From: cep4478@ultb.isc.rit.edu (C.E. Piggott )
Subject: Re: Building IBM PC TDD (need BAUDOT & UART info)
Originator: cep4478@ultb
Keywords: IBM PC TDD UART BAUDOT
Organization: Rochester Institute of Technology
Date: Mon, 21 Dec 1992 13:51:03 GMT

I'll try to keep this shorter than my normal, babbling self, but I'll leave
it here, because it'll probably be interesting to other people than Mike.

Note that wsmr-simtel20.army.mil (by anonymous ftp) has a program called
TDD540 for the IBM PC.  This program uses the built-in cassette port on the
original PC's to make the hardware extremely simple.  I never had a PC with
a cassette port, but I've built a simple XR2206/2211 modem for 170 Hz shift
and used it on RTTY, with the PC doing all of the demodulation.  I wrote my
own split-screen software for this purpose, but i don't think that's really
necessary - a few of the off-the-shelf terminal programs support 5-bit words.

In article <1992Dec19.021143.8280@networx.com> mikel@networx.com (Mike Lempriere) writes:
>a)  TDD baud rate is 45.5 baud (sheesh).
>b)  Basic carrier (space tone) is 1400Hz.
>c)  Mark frequency is 1800Hz.
>d)  There is always 1 mark start bit.
>e)  There are 5 data bits.
>f)  There is always 1 space stop bit.  (Would 1.5 or 2 work too?)

I'm not 100% sure of the frequencies (it's been several years) - on HF
you're using SSB, so it's the shift that counts.  I think I set the tones
for 170 Hz shift centered around 1500'ish.

For ARQ, a "good" modem would be able to work with the two tones, and also
with the presence of one tone and the absence of the other.  This is to
counter the effects of selective fading, which is a very "real" problem.
People who don't fully understand radio often don't believe you when you
tell them that a signal a few hundred hertz away could be faded down below
the noise while a few hundred hertz up is armchair, under the "right"
conditions.  On RTTY, however, the only thing I've been able to figure out
is to make DCD pull the receive data line to the "space" condition.  I
don't think you can live without the "mark" tone, since it's asynchronous
(I guess you could assume mark if you're hearing occasional spaces).
My one-tone circuits didn't work very well, because the DCD circuit is
way too slow with the modem design I used.  Sounds like a neat DSP
application, though.

>a)  What UART chips do the default OS/2 drivers talk to directly?
>b)  What UART chips do the default MS-DOS drivers talk to directly?

Same hardware either way, it's an 8250 or compatible.  Most AT's
have a 16450, and the "really good" cards use 16550a's, but the basic
chip is programmed the same way.  If you can, find your local National
Semiconductor rep (if you're a student, call up a Hamilton/Avnet or Arrow
office from the phone book.  They're usually nice to college students,
and will give you a book).  The 8250 has a baud rate generator built
in, so getting funky rates is no problem.

>a)  I have a 4-port serial card made by "STB Peripherals" containing
>    a "StarTech ST16C554CJ" chip for all UART functions.  Does this chip
>    allow access to the "External clock rate" pins, or did they just
>    dump that functionality when they crammed 4 UARTs onto the one die?
>    If so, what pins?

About these goofball cards...I hate them.  The last two serial cards I
bought were a PC card with one giant chip in the middle, which had two
serial ports, one parallel port, and a game port.  I have had a lot of
problems with them.  The worst one is that the port sometimes comes up
dead as a doornail, and won't reset without powering cycling the computer.
They seem to get along with NOS, but things like mouse drivers and
off-the-shelf terminal programs won't wake the card up.  (I am using a
good bios, it's AMI, so I can't blame that.)  Getting a "real" serial
card is worth the extra bucks, and it's still cheap.  If you're running
NOS, it's even MORE worth it, because you can pull out the chip and put
in a 16550a and get reduced interrupt overhead.  This makes an AMAZING
difference with Windows 3.1 running NOS in the background (although you
have to play some games to get NOS in direct control of the serial port).

Good luck with the TDD project.


Chris
-- 
Christopher E. Piggott, WZ2B                    cep4478@ultb.isc.rit.edu
President                                       wz2b.ampr [44.69.0.1]
Rochester Institute of Technology               wz2b @ WB2PSI.#WNY.NY.USA.NA
Amateur Radio Club K2GXT                        CEP4478@RITVAXA.BITNET
Newsgroups: rec.radio.amateur.misc