Xref: utzoo comp.misc:4634 comp.sys.ibm.pc:22912
Path: utzoo!attcan!uunet!lll-winken!ames!xanth!mcnc!rti!bcw
From: bcw@rti.UUCP (Bruce Wright)
Newsgroups: comp.misc,comp.sys.ibm.pc
Subject: Re: TDD protocol on PC?
Summary: TDD protocols
Message-ID: <2703@rti.UUCP>
Date: 7 Jan 89 22:47:44 GMT
References: <210@imspw6.UUCP> <109@csd4.milw.wisc.edu> <77200@felix.UUCP> <4521@cs.utexas.edu>
Organization: Research Triangle Institute, RTP, NC
Lines: 50

In article <4521@cs.utexas.edu>, bunda@cs.utexas.edu (John Bunda) writes:
> 
> > ...  The crushing problem with things
> > like TDD's (Telecommunications Device for the Deaf) is that they are
> > totally incompatible with everything else (they use an ancient protocol
> > predating even 110 baud modems) - the other party has to have one in
> > order to talk to them, and they aren't very common except for the
> > deaf and certain "core" social services (911 and similar services, at
> > least in some cities).
> 
> I have run into this problem before, and I'm wondering how difficult
> it would be to get my PC to emulate a TDD - I'm guessing that it's 
> trivial but would require hardware - I'm supposing that it wouldn't
> be possible to convince my modem to talk to the things.
> 
> Probably easier to send a PC and modem to my deaf relative...

TDD's use a modem signalling protocol known as "simplex".  There is no
carrier signal (as you may already know).  There are two separate
frequencies used for Mark and Space, which are only generated when one
side or the other wants to send something.  The frequencies are the same
whether you are the Answer or the Originate side of the connection -
and there is no provision for relinquishing the line (as there is in
half duplex), so if both parties try to talk at once you get garbage.
The frequencies used for Mark and Space are different from the frequencies
used by any other modem signalling technology that I'm aware of.  Simplex
was abandoned by most communications carriers back in the 50's and 60's
because it is just too primitive to be useful for most purposes.

Now once you get the signal demodulated, the problem becomes easy:  they
use a Baudot-type character code (actually, there is no single "Baudot"
character code, there's a whole family of them;  I forget which one the
TDD's use.  The differences are relatively minor, though they sometimes
move around the Shift-In and Shift-Out codes so you may not get anything
meaningful by applying the wrong Baudot-ASCII converter).

If you have a fair amount of ability with signalling hardware it would
probably not be too difficult to build a Simplex modem.  They don't run
at any kind of speed (something like 50 baud) so there isn't any heavy
timing constraint.  Unfortunately I don't remember enough specs on the
subject to be able to tell you what you would have to do to recognize
the signals, but this may give you enough to try to look up references.

Better yet, it is likely that one or another TDD manufacturer has a
Simplex -> ASCII (or at least Simplex -> RS-232) converter;  if this
exists it would simplify your task considerably.

Good luck!

						Bruce C. Wright