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From: guy%gorodish@Sun.COM (Guy Harris)
Newsgroups: comp.unix.questions
Subject: Re: Async terminal as console on a Sun
Message-ID: <26419@sun.uucp>
Date: Mon, 24-Aug-87 18:31:30 EDT
Article-I.D.: sun.26419
Posted: Mon Aug 24 18:31:30 1987
Date-Received: Wed, 26-Aug-87 00:59:44 EDT
References: <306@pvab.UUCP>
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> I'm about to use an asynchronus terminal as console to a Sun-2/130.
> I 've heard that this dumb machine will ehter the prom monitor if I'll
> ever try to unplug the console, plug out its power cord or if I press
> the Break button. Is there any way to circumvent this strange behaviour?

There is nothing intrinsically strange about this behavior.  Modern machines
tend to use a terminal, rather than a bunch of buttons and lights, as a control
panel.  Such machines often tend to "double up", using the same terminal for
the system console ("/dev/console" for UNIX) and for the control panel.  This
is often done by having some magic key or keys that switch the terminal between
the two modes.

Using a control character can be a nuisance if this behavior cannot be
disabled; the character tends to be ^P on DEC systems, which would probably
annoy people whose systems have a CRT as their console and who use EMACS on
that console.  (One Sun I use has a CRT as its console and yes, I do use EMACS
on it.)

Using a "break" as the signal in question has some advantages; the BREAK key
less likely to be typed as part of normal use than might a control key, and
it's easier to produce if your terminal is somehow gronked - just unplug the
terminal and plug it back in again.

If you have a "console processor", jumping to console mode may not be
noticeable, as the CPU will continue.  Suns (and many other machines) don't
have console processors, so this magic signal has to stop the machine.

If you end up going to the PROM monitor as a result of a BREAK signal, you can
type the "c" (continue) command, followed by a RETURN, and it will pick up
where it left off (in most cases, anyway; I haven't seen any cases where it
couldn't do this and it wasn't already wedged, but they may exist).

Perhaps there should have been a "disable" switch; I don't know whether this
was not considered or was considered and rejected (for e.g. cost reasons).

> Are there any pins on the ttya port (the one I use as console port for
> my async terminal) I can fool to make the machine think the console
> terminal is connected all the time?

The pin on the tty port that it uses for break detection is, of course, the RxD
pin; you probably don't want to fool that pin....
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com