Xref: utzoo comp.os.minix:6606 comp.sys.amiga:37559 comp.sys.amiga.tech:6377
Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!husc6!bloom-beacon!usc!elroy.jpl.nasa.gov!ucla-cs!uci-ics!zardoz!tgate!ka3ovk!drilex!axiom!linus!gomez!gwr
From: gwr@gomez.uucp (Gordon W. Ross)
Newsgroups: comp.os.minix,comp.sys.amiga,comp.sys.amiga.tech
Subject: Re: 75/1200 baud modem in software (Yes!)
Keywords: modem break signals
Message-ID: <60796@linus.UUCP>
Date: 26 Jul 89 15:28:26 GMT
References: <539@vice2utc.chalmers.se>
Sender: news@linus.UUCP
Reply-To: gwr@gomez.UUCP (Gordon W. Ross)
Organization: The MITRE Corporation, Bedford, MA.
Lines: 36

In article <539@vice2utc.chalmers.se> d5kwedb@hacke9.dtek.chalmers.se
(Kristian Wedberg) writes:

>I've written a Videotex program (Prestel type) for the Amiga, a kind of
>communication program. Trouble is, here in Sweden some folks have 75/1200
>modems instead of the more normal 1200/1200.
>[...] is it possible to transmit 75 bit/s and receive 1200 bit/s over the
>serial port? At the same time? Or would you need some kind of transformer?
>	wonders kitte			d5kwedb@dtek.chalmers.se

Yes, it should be possible to transmit at a low speed using a
serial port capable only of matched transmit and receive rates.

The best way to do it is probably to write a special driver which
generates each bit of the 75 bps data under software control.
This is made relatively easy if the UART (Universal Asynchronous
Receiver/Transmitter) can be programmed to generate a "break" signal
of any desired length or to generate a "continuous break" until
software tells it otherwise.  Most UART's can do this.

A software driver would have to handle timing for the 75 bps output
and generate a "break" of the correct length for each zero bit
to be transmitted (the start bit is also a zero bit).

If the UART cannot generate "break" signals as described above,
one could use some other, controllable output signal for the TD
(Transmitted Data) line to the modem.  A special, non-standard cable
would be required to connect (for example) DTR to the modem TD pin.

Again the driver software would have to handle the timing and
generation of each bit in the 75 bps transmitted data, and would
generate the transmitted data on the (for example) DTR line.

Are these dual-rate modems very common?
Gordon W. Ross    gwr@gomez.mitre.org    (617) 271-3205 (daytime)
The MITRE Corp. (M/S E025)  Burlington Road, Bedford, MA 01730