[GreenKeys] Another button question

[Jeffrey Angus]

On 6/16/19 4:11 PM, Jim Haynes wrote:
> But the polarity is such that the motor is turned off.?? When the
> central office connects to the out station the line polarity is
> reversed and that turns the motor on.?? This was popular with
> Western Union.
> It may also be used in Telex - I don't remember.
That's exactly how it was done with the Telex system.
They used a 7-10 mA line current at reverse polarity to indicate
that there was indeed a machine connected at the far end.
An incoming call would reverse the line polarity back to normal,
The receiving call control unit would short the internal resistor
allowing the line current to increase to 40 mA and turn the
motor on.
Out going calls were initiated by pressing the AT key on the call
control unit. This would increase the line current to 40 mA and
unlock the dial. Once the dialed party was connected, the central
office would reverse the line polarity back to normal, and the
machine motor would turn on again.
Once the conversation was over, either station could terminate
the call by either hitting the ST key on the call control unit or
hitting the break key.


-- 
"I am a river to my people."
Jeff-1.0
WA6FWI
www.foxsmercantile.com