Re: History AT&T early modem developments? [telecom]

On Mar 13, 10:57 pm, Bill Horne <b...@xxxxxxxxxxxx> wrote:

Please provide more details on WU's TDM system(s). I didn't know that
TDM was in use before the 50's, so I'm very curious about these.

As a reminder, we have the Western Union Technical Review on file
here. I don't know if it's indexed or if there's an issue
specifically on this subject, but it is interesting going through
them. The first page is a contents of the issue. Please see:

http://massis.lcs.mit.edu/telecom-archives/archives/technical/western-union-t
ech-review/


Amazon has the switching book for about $30, but there's a "Transmission
Technology" volume that starts at $65.00. Do you have that one? Is it worth
it?

I'm not familiar with that one. Is that vol 4? I think more books
were planned but then divesture stopped the effort.


The modems used for the 4-row side of TWX had an important feature not
found in equivalent units from other manufacturers: because they had to
inter-operate with the 3-row machines running at 60-speed, they could
respond to the presence of BOTH a mark and a space tone being received
at the same time, which was a RESTRAIN signal that turned off the tape
reader and locked the keyboard until the 60-speed machine at the other
end of the connection could catch up.

Initially, the Teletype 33 ASRs we used for computer time sharing
terminals had the built-in modem and automatic dialing; and had an
orange lamp "REST". We never knew what it was for. Also, for some
reason our machine was Touch Tone, which was unusual in March 1968
(for me, marks forty years of this stuff.) It was also all number,
which for us used to lettered exchanges was a little cumbersome.

Our later Teletype 33s didn't have the automatic modem. Some had the
acoustical coupler. Some had the interface block and a modem which
was activated by pulling up the 'hold' knob on the phone. (They used
a two-line phone which had a pull-up knob.) Apparently this
arrangement was cheaper than renting the TTY from the phone company.
(But maint service was free, an important consideration given high
school kids were rough users).


***** Moderator's Note *****

I'm surprised that you were able to get that model of 33 ASR to work
with time sharing systems, because AFAIK all the "TWX" machines (if
you had a RESTrain light, that's what it was) were wired for
half-duplex and local echo. I know, because I used the one at Back Bay
Toll in Boston to work with Ward Christianson's bulletin board in
Chicago, and it ddoouubblleedd eevveerryy cchhaarraacctteerr I typed,
because in addition to the local echo, the BBS echoed everything it
received.

Although it was possible to set the bulletin board so as to turn off
character echo, there was no way to do it on a call-by-call basis, so
Ward couldn't cure the problem for me: either your time sharing guys
had gotten that figured out or all your machines were doing
local-echo.

The first time I logged on (from school) with a 300 baud modem, Ward
cut in and said "Hey, Speedy!". ;-)

Bill Horne
Temporary Moderator

(Please put [Telecom] at the end of the subject line of your post, or
I may never see it. Thanks!)

.