Xref: utzoo comp.misc:4692 comp.sys.ibm.pc:23171
Path: utzoo!attcan!uunet!lll-winken!ames!mailrus!cornell!uw-beaver!tikal!toybox!msa
From: msa@toybox.UUCP (Mark Anacker)
Newsgroups: comp.misc,comp.sys.ibm.pc
Subject: Re: TDD protocol on PC?
Summary: well, it can be done....
Message-ID: <189@toybox.UUCP>
Date: 12 Jan 89 19:34:00 GMT
References: <210@imspw6.UUCP> <109@csd4.milw.wisc.edu> <77200@felix.UUCP> <5604@sdcrdcf.sm.unisys.com>
Organization: USWest NSI, Bellevue, WA
Lines: 42

In article <5604@sdcrdcf.sm.unisys.com>, darrelj@exec.sm.unisys.com (Darrel VanBuer) writes:
: 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
    [ text deleted...]
: Darrel J. Van Buer, PhD; unisys; 2400 Colorado Ave; Santa Monica, CA 90406

Well, if you absolutely MUST make your PC talk to a TDD, it appears
that you could fairly easily modify a ham-type RTTY interface to do
it.  For instance, I have a Heathkit RTTY box which takes the audio
FSK tones from my shortware receiver and puts out RS-232.  From there,
it's all software.  I have no trouble converting the BAUDOT to ASCII
on a PC or Model 102, at a variety of speeds.  The beauty of this
particular box is that it's easy to modify the filters for whatever
mark/space tones you are using.  All you would need to couple to the
phone line is a simple isolation and gain adjust circuit, or even an
acoustic coupler (remember those?).
 
 It's a pity Heathkit doesn't sell this kit anymore, but they should
 be available from ham's who've upgraded to more sophisticated
 units.  Might be something to look into.

-- 
Mark Anacker, US West Network Systems Inc., Bellevue, WA
{...uw-beaver!tikal,...!thebes}!toybox!msa -or- msa@toybox
    "Their mission read, To Boldly Go,
     But how ill-bred, To phrase it so." - Phil Garland