[GreenKeys] TTY loop interface

[Jim Haynes]

On Thu, 12 Aug 2021, Nick England wrote:

> Well perhaps Jim is the father of low level +/-6v signaling that was
> implemented for TEMPEST purposes. The TH-83 hubbing repeater is exactly what
> he describes. See

At the time I was working on the low-voltage selector magnet driver, when
I had been asked to work on a solid state replacement for a polar relay.
And I felt that these were two separate problems - if we need an interface
to a 60ma loop coming in from the outside world, and we need a way to
drive selector magnets with solid-state circuits.  TEMPEST didn't come up
as an issue in that little project.  But I did realize that the low 
voltage hