Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cbmvax!grr
From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.dcom.modems
Subject: Re: Minitel videotex terminals and v.23
Keywords: videotex v.23 CCITT
Message-ID: <7383@cbmvax.UUCP>
Date: 21 Jul 89 02:27:37 GMT
References: <431@siswat.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 32

In article <431@siswat.UUCP> buck@siswat.UUCP (A. Lester Buck) writes:
> 
> It seems that v.23 modems are quite expensive, around $800 and available
> only in Europe.  Since the Minitel only costs something like $70-80, this is
> something of a problem.  Assuming v.23 is basically 1200 baud in, 75 baud
> out, what are the possibilities of using standard 1200 baud modem chips?
> For example, on XT prototype cards, with some software glue for the startup
> protocol?  As a last resort, wouldn't a software only solution work,
> as with the Commodore software serial port support?

I think you'll find that the V.23 standard is much simpler than that
used in the 212A style 1200 baud modems.  It's a FSK half duplex
scheme with a 75 baud reverse channel.  This sort of thing can be
handled by the simple modem chips used for 300 baud class modems, with
no real handshaking is needed beyond a carrier detect for the originating
modem.  The closet us equivalent is the 108 series of datasets, though
the frequencies are probably a bit different.

Something like the AMD7911 "World Chip" FSK modem chip is pretty close
the the state of the art "one chip does all" for this sort of thing,
though you'll find plenty of alternatives needing more or less external
analog circuitry.

You also need some sort of serial interface capable of doing split
transmit / recieve speeds, though a kludge of using two serial
interfaces or using the uart for the receive end and bit bashing the 75
baud would be possible with a bit of cleverness.

-- 
George Robbins - now working for,	uucp: {uunet|pyramid|rutgers}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@uunet.uu.net
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)