IJJT500@INDYCMS.BITNET (Mike Wheeler) (02/09/91)
Index Number: 13600 I was wondering if anyone knows where I can get a TTY/TDD card or RS-232 i nterface for my computer. I don't want to spend @$250 for something when I can use the hardware in my computer. I can write software to make it work better. My old TTY is broke it is about 16 years old, An old AT&T (Bell systems) TDD. (when will they get the name right TTY). An XT card or RS-232 will work. It shouldn't be hard to convert the 5 bit to 8 bit and feed it as a comm port the b sor xxx the software could do any translation from there. Basic interpeter on original PC fast enough for that. Been llooking but all I found was schemat ics/program for original PC (hooked to the cassette port) no chance here. Have Amiga 2000 with PC Bridge card. (Hardware so it can run PC software and even cards). Mike
rbarth@tumtum.cs.umd.edu (Dick Barth) (02/21/91)
Index Number: 13676
In Handicap Digest #1832, IJJT500@INDYCMS.BITNET (Mike Wheeler) writes:
I was wondering if anyone knows where I can get a TTY/TDD card or
RS-232 i nterface for my computer. I don't want to spend @$250 for
something when I can use the hardware in my computer. I can write
software to make it work better. My old TTY is broke it is about
16 years old, An old AT&T (Bell systems) TDD. (when will they get
the name right TTY). An XT card or RS-232 will work. It shouldn't
be hard to convert the 5 bit to 8 bit and feed it as a comm port
the b sor xxx the software could do any translation from there.
Basic interpeter on original PC fast enough for that. Been
llooking but all I found was schemat ics/program for original PC
(hooked to the cassette port) no chance here. Have Amiga 2000 with
PC Bridge card. (Hardware so it can run PC software and even
cards).
You're right about it being easy to convert ASCII characters to Baudot
(8 bits to 5 bits) and vice versa. That's an easy programming job. The
hard part comes from the fact that the TDD uses a modem that is not
compatibile with, and won't talk to, an ASCII modem of the type usually
used with computers. You *must* have a TDD modem, or else emulate one
in some way. That's what the software does that you mentioned - it
uses the cassette port hardware in an original PC to generate TDD tones
so that you don't have to have a modem. If your Amiga doesn't have a
casette port or some other hardware to generate and receive tones, you
do need the modem. These can be bought, but if you don't want to spend
the money you can also build your own. TDD modems use frequency shift
keying (FSK) of the same type used in the old Bell-103 (300 baud ASCII)
modems. The tones are different, thought; for TDD, MARK is 1400 Hz and
SPACE is 1800. Also, the tone has to shut off quickly (say, 100 milli-
seconds) after you stop typing because the same tones are used in both
directions.
The HEX BBS, which accepts TDD calls as well as ASCII, ran for years
using a modified Bell-103 modem card. Change the frequencies, install
a simple circuit to cut off tone after the keying stops, and you're in
business.
Of course you do need some technical ability in the circuit design area,
which is why a lot of people prefer to buy these modems rather than
build 'em.
---
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)