[GreenKeys] Given that energy can be neither created or destroyed ...

[Jeffrey Angus]

> I think the only sure way to find out is to try it. The machines need to 
> be in series with a single loop control, so it is the total reactance of 
> the coils that limits the number. Most early loop supplies were around 
> 135V, with very low impedance, so there was enough power for several 
> devices in a loop. Still, since there is significant inductance 
> involved, nearly half a henry per selector IIRC, the signal distortion 
> increases much faster than the resistance alone would suggest.

The short answer to the loop supply is that you are trying to push a square
wave through a large inductance, and to do that properly, you need a 
current source rather than a voltage source to accomplish that.

The 120-135 VDC with a series resistance to limit the current is a sort of
poor man's current source with a "hig compliance" (meaning it can go to
120-135 VDC in an effort to keep the current up as it switches from off to
on.)

By the way, this is why they used polar relays on longer loops, was to have
a current source to force the off condition as well as the on condition and
keep the signal as close to a square wave as possible. This gave the TTY
machine the widest range to deal with signal distortion.

Local loops can be "neutral" (i.e.on off keying) because they typically
only had a selector magnet to deal with rather than several miles of twisted
pair. 

Jeff-1.0
wa6fwi