terminal -- Technical Information

This article  describes how  you can  hook up a  terminal to  your COHERENT
system via  a serial  port.  It also  discusses common problems  that arise
with this  procedure, as diagnosed daily by the  technical support staff at
Mark  Williams Company.   For  information on  connecting a  modem to  your
computer's serial port, see the article modem.

Terminals for Beginners

To a  beginner, a terminal  -- with its  many wires and controls  -- may be
daunting.  However, connecting  a terminal or PC-based terminal emulator to
a COHERENT system  is really very easy.  To make  matters even easier, this
section discusses  how to  use a simple  configuration, using only  one COM
port.

What you need:

1. A COHERENT system with a COM port.

2. Either a terminal  or a PC with a COM  port and communications software.
   An old PC or PC-AT is fine.  (You can also use a Macintosh or other such
   computer, but that is beyond the scope of this discussion.)

3. A  ``null modem  cable''  purchased from  your  nearest computer  supply
   store.  This  should cost  between $10 and  $20, in U.S.  currency.  The
   only tricky part  about the cable is making sure  that the connectors on
   the ends match  the connectors on your COM ports  (i.e., nine pin vs. 25
   pin, and male vs female.  Adaptors are also available.

What you do:

1. Connect the cable to the COM port on the COHERENT system.  Note that the
   COM port on your COHERENT system  is always a male plug (that is, it has
   pins rather  than sockets).  Do  not plug your connection  into a female
   plug, as this  is the parallel port.  If you  do so, you can damage your
   equipment.

2. Connect the cable to the COM  port on the terminal/PC.  If you are using
   a PC  as a terminal, the COM  port on it will also be  male.  If you are
   using a stand-alone terminal, the plug could be either male or female.

   If you are using a stand-alone terminal, there may be a plug on the back
   that is labelled ``AUX''.  Do not use this plug; use the other one.

3. Configure  the  terminal/PC to  use  9600 speed  (or  ``baud'') and  8N1
   character setting  (that is, eight bits, no parity,  one stop bit.  Note
   that you do not need a telephone number: you won't be dialing anywhere.

4. Log in on your COHERENT system as the superuser root. Type the following
   command:

       /etc/enable com?l

   where `?'  is the number  of the COM  port on your  COHERENT system into
   which you are  plugging the cable from the PC  or terminal Note that the
   last character is lower-case `L', not a one.

5. Now come  a tricky  part: use  your favorite text  editor and  edit file
   /etc/ttytype. This file gives the  default type of terminal that will be
   connecting to COHERENT via a given COM port.  This is important.  If you
   don't do this, such screen-oriented programs as editors and vsh will not
   work properly.

   Each entry  in /etc/ttytype has two  columns: the first gives  a type of
   terminal, and the second names the port.  The following shows an example
   entry:

       vt100   com3l

   In this  instance, COM port  com3l has a  DEC VT-100 plugged  into it by
   default.  Look for an entry for the COM port into which you plugged your
   terminal  device.  If  you can't  find  you, you  can create  one easily
   enough; however,  having two  entries for  the same port  is not  a good
   idea, as COHERENT will become confused.

   If you  are plugging in  a PC, use  the terminal type vt100.  If you are
   using a  stand-alone terminal, name the type of  terminal you are using,
   e.g., wyse370 for  a Wyse 370 color terminal.  If  you cannot figure out
   what type of  terminal you are using, use dumb.  This will not allow you
   to use  screen-oriented programs like MicroEMACS, but  at least you will
   be able to connect to COHERENT;  you can figure out the type of terminal
   later.

6. Return  to your  terminal device.   If you  are using  a PC,  invoke the
   terminal emulator, and put it into ``connect'' mode.  If you are using a
   stand-alone terminal, turn it on.  Press <return>.  After a short pause,
   you should see the prompt

       Coherent login:

   on your screen.  You can then log in and run normally.

That's all  there is to it.   In this way, you  can get more use  out of an
old, obsolete  PC.  After you get  it working, you may  need to adjust some
other settings, particularly if you are using a communications package on a
PC for a terminal.

If  you see  garbage characters  when you  log in,  probably the  speed (or
``baud rate'') is not set correctly either on the COM port of your COHERENT
system or  on your terminal device.  The Lexicon  article on ttys describes
the magic  character string used to  describe each com port  and one of the
letters is the port speed.

Problems with Terminal Hookup

As noted  above, it is easy  to hook up a  terminal.  However, problems can
arise if you overlook any details.

When problems arise, they usually  come from some form of confusion.  These
can include send/receive confusion, baud rate confusion, and shell/no shell
confusion.   The following  sections describes  eacy  type of  confusion in
turn.

Send/Receive Confusion

This most  often happens when you've soldered your  own connectors, and you
made a  mistake.  If  you purchased a  connector, as we  recommended above,
then skip to the next section.

