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]