.  (It actually is a little more
  complex than this since several samples of a bit are often taken and
  this requires additional timing ticks.)  Then the stop bit is read,
  the clock stops and the receiver waits for the next start bit.  Thus
  async is actually synchronized during the reception of a single byte
  but there is no synchronization between one byte and the next byte.


  21.6.2.  Defining Asynchronous vs Synchronous

  Asynchronous (async) means "not synchronous".  In practice, an async
  signal is what the async serial port sends and receives which is a
  stream of bytes each delimited by a start and stop bit.  Synchronous
  (sync) is most everything else.  But this doesn't explain the basic
  concepts.

  In theory, synchronous means that bytes are sent out at a constant
  rate one after another in step with a clock signal tick.  There is
  often a separate wire or channel for sending the clock signal.
  Asynchronous bytes may be sent out erratically with various time
  intervals between bytes (like someone typing characters at a
  keyboard).

  There are borderline situations that need to be classified as either
  sync or async.  The async serial port often sends out bytes in a
  steady stream which would make this a synchronous case but since they
  still have the start/stop bits (which makes it possible to send them
  out erratically) its called async.  Another case is where data bytes
  (without any start-stop bits) are put into packets with possible
  erratic spacing between one packet and the next.  This is called sync
  since the bytes within each packet must be transmitted synchronously.


  21.6.3.  Synchronous Communication

  Did you ever wonder what all the unused pins are for on a 25-pin
  connector for the serial port?  Most of them are for use in
  synchronous communication which is seldom implemented on PC's.  There
  are pins for sync timing signals as well as for a sync reverse
  channel.  The EIA-232 spec provides for both sync and async but PC's
  use a UART (Universal Asynchronous Receiver/Transmitter) chip such as
  a 16450, 16550A, or 16650 and can't deal with sync.  For sync one
  needs a USART chip or the equivalent where the "S" stands for
  Synchronous.  Since sync is a niche market, a sync serial port is
  likely to be quite expensive.

  Besides the sync part of the EIA-232, there are various other EIA
  synchronous standards.  For EIA-232, 3 pins of the connector are
  reserved for clock (or timing) signals.  Sometimes it's a modem's task
  to generate some timing signals making it impossible to use
  synchronous communications without a synchronous modem (or without a
  device called a "synchronous modem eliminator" which provides the
  timing signals).

  Although few serial ports are sync, synchronous communication does
  often take place over telephone lines using modems which use V.42
  error correction.  This strips off the start/stop bits and puts the
  date bytes in packets resulting in synchronous operation over the
  phone line.


  21.7.  Block Mode

  21.7.1.  Intro to Block Mode

  Block mode is seldom used with Linux.  In block mode when one types at
  a terminal, the results are saved in the terminal memory and are not
  sent just yet to the host computer.  Such terminals often have built-
  in editing capabilities.  When the user presses certain keys (such as
  the send key) what has been saved in the terminal memory is sent to
  the host computer.  Now the Linux editors vi and emacs, react
  instantly to pressing certain keys but in the above situation such
  keys will be pressed and nothing will happen since nothing is sent
  when a key is pressed.  Thus using a block mode terminal will not
  allow the use of such interactive programs.  The old IBM mainframe
  interface uses block mode (see ``IBM Terminals '' so many IBM
  terminals are block-mode only and also synchronous (see Section
  ``Synchronization & Synchronous'').


  21.7.2.  Types of Block Modes, Forms

  Block mode may itself have various sub-modes such as "page" (a page at
  a time) and "line" (a line at a time).  Some terminals have both block
  transmission modes and conventional character modes and may be
  switched from one mode to another.  Async terminals which have block
  modes include HP2622A, VT130, VT131, VT330, VT340, and Visual500.
  Many later model terminals can emulate block mode.  Block modes may
  include a forms capability where the host computer sends a form to the
  terminal.  Then the user fills it out and hits the send key which
  sends only the data in the form back to the host computer.  The form
  itself (not the data) is displayed on the screen in protected fields
  which don't get transmitted to the host.


  21.7.3.  Efficiency

  Block mode takes a great deal of load off the host computer,
  especially if the host computer's hardware is designed for block modes
  (as IBM mainframes were).  In character mode every character typed is
  sent immediately to the serial port and usually causes an interrupt at
  the host computer.  The host that receives the byte must stop whatever
  it is doing and fetch that character from the port hardware.  Even
  with UART's that have FIFO hardware buffers, the hardware timeout is
  normally only the transmission time of 3 bytes so that an interrupt is
  usually issued for every character typed.

  In true block mode a long block of characters is received using only
  one interrupt.  If block mode is used with conventional async FIFO
  serial ports, an interrupt is needed only every 14 bytes since they
  have 16-byte hardware buffers.  Thus much of the load and overhead of
  interrupt handling is eliminated and the computer has more time to do
  other tasks when block mode is used.

  A significant savings for block mode occurs if the terminal is
  connected to its host via a network.  Without block mode, every
  character (byte) typed is sent in its own packet including all the
  overhead bytes (40 in a TCP/IP packet as used on the Internet).  With
  block mode, a large number of characters are sent in a single packet.


  21.8.  EIA-232 (RS-232) Books

  (Note: The first book covers much more than just EIA-232.)

  o  Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
     Computer Society Press, Los Alamitos, CA, 1996.

  o  Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.

  o  Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.

  o  Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall, 1991.


  21.9.  Serial Software

  See Serial Software <ftp://sunsite.unc.edu/pub/Linux/system/serial/>
  for Linux software for the serial ports including getty and port
  monitors.


  22.  Appendix D: Notes by Brand Name

  Here are notes by brand name that were too specific to a certain
  terminal to be put elsewhere in this HOWTO.  If you have some info to
  contribute on a certain terminal that is not covered elsewhere, it
  could go here.  Various models and brands often have much in common
  which only need be written about in one place.  It would be nice to
  have for each terminal model, a large set of links linking to the
  documentation relevant to that model (including escape codes).  There
  are so many models of terminals that such a task would be quite
  onerous and I, David Lawyer (as of 1998), have no intention of
  attempting this.  If terminal manufacturers would only make their
  manuals available on the net, then all this might not be needed.


  22.1.  CIT

  CIT terminals were made in Japan in the 1980's for CIE Terminals.
  They ceased to be imported in the late 1980's.  The company, CIE,
  still makes CItoh printers (in 1997) but has no parts for its
  abandoned terminals.  Ernie at (714) 453-9555 in Irvine CA sells (in
  1997) some parts for models 224, 326, etc. but has nothing for the 80
  and 101.  (The document you are now reading was written mostly on the
  101e.)

  To save the Set-Up parameters press ^S when in Set-Up mode.  cit80:
  Contrast: knob on rear of terminal, cit101e: Brightness: use up/down
  arrow keys in Set-Up mode.


  22.2.  IBM Terminals

  Don't confuse IBM terminals with IBM PC monitors.  Many IBM terminals
  don't use ASCII but instead use an 8-bit EBCDIC code.  It's claimed
  that in EBCDIC the bit order of transmission is reversed from normal
  with the high-order bit going first.  The IBM mainframe communication
  standards are a type of synchronous communication in block mode (sends
  large packets of characters).  Two standards are "BISYNC" and "SNA"
  (which includes networking standards).  Many of their terminals
  connect with coax cable (RG62A/U) and naive persons may think the
  "BNC" connecter on the terminal is for ethernet (but it's not).

  While this IBM system is actually  more efficient than what is
  normally used in Linux, terminals meeting this IBM standard will not
  currently work with Linux.  However, some IBM terminals are
  asynchronous ASCII terminals and should work with Linux on PC's.  The
  numbers 31xx may work with the exception that 317x and 319x are not
  ASCII terminals.  Before getting an IBM terminal, make sure there is a
  termcap (terminfo) for it.  If their isn't, it likely will not work
  with Linux.  Even if there is a terminfo, it may not work.  For
  example, there is a termcap for 327x but the 3270 is an EBCDIC
  synchronous terminal.

  The 3270 series includes the 3278 (late 1970's), 3279 with color and
  graphics, and the 3274 terminal controller (something like the 3174).
  They may be used for both BISYNC and SNA.  The 3290 has a split screen
  (splits into quarters).

  The synchronous IBM terminals don't connect directly to the IBM
  mainframe, but connect to a "terminal controller" (sometimes called
  "cluster controller" or "communication controller").  Some of these
  controllers can convert a synchronous signal to asynchronous so that
  in this case a synchronous terminal could indirectly connect to a
  Unix-like host computer via its serial port.  But there is still a
  major problem and that is block transmission.  See section ``Block
  Mode''.


  22.2.1.  IBM 3153

  It's claimed that the Aux port is DCE and uses a straight-thru cable.


  22.3.  Teletypes

  These are antiques and represent the oldest terminals.  They are like
  remotely controlled typewriters but are large and noisy.  Made by the
  Teletype Corp., the first models were made in the 1920's and predate
  the computer by over 30 years.  Early models used electro-mechanical
  relays and rotating distributors instead of electronics.  Their Baudot
  code was only 5-bits per character as compared to 7-bit ASCII.  See
  the book "Small Computer Systems Handbook" by Sol Libes, Hayden Books,
  1978: pp. 138-141 ("Teletypes").


  22.4.  VT (DEC)

  Digital Equipment Corporation made the famous VT series of terminals
  including the commonly emulated VT100.  In 1995 they sold their
  terminal business to SunRiver which is now named Boundless
  Technologies.  More information is available at Shuford's Website
  <http://www.cs.utk.edu/~shuford/terminal_index.html> Information on
  current products is available from the Boundless website.  See
  ``Terminal Info on the Internet''.

  VT220: Some have a BNC connector for video output (not for input).
  Sometimes people erroneously think this is for an ethernet connection.

  VT520: Supports full DTR/DSR flow control.