A serial connection between your  computer and a terminal requires at least
three wires:  one each  for pins  2, 3, and  7.  These  pins, respectively,
control send (TD), receive (RD),  and signal-ground (Gnd or SG).  These pin
numbers  correspond  to  the  25-pin  ``DB-25''  connectors  used  on  most
equipment.  If  your system has the  AT-style nine-pin ``DB-9'' connectors,
you will need to wire to  the corresponding signals.  See the Lexicon entry
for RS-232 for details of the pin-outs for these two connectors.

When hooking up a terminal to  a serial port using a three-wire connection,
you must  cross pins 2 and  3, so that each device's send  pin talks to the
other device's receive pin.  You can  plug a device called a ``null modem''
between the cable and the serial port, to do this automatically.

Note  that the  only symptom  of  a problem  in the  cable is  that nothing
appears on your terminal when you type.

Baud-Rate Confusion

The terminal and  the computer must speak to each  other at the same speed,
or  ``baud rate''.   A typical  symptom of  baud-rate confusion  is garbage
characters on the screen.  When the  wiring is wrong, you see nothing; when
the baud  rate is wrong,  you see random  collections of characters  on the
screen, but nothing meaningful.

You can fix baud-rate problems by  using the command stty to reset the baud
rate on the port, or resetting  the baud rate on the terminal.  The problem
should also be  solved by editing file /etc/ttys. For  directions on how to
reset the  baud rate for  a port, see  the Lexicon entry for  stty; see the
Lexicon entry for ttys for information on how to edit /etc/ttys.

Please note,  too, that COHERENT  supports the following  configuration for
terminals:

    8 word bits
    1 stop bit
    No parity bits

These settings, as  well as the baud rate, must  match before your terminal
will work correctly.

The Old Shell Game

Before a terminal is useful to  you, you must enable the port into which it
is plugged.  Enabling a port means that the COHERENT system creates a shell
for that port: this, in turn,  means that COHERENT prints a login prompt on
the device  plugged into that  port, and reads  and processes interactively
commands  that  are  entered from  that  port.   The  COHERENT system  also
restricts  permissions  on  all enabled  serial  ports,  so  that only  the
superuser root can  read and write to the port.   This prevents other users
who may be using the system from accessing the serial port.

Note that not all ports need be enabled: for example, you should not enable
a COM port into which you've plugged a printer.

When  you boot  the COHERENT  system,  it reads  system file  /etc/ttys and
creates a shell  for each serial port that needs  one.  One way to enable a
port is to  log in as the superuser root,  then use a text editor to change
the port's entry in  /etc/ttys, as described its Lexicon article.  Finally,
typing the command

    kill quit 1

forces COHERENT  to re-read /etc/ttys and  so create a shell  for the port.
Note that doing this will ensure that the port is re-enabled every time you
boot.

A better way to enable a port is to use the command enable, as described in
its  Lexicon  article.   For  example,  to  put up  a  shell  on  COM  port
/dev/com1r, log in as the superuser root and type the command:

    /etc/enable com1r

Exiting Raw Mode

A terminal  is in cooked mode.   In cooked mode, the  tty driver interprets
and  correctly  processes such  predefined  characters  as the  end-of-file
character or the quit character.   In raw mode, however, processing of such
characters  is  turned  off;  and  in  general  the  terminal  will  behave
bizarrely.  Raw mode is used by programs that do not want the tty driver to
interpret characters; for example, a program  that uses a tty to transmit a
binary to another  machine does not want the tty  driver to be interpreting
the binary information being passed through it.

Occasionally, a  program will exit  abruptly and leave the  terminal in raw
mode.  To  return to cooked mode, use the  command <ctrl-J> stty sane
<ctrl-J>. This  invokes the command  stty, which lets  you manipulate
terminal settings,  to restore the previous cooked  state.  See the Lexicon
entry  on stty  for details  on  raw and  cooked modes;  this article  also
describes the options of this most useful command.

See Also

Administering COHERENT,
asy,
device drivers,
hs,
modem,
RS-232,
sgtty,
stty,
termcap,
terminfo,
termio,
termios,
ttys

Notes

One final  bit of hard-won  wisdom: once you have  something working, write
down what you  did, and store it in a  place where you won't lose it.  Note
especially  what  connectors  are  where  and  how they  have  been  cabled
together.  It  makes life easier  just knowing that  you are looking  for a
female-to-female cable  instead of male-to-female or  male-to-male.  If you
know whether to insert a null modem, you are even better off.

COHERENT supports multi-port serial cards as well as COM ports 1 through 4.
See  the Lexicon  entry on  asy  for a  list of  the devices  that COHERENT
supports, and for details.

Thanks  to   Dave  Hough  (tecdah1@sdc.cs.boeing.com),   whose  posting  to
comp.os.coherent is the basis of the ``Beginners'' section, above.

© 2012 by Stephen A. Ness