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From: darrelj@exec.sm.unisys.com (Darrel VanBuer)
Newsgroups: comp.misc,comp.sys.ibm.pc
Subject: Re: TDD protocol on PC?
Message-ID: <5604@sdcrdcf.sm.unisys.com>
Date: 8 Jan 89 06:33:58 GMT
References: <210@imspw6.UUCP> <109@csd4.milw.wisc.edu> <77200@felix.UUCP> <2700@rti.UUCP> <4521@cs.utexas.edu> <6033@mailgw.cc.umich.edu>
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Reply-To: darrelj@sm.unisys.com (Darrel VanBuer)
Organization: Unisys Santa Monica
Lines: 29

TDDs differ from more common computer communications as follows:
Baudot instead of ASCII (handle in software with a couple of tables
and shift state flags) 

45 baud (60 WPM in the Baudot 5 level code) instead of 110, 300, 1200,
2400 ...  (IBM PC serial I/O chip can be programmed for this, but not
via the BIOS)

5 data bits instead of 7 or 8, no parity (IBM PC serial chip can do it)

1.5 stop bits instead of 1 or 2 (any serial I/O chip which does 5 data
bits usually provides 1.5 stop capability, even those that don't will
interoperate by programming 2 stop bits since the receiver only needs
one, and sending 2 instead of 1.5 justs costs 7% of speed)

Modem uses FSK half duplex signalling which uses frequencies
incompatible with any other modem (something like 1400 Hz mark, 1800
Hz space, silent when idle).  I faked it once by using a Motorola
Modem IC, a fast crystal (to shift freq up) and a couple of inverters
to fix the data sense [it was a little off spec, but just fine with
the wide tolerance detectors used at such a slow data rate].
The deaf community is beginning to use dual standard machines (old
plus ASCII/Bell 103), but large numbers of existing equipment in the
hands of many who cannot afford new equipment make this a very slow
process.  (And the need to change is low for them since even 60 WPM is
faster than most people can type).
Darrel J. Van Buer, PhD; unisys; 2400 Colorado Ave; Santa Monica, CA 90406
(213)829-7511 x5449        KI6VY        darrel@CAM.UNISYS.COM   or
..{allegra,burdvax,cbosgd,hplabs,ihnp4}!sdcrdcf!darrelj