Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!bunker!wtm
From: rbarth@tumtum.cs.umd.edu (Dick Barth)
Newsgroups: misc.handicap
Subject: Re: TDD for Amiga
Message-ID: <13790@bunker.UUCP>
Date: 4 Sep 90 02:20:50 GMT
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Reply-To: rbarth@tumtum.cs.umd.edu (Dick Barth)
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Index Number: 10050

In Digest # 1454, Scott Bechtel writes:

   I have an Amiga 2000 and was wandering if anyone would know of a
   program/hardware addition that I could use to talk to my tdd friends with?
   I have heard of this for some ibm's so I think it would be possible.  If
   not, maybe I will be able to make something someday.  SB

I have to assume that your Amiga does NOT have a cassette port; most newer
computers don't. If my assumption is wrong, let me know; I'll have a couple
of other suggestions for you.

TDDs use the older Baudot code, not ASCII. They also use a modem that isn't
compatible with standard ASCII modems. So, you need to get a TDD modem and,
depending on how you do that, you may need to write a comm program that does
code conversion.

The simplest solution is to buy a commercial TDD modem that has built-in
code conversion. These connect to a standard serial port and to a phone line.
They provide both TDD and Bell-103 compatibility and are available from two
sources I know of. These are Ultratech and Krown Research. If you were
using an MS-Dos machine, you could also use a unit from Phone-TTY that comes
with software to run on that kind of a computer.

The TDD uses frequency-shift keying (FSK) same as the -103 type modem.
You can modify an older Bell-103 modem to use TDD frequencies (1400 MARK
and 1800 SPACE), recognizing that the send tones have to shut off within
a couple hundred milliseconds after the character has been sent. This is
because TDD uses the same tone pair in both directions, therefore tones
have to squelch so you can hear the other guy.

You can also build a simple FSK modem from scratch using such chips as
the EXAR XR2206 and XR2211. Not difficult; ask EXAR for the modem
designer's manual and poop sheets for these chips.

Code conversion is just table lookup; if you can write a simple comm
program (which doesn't need to be interrupt driven at 45 baud) you
should have no trouble doing the rest.

For a description of the commercial TDD modems on the market, download
the file COMPARE.TTY from file area two of the HEX BBS. For code
conversion software in the C programming language, download BAUDOT.C
from the same place. And if you finish this project and want to share
your design, please let me know.

Happy hacking.
-----------------------------
Richard Barth, W3HWN,  rbarth@tumtum.cs.umd.edu
Handicapped Educational Exchange... (301) 593-7033 (TDD & 300 baud ASCII)
                                    (301) 593-7357 (300/1200 baud, 8N1)