[GreenKeys] Suppression - M28 printers

[Jim Haynes]

On Tue, 8 Mar 2022, Nick England wrote:
> So what’s the story on the suppression solenoid - what was it used for?

FAA facilities such as Flight Service Stations had several Teletype
machines on several different services.  Service A is for weather
reports and forecasts, and Service B is for flight plans and 
administrative traffic.  They wanted to be able to use the same
Teletype machines on all their services, so they could swap in and
out spare machines.

For Service A they want to copy everything.  For Service B they want
to copy only messages directed to that particular station.  So the
machines are set up for selective calling, and then the solenoid is
there to disable print suppression when you want to use the machine
on a service that copies everything.  Usually there is a local-remote
switch on the front of the machine that controls the solenoid.
So they can instantly switch selective calling on and off as needed,
rather than having to open the machine and move a little part to
disable print suppression.