[GreenKeys] 18 volt Internal Loop Supply

[Jim Haynes]

On Fri, 8 Jul 2022, Jeffrey Golas wrote:
>
> The polarity thing on the 32 had to do with the switching gear; it was 
> still current loop from the carrier, so a temp reversal I think meant an 
> ended call (or answered I forget). A reversed current from idle 
> indicated an incoming call. The 32/33 manual describes the call control 
> states, which was via telegraph lines/loop yet.
>
For non-TWX service it was pretty popular to use polarity reversal to
control the motor.  A typical W.U. setup with the teleprinter in the
customer's office had loop current on all the time, but of a polarity
that would keep the motor turned off.  If the customer wanted to originate
a connection she could push a button that would break the line current.
This would release a relay in the WU office and light a signal lamp.
The WU operator would insert a plug into the jack for the caller, and that
would be of opposite polarity so it would operate a polar relay in the
customer's set and turn on the motor.  Then the customer would know she
could start typing.  And if the WU office wanted to originate a connection
it was just a matter of plugging into the jack and that would reverse the
current polarity and turn on the motor.

The polar relay that did this line current detection was very similar to
an old model telephone ringer, with two magnet coils and a permanent
magnet to polarize it.  The armature instead of carrying the bell clapper
would operate a contact that would energize a relay big enough to handle
the motor current.