[GreenKeys] Detroit's 'TELEX SYSTEM' of the late 1960s and later

[Jim Haynes]

There was of course the TTD  Teletype for the Deaf system before that,
and still in use.  It mostly used acoustic couplers to avoid the telco
restriction on direct connection to their circuit.  And the modem is
much simpler, as it is not full duplex so doesn't require originate/answer
distinction.

Then I was told of a system operated in the North suburbs of Chicago.
It used telco private lines which were marketed as burglar alarm lines
at a very low charge, but were in fact ordinary voice grade lines.  Until
the telco discovered what was being sent over them and put capacitors
across them at the central office to limit the bandwidth to something a
burglar alarm system could use.

The 101 modem family was a real kluge, with most of the works of a 
telephone built into the Teletype call control unit.  The 101 data sets
had a 99-wire interface between the modem and the Teletype.  Some of
the Bell operating companies decided to save money by using the 103
family of modems, which had the telephone part self-contained, and
Teletype machines designed for private line use.  Which is the way Dial
TWX should have been done from the get-go rather than using the 101 
modems.

But one reason for the expensive modems was that the system was expected
to work over almost any dialed-up connection, and they varied widely in
quality back then.  I remember an article or maybe it was a published
Technical Reference where Bell Labs had dialed up connections all over
the U.S. and measured the quality of signals, or the error rates.  That
was a necessity before converting TWX to dial operation over the voice
switched network.

I don't know about the 101 modems, but the 103 modems had three different
sets of options: which tone pair is originate and which is answer, which
tone is mark and which is space in the lower-frequency tone pair and
which is mark and space in the higher-frequency tone pair.  This allowed
setting up modems for eight different mutually non-interoperable services.
There was TWX, and Dataphone, and WADS and something called WADS-prime
using up four of them.  Thus a TWX station could not communicate with a
DataPhone station just by dialing the correct number.  (There was the
further segregation of five-level and eight-level TWX by giving them
a different area code.