[GreenKeys] WU 1-A

[Jim Haynes]

I don't know dates on this stuff, but W.U. used the time-division 
multiplex up until about 1950 when the switched to frequency division
multiplex.  The mux printers, their Model 21-A tape printers, were
used by hams in the very early days of amateur RTTY.  You could drive
one from the receiving distributor of a Model 12, or there was a design
for an electronic receiving distributor to drive them from start-stop
signals.  Since they used the standard Murray 5-level code they probably
began in the 1920s.  In Western Union Technical Review you can read a
lot about the technology, motor-driven faceplate distributors and it
was high technology for the day to keep the distributors at both ends
of the wire running in phase at all times.  And quite a pain to restart
the system after a shutdown, which is why use of that multiplex was 
limited to large offices.  What made Morkrum-Kleinschmidt-Teletype
successful was the start-stop synchronization, which allowed printing
telegraphy on individual lines of lower traffic capacity, and without
the need for skilled technical people to keep them running.