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From: rbarth@tumtum.cs.umd.edu (Dick Barth)
Newsgroups: misc.handicap
Subject: Re: TDD TO MODEM SOFTWARE
Message-ID: <14797@bunker.UUCP>
Date: 9 Oct 90 02:00:14 GMT
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Reply-To: rbarth@tumtum.cs.umd.edu (Dick Barth)
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Index Number: 10946

In Digest # 1543  mbeck@rice-chex.ai.mit.edu (Mark Becker) writes:

   > I think that the now obsolete Hayes 300 bauder had that ability as well to
   > act as a TDD system with a certain command. Write to or call Hayes and see
   > if I'm right, I suspect I might be close with this one.
   >

   I knew there was a varient of some Novation modem that, with some
   special ROM's, could handle Baudot.

True. It was the Novation Applecat, and came with an in-ROM program
called DEAFWARE. Another package was written by Bill Blue to use
the hardware, and some users thought it an improvement over what
came with the modem card. It was called "ASCII Express", if I
remember correctly.

   I found out (the hard way) that a conventional ASCII modem will lock
   to a TDD signal.. and then (if the default parameter settings are
   used) disconnect when the caller let go of the keyboard.

TDDs are half duplex. They talk and listen on the same tone pair,
and so have to shut off their transmit tones when you stop keying.
If your -103 modem is broad enough (and doesn't filter out the
mid-band frequencies at 1400 and 1800 where TDDs operate) it will
think a TDD is an ASCII carrier.  Until you stop typing, the
carrier drops, and the modem disconnects.  Some Bell-103 type
modems work the same way. They can't tell the difference between a
300 baud carrier and a 1200 or 2400 baud carrier. They'll tell the
computer a carrier has called and the computer will collect a
snootful of garbles before the caller hangs up.

   This nearly drove me bananas when developing my TDD BBS system..
   TDD callers started whomping on their keyboards before the system
   discovered it was dealing with a Baudot caller.

Funsies.

   After wrestling with the problem for awhile, I gave up.  The
   documentation was changed to tell TDD callers to wait about 10-15
   seconds for the TDD banner to pop up. 

I haven't found a better solution either.

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