  22.5.  Wyse

  Wyse has some FAQ's for terminal numbers under 100 (such as WY60).
  See  <http://www.wyse.com/service/faq/wysetter.htm> For the specs on
  more recent terminals see See  <http://www.wyse.com/terminal/>.
  22.5.1.  Wyse 60

  Display adjustments (must remove cover): Brightness VR202, Height
  VR302, Width VR101 (also affects height).


  22.5.2.  Wyse 99-GT

  Here is the setup Menus of the Wyse99GT (late 1980's).  Note that TERM
  means "termination" (character) and not "terminal".


    WYSE 99-GT Terminal Set-Up as used at the University of CA, Irvine
                  by David Lawyer, April 1990

                          F1 DISP:
  COLUMNS=80              LINES=24                CELL SIZE=10 X 13
  STATUS LINE=STANDARD    BACKGROUND=DARK         SCROLL SPEED=JUMP
  SCREEN SAVER=OFF        CURSOR=BLINK BLOCK      DISPLAY CURSOR=ON
  ATTRIBUTE=CHAR          END OF LINE WRAP=ON     AUTO SCROLL=ON
  ----------------------------------------------------------------------------
                          F2  GENERAL:
  PERSONALITY=VT 100      ENHANCE=ON              FONT LOAD=OFF
  COMM MODE=FULL DUPLEX   RCVD CR=CR              SEND ACK=ON
  RESTORE TABS=ON         ANSWERBACK MODE=OFF     ANSWERBACK CONCEAL=OFF
  WIDTH CHANGE CLEAR=OFF  MONITOR=OFF             TEST=OFF
  ----------------------------------------------------------------------------
                          F3 KEYBRD:
  KEYCLICK=OFF            KEYLOCK=CAPS            KEY REPEAT=ON
  RETURN=CR               ENTER=CR                FUNCT KEY=HOLD
  XMT LIMIT=NONE          FKEY XMT LIMIT=NONE     BREAK=170MS
  LANGUAGE=US             MARGIN BELL=OFF         PRINTER RCV=OFF
  ----------------------------------------------------------------------------
                          F4 COMM:
  DATA/PRINTER=AUX/MODEM    MDM RCV BAUD RATE=9600  MDM XMT BAUD RATE=9600
  MDM DATA/STOP BITS=8/1    MDM RCV HNDSHAKE=NONE   MDM XMT HNDSHAKE=NONE
  MDM PARITY=NONE           AUX BAUD RATE=9600      AUX DATA/STOP BITS=8/1
  AUX RCV HNDSHAKE=NONE     AUX XMT HNDSHAKE=NONE   AUX PARITY=NONE
  (There is a main port (Modem=MDM) and an Auxiliary Port (AUX)
  ----------------------------------------------------------------------------
                          F5 MISC 1:
  WARNING BELL=ON         FKEY LOCK=OFF           FEATURE LOCK=ON
  KEYPAD=NUMERIC          DEL=DEL/CAN             XFER TERM=EOS
  CURSOR KEYS=NORMAL      MARGIN CTRL=0           DEL FOR LOW Y=ON
  GIN TERM=CR             CHAR MODE=MULTINATIONAL
  ----------------------------------------------------------------------------
                          F6 MISC 2:
  LOCAL=OFF               SEND=ALL                PRINT=NATIONAL
  PORT=EIA DATA           SEND AREA=SCREEN        PRINT AREA=SCREEN
  DISCONNECT=60 MSEC      SEND TERM=NONE          PRINT TERM=NONE
  PRINT MODE=NORMAL       VT100 ID=VT100          POUND=US
  ----------------------------------------------------------------------------
  F7 TABS: You should see several "T" characters spaced 8 dots apart.
      If you don't, hit backspace.
  F8 F/KEYS: Normally you will see no definitions for the Function Keys
      here (unless someone has set them up and saved them).  This means that
      they will normally generate their default settings (not displayed here).
      <ctrl><F5> shows the "user defined definition" of the F5 key, etc.
  F9 A/BACK: Normally not defined: ANSWERBACK =
  F10 EXIT: Selecting "DEFAULT ALL" will make the factory default settings
      the default.



  HINTS on use of WY-99GT User's Guide: Note that much that is missing
  from this Guide may be found in the WY-99GT Programmer's Guide.  The
  VT100 emulation (personality) is known as ANSI and uses ANSI key codes
  per p. A-10+ even though the keyboard may be ASCII.  A sub-heading on
  p. A-13 "ASCII Keyboard" also pertains to VT100 because it has an
  "ANSI KEY ..." super-heading a few pages previously.  But not all
  ASCII keyboard headings pertain to VT100 since they may fall under a
  non-ANSI personality super-heading which may found be a few pages
  previously.  Appendix H is the "ANSI Command Guide" except for the
  VT52 (ANSI) personality which is found in Appendix G.


  22.5.3.  Wyse 150

  When exiting set-up using F12, hitting space changes "no" to "yes" to
  save the set-up.  The sentence to the left of this no/yes is about
  "vertical alignment" and has nothing to do with this no/yes for saving
  the set-up (confusing menu design).

  END OF Text-Terminal-HOWTO
















































