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From: smh@mit-eddie.UUCP (Steven M. Haflich)
Newsgroups: net.micro
Subject: Re: 1200 baud modem problems with break
Message-ID: <1201@mit-eddie.UUCP>
Date: Wed, 18-Jan-84 08:37:14 EST
Article-I.D.: mit-eddi.1201
Posted: Wed Jan 18 08:37:14 1984
Date-Received: Thu, 19-Jan-84 02:22:16 EST
References: <15373@sri-arpa.UUCP> <778@qubix.UUCP>
Organization: MIT, Cambridge, MA
Lines: 37

	I do not understand what the modem has to do with the ability or
	inability to send a break.  A break, as its name implies, is a long
	pulse (at least .25 seconds) on the data line.  It looks the same as a
	momentary physical break in the line.

	As long as your computer or terminal can generate a break there
	shouldn't be any problem in it being passed through the modem.

True, one imagines that a modem simply and instantaneously translates
the state of the RS232 Transmitted Data line (a digital signal) to an
frequency-encoded (analog) audio signal quite independantly of the
frequency of bit transitions, and any modem which does this ought to be
able to send BREAK.  Many 300 baud modems indeed work this way.  Notice
that specs for 300 baud modems usually specify baudrate as 0-300 baud.

Getting 1200 baud down a voice-grade line is entirely another matter.
Very sophisticated filtering and phase-manipulation techniques are
required both sending and receiving -- those messy analog circuits are
why 1200 baud modems have been so expensive.  Basically, the modem
transmitter is a filter which converts Transmitted Data into control
voltage(s) for an oscillator.  This filter is very frequency dependant
and is tuned to transitions at 1200 baud.  (Ever notice that the
baudrate specification for dual-speed modems is often given as "0-300,
1200 baud"?  The Racal Vadic 3451 specs in front of me give:  103 mode:
0-300 baud; 212 mode: 1182-1212 baud.)  Anyway, this filter can do
unanticipated things when presented with a non-1200-baud signal.
Clearly a modem must operate reasonably for arbitrarily long MARKs (idle
state) but it is entirely possible that this filter might not pass a
long SPACE -- which, as has been pointed out, translates into the BREAK
"character".

The filtering on most modems do indeed seem able to translate MARKS.
Although I am not familiar with the modems in question, I suspect the
nature of the filtering used to achieve low cost may be the reason for
inability to send BREAK.

Steve Haflich, MIT