Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!pasteur!ucbvax!decwrl!sun!imagen!atari!portal!cup.portal.com!thad
From: thad@cup.portal.com
Newsgroups: comp.dcom.modems
Subject: Re: Autobaud Matching
Message-ID: <9262@cup.portal.com>
Date: 19 Sep 88 10:06:38 GMT
References: <56.015163@adam.DG.COM>
Organization: The Portal System (TM)
Lines: 36
XPortal-User-Id: 1.1001.2826

Whew!  A "loaded" question (re: autobaud)

In general, autobaud detection necessitates your system being set up at some
"high" baud and analyzing the bits that arrive when a caller types some
character (typically a carriage return).

Using a UART, you'll ALSO get framing errors (from the UART) unless the bauds
are already matched.  I developed some autobaud detection schemes by trial and
error and discovered that if the baud disparity is greater than 4x you're
gonna be SOL unless you adopt a technique as used by DEC.

DEC's scheme (as with VAX/VMS), appears to start at 19,200 baud and steps down
until a recognizable character is seen ... the serial channel's baud is then
"set" to that rate.  This may neccessitate typing 4 or 5 CRs until successful
communication is established.

My scheme (basically) matches incoming bits (ignoring any framing errs)
against data tables determined experimentally; usually a match is found
within one CR, sometimes two.

Autotraining modems (such as Hayes' (tm)) use, I believe, analog circuitry.
Not so bad, considering the expected range of bauds is 300, 1200 or 2400.

The "older" AT&T 212A modems used pin 12 (on the RS-232 connection) as a
high speed indicator.  Asserted  TRUE, the baud was (presumed) 1200; otherwise
it wa assumed 103J (0-300)  (and, note, the assertions were negative logic; but
this is clear from the modem's manuals).

As a side note, has anyone else used a MC68681 (dual UART) as an adjunct to
an MC6801/6803?  I still am tickled pink having interfaced this in a commercial
product (1000's sold) when, normally, people interface 8-bit peripherals
to, say, the MC68000 family.  Oh well!  :-)



Thad Floryan [thad@cup.portal.com (OR) ...!sun!portal!cup.portal.com!thad]