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terminal              Technical Information              terminal




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 mmooddeemm.

***** Hooking Up a Terminal to COHERENT *****

This  process is  straightforward, but  can  be confusing  if you
overlook  any  details.   Typical problems  include  send/receive
confusion, baud rate confusion, and shell/no shell confusion.

***** Send/Receive Confusion *****

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 RRSS-223322 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.  Unless someone has sat down and
taught  you how  to solder  connectors, we  strongly urge  you to
purchase  the  necessary  cable  and  null  modem at  your  local
computer store or electronics shop.

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  _b_a_u_d _r_a_t_e.  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 something, but
nothing meaningful.  You  can fix baud-rate problems by using the
command ssttttyy to reset the baud rate on the port, or resetting the
baud rate  on the terminal.   For directions on how  to reset the
baud rate for a port, see the Lexicon entry for ssttttyy.

***** The Old Shell Game *****

Before a terminal is useful to you, you must _e_n_a_b_l_e 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


COHERENT Lexicon                                           Page 1




terminal              Technical Information              terminal



entered  from  that port.   The  COHERENT  system also  restricts
permissions  on  all  enabled  serial  ports,  so that  only  the
superuser rroooott  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:  printer ports,  for
example, should  not be enabled;  nor should you  enable any port
whose device you want to accept data passively.

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


        kill quit 1


forces COHERENT  to re-read /eettcc/ttttyyss  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 eennaabbllee, as
described in its Lexicon article.  For example, to put up a shell
on COM port /ddeevv/ccoomm11rr, log in as the superuser rroooott and type the
command:


        /etc/enable com1r


***** See Also *****

mmooddeemm, RRSS-223322, ssttttyy, tteecchhnniiccaall iinnffoorrmmaattiioonn, tteerrmmccaapp, tteerrmmiioo, ttttyyss

***** 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 Lexicon                                           Page 2

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