[GreenKeys] TTY-Connect boards/micros

[Dave Emery]

On Wed, Jun 16, 2004 at 09:45:57AM -0700, gil smith wrote:
> 
> You may have the only polar machine out there.  I'm not quite sure what is 
> involved to drive it.  There are "polar relays," but I am not sure how they 
> connect to equipment.  Jim Haynes thought the loops used +/- 30 mA (not 
> 60).  I don't know much about polar loops at all.  I tried to post a 
> message (about ten different ways) to greenkeys about polar loops, and it 
> was rejected as spam -- you were CC'd on it -- did you ever see it?  I 
> should try posting it again.

	Polar loops were pretty much the standard method of transmitting
tty and other telegraph traffic over long distances on wire lines.   
The standard was plus and minus 20 ma over twisted cable pairs that
could run into tens of miles.

	Polar transmission is symetrical and aviods a lot of kinds
of distortion that can happen with on off keying, especially if the
loop is actually open during the current off interval.

	Generally, of course, there was a polar relay between the line
and the Teletype machine that converted the plus and minus current
swings to contact openings and closings of a local neutral loop
with the actual selector magnets on it.  I have heard of very few
if any machines that actually used selector magnets driven off a polar
20 ma source.

	Back in the early days of minicomputers when ttys were common
as console devices,  plus and minus 20 ma signalling over a current
loop was the prefered way of interfacing to the model 33 or 35.
In theory this signalling would work over much longer distances than
voltage driven relatively high impedance RS-232.

	But 33's and 35's all had transistorized magnet drivers that
actually drove the selector magnets - they did not have mechanical
polar relays or high voltage neutral loops.   And the magnet drivers
had resistors in series with the loop to sense current, not inductive
relay coils so they could actually be driven by simple low voltage
interface circuits without worry about inductive spikes.


-- 
   Dave Emery N1PRE,  die at dieconsulting.com  DIE Consulting, Weston,
Mass 02493