Xref: utzoo comp.sys.amiga:39182 comp.os.minix:6872
Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wasatch!cs.utexas.edu!uunet!mcsun!ukc!tcdcs!csvax1.cs.tcd.ie!ecarroll
From: ecarroll@csvax1.cs.tcd.ie (Eddy Carroll)
Newsgroups: comp.sys.amiga,comp.os.minix
Subject: Re: 75/1200 baud modem in software?
Message-ID: <53150@csvax1.cs.tcd.ie>
Date: 27 Aug 89 17:36:48 GMT
References: <539@vice2utc.chalmers.se> <1052@kuling.UUCP> <431@tardis.Tymnet.COM> <431@xdos.UUCP>
Followup-To: comp.os.minix
Organization: Computer Science Department, Trinity College Dublin
Lines: 28

In article <431@xdos.UUCP>, doug@xdos.UUCP (Doug Merritt) writes:
> In article <431@tardis.Tymnet.COM> jms@tardis.Tymnet.COM (Joe Smith) writes:
>>In article <1052@kuling.UUCP> marten@kuling.UUCP (M}rten Norman) writes:
>>When you send 8 bits to an async line, the hardware sends 10 bits by
>>adding a start bit and a stop bit.  Although one byte at 75 baud takes
>>the same amount of time to transmit as 16 bytes at 1200 baud (160 bit times
>>total), you can only specify 128 of the 160 bits.  The remaining 32 bits
>>will screw up the receiver at the far end.
> 
> I would think there's a chance that they wouldn't screw it up, since
> we're basically talking about what would look like a glitch in 1/16th
> of the received waveform. Would depend on how the receiver hardware was
> designed, wouldn't it? Might still be worth a try.
> 	Doug

I posted a message a while ago in comp.sys.amiga.tech about this, but
some folks may have missed it. In fact, the method outlined above works
pretty well, and it formed the basis of a modem driver I wrote for use with
Compunet (a UK 1200/75-based service). A little tweaking of the output
signal away from what might look like the optimum can help quite a lot in
fooling the receiver that you are sending at 75 baud when you're really
sending at 1200 baud.

Anyway, I have a document I did up which explains the whole thing in more
detail, gives example C code etc. If anyone wants a copy, send me email to
the address below.

Eddy Carroll                                         ecarroll@vax1.tcd.ie