Path: utzoo!attcan!uunet!lll-winken!ames!mailrus!csd4.milw.wisc.edu!uxc!uxc.cso.uiuc.edu!clio!berger
From: berger@clio.las.uiuc.edu
Newsgroups: comp.sys.ibm.pc
Subject: Re: TDD protocol on PC?
Message-ID: <16800406@clio>
Date: 12 Jan 89 22:05:00 GMT
References: <6033@mailgw.cc.umich.edu>
Lines: 29
Nf-ID: #R:mailgw.cc.umich.edu:6033:clio:16800406:000:1049
Nf-From: clio.las.uiuc.edu!berger    Jan 12 16:05:00 1989


The protocol is a half-duplex protocol, not simplex.  A simplex
transmission is in one direction only.  In half-duplex, each side
can transmit and receive in turn.

Due to minor differences in electromechanical hardware, some machines
used 1.5 stop bits (Western Union), and others used 1.42 stop bits
(military), while some machines used 2 stop bits.  So machines ran
at 45, 45.5, and 50 baud for 60-66 WPM traditionally.

The fsk modem is very much like the 103 type 300 baud modem
popular among computer users, at least in signalling method.
The tones and frequency-shift (difference between the tones)
are different.

Noise is not a big problem over phone lines, so it need not be
difficult to build a modem for TDD - the frequency shift is
more important than the actual tones used.

Surplus teleprinters that use baudot code are available for
free all over... but nobody wants to keep them running.


			Mike Berger
			Department of Statistics 
			University of Illinois 

			berger@clio.las.uiuc.edu
			{convex | pur-ee}!uiucuxc!clio!berger