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From: stepp@uiuccsb.UUCP
Newsgroups: net.micro
Subject: Re: 1200 baud modem problems - (nf)
Message-ID: <4957@uiucdcs.UUCP>
Date: Wed, 18-Jan-84 22:31:53 EST
Article-I.D.: uiucdcs.4957
Posted: Wed Jan 18 22:31:53 1984
Date-Received: Sat, 21-Jan-84 06:54:56 EST
Lines: 64

#R:sri-arpa:-1540200:uiuccsb:4400035:000:3621
uiuccsb!stepp    Jan 18 09:26:00 1984

First, what is a 'break character'?

The 'break character' is a character only in the sense that there is usually
a key on the keyboard for that function.  Like a regular character, break
is a sequence of changes on the communication link of finite duration.
Unlike a character, the break pattern on the link has no parity and is
of an incorrect (too many) number of bits.  Each regular character is sent
as a pattern of (10 or 11) bits, each character composed of exactly the
same number and with leading and trailling bits called 'start' and 'stop',
respectively.  The serial line idles in the 'mark' state (1 state, dating
from the time when telegraphs were connected together in a series loop circuit,
data was sent by interrupting the current briefly) with start and stop
bits always being in the 'space' state (0 state).  Note, on RS232 lines
'mark' is a negative voltage, 'space' is a positive voltage.  One or more
mark bits separate characters.

A break is just that, an opening of the connection (a la telegraphs) for
a long time.  On modern serial data links, the break is a sequence of
space bits much longer that any possible sequence that could be data
surrounded by start and stop bits with mark bits between characters.
UARTS can detect break because they get a 'framing error', that is when
they expect to see a stop bit (ending a character) followed by one or
more marks between characters, they see instead lots of spaces (the break).

On modems that encode one bit as one baud, everything is simple.  Regular
characters go in at the transmitting modem and come out from the receiving
modem bit-by-bit with no interpretation of the bits by either modem.
Break goes in as lots of space bits and comes out the same way.  The speed
of the bits going in equals the speed of the bits coming out, the modem
does not have to know what that speed is as long as its not too fast
(e.g., less than 300 bps, though 600bps is possible).

The 212A style of modem is much different!

The 212A (1200bps full duplex) is not a baud-for-bit modem.  The modem
takes in 2 bits and outputs (on the phone line) one baud.  It can send
four different 'bauds' encoded by phase (PSK).  To decode PSK the receiving
modem must be in constant synchronization with the transmitting one.
The asynchronous data is actually sent synchronously between modems.
It comes out looking like asyncoronous data, but thats just to fool you.

Now consider the break signal.  All is not so simple at all.  Break is
handled specially on such modems and it is possible for a modem to get lost
handling the break, basically because the modem may lose track of correct
timing if it is not careful.  (Note:  on the 212A the data must be presented
to the modem at exactly 1200 bps (+- 1%) for the conversion from asyn. to
syn. transmission to work; the modem fills in garbage data during the
varying time periods between characters which the receiving modem swallows,
but the timing WITHIN A CHARACTER must be exact).

Thus to send break over a 1200 bps line the modems at each end must be
capable of understanding that a sequence of spaces coming in, then being
synchronously transmitted is a break and the receiving modem must then
temporally violate its idea of well-formed-character and hold the outgoing
serial line at the space state 'too long', all without losing synchronization
with the modem at the other end or failing to properly recover the next
real character.

Good 212A modems should do this trick with ease.  I suppose it would be
possible to have a modem that thinks break is something to swallow.  I
would tend to avoid such a modem.