[GreenKeys] TTY protocol

[Jim Haynes]

On Mon, 18 Aug 2008, Teletypeparts at aol.com wrote:

> I recall lots of 28's running, but not typing, waiting for the proper
> characters to turn on the printing.  Wore out lots of mainshafts and motors.  Am I
> correct about this?

Yep.  It was called selective calling.  Very popular with the FAA, among
others.  All these machines were sitting on a multipoint line and a
message would be prefixed with selection characters to turn on printing
at the desired stations, leaving it off at the others.  I guess it was
used in Bell System services as well.  Seems like the commercial-type
customers were a lot less tolerant than the FAA of having the machine
running all the time, making busy sounds but not printing anything.  I'm
told there was eventually an "electronic stunt box" to do this kind of
selecting electronically without the machine having to be on.

Before the Model 28 stunt box there was a product called the SOTUS which
did that kind of function, in a separate non-printing machine.  And
there was ultimately a replacement for the SOTUS, consisting of a Model 28
printer with the front plate mechanism left off, so that all it did was
run the stunt box.  These machines could be put inside a sound-deadened
cabinet away from the printers.

Some of the selective calling 28s have a solenoid on the left side
plate that disables the print suppression.  The FAA wanted to be able
to have all their machines alike, and switch them around to different
services in a station.  When a printer was used in weather service it
needed to copy all the traffic on the line; while in flight plan service
it needed to copy only traffic addressed to the particular station.
So the solenoid override made it easy to switch between these two
operating requirements.