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From: roy@phri.UUCP
Newsgroups: comp.dcom.modems
Subject: Re: Answering modems should need no commands! // 2400 answer is fragile
Message-ID: <2726@phri.UUCP>
Date: Mon, 25-May-87 15:25:06 EDT
Article-I.D.: phri.2726
Posted: Mon May 25 15:25:06 1987
Date-Received: Wed, 27-May-87 06:48:58 EDT
References: <2130@hoptoad.uucp> <913@runx.ips.oz>
Reply-To: roy@phri.UUCP (Roy Smith)
Organization: Public Health Research Inst. (NY, NY)
Lines: 47
Summary: commands vs. modem control

In article <913@runx.ips.oz> dave@runx.ips.oz (Dave Horsfall) writes:
> I get annoyed when making several expensive phone calls to remote
> machines, hearing the answer tone, then zilch.  If the machine has
> crashed or is turned off, the modem doesn't know this, and quite happily
> answers the phone for you.

	I think we're talking about two different things.  On the one hand,
we have commands -- characters sent over the RS-232 data lines which a
micro in the modem interprets.  This includes the "ATDT" kind of stuff
familiar to anybody who's used a Hayes-style modem.

	On the other hand, we have modem control signals, such as DTR (Data
Terminal Ready) and DSR (Data Set Ready).  A (properly designed) data set
won't answer an incoming call unless DTR was asserted; this is as true
today as it was 20 years ago.  The modem control signals are wired such
(i.e. with pullup/down resistors) that if the Data Terminal (which means
whatever is connected to the modem, be it a VAX or a VT-100) is turned off
off, DTR is negated.  The end effect is that if I turn the power off the my
VAX, the modems won't answer a call, without my having to do anything
specific to disable them.

	I believe most serial boards will also negate DTR if they see a bus
reset (or init, or whatever you call it on your particular machine).  Thus,
if my VAX panics, when it tries to reboot and does a Unibus reset, all the
modem lines get disabled until /etc/init explicitly re-enables them.  If
for some reason the machine can't reboot itself, the modem won't answer
calls until somebody comes and manually fixes things up.

	It is of course possible for the system to get wedged in such a
state that DTR is left asserted even though nothing is running.  In this
situation, the modems will answer the phone and you will get nothing.  Once
carrier is established, the modems are perfectly happy to keep the line
open forever, regardless of the fact that the machines at the two ends are
not doing anything.  Fortunately, this doesn't happen often.

	You could prevent this last case by having a watch-dog timer
running on the serial line controller board -- if the CPU doesn't reset the
timer every whatever (O(1 minute) seems reasonable), the board drops DTR.
Of course, you want the option to disable the timer if you O/S can't deal
with it, and if you opt for such a feature, you certainly want it in the
serial controller, not in the modem.  Modems are already too smart for
their own good.  Auto-dialers are nice, but in some ways the good-old 801
external dialer had the upper edge.
-- 
Roy Smith, {allegra,cmcl2,philabs}!phri!roy
System Administrator, Public Health Research Institute
455 First Avenue, New York, NY 10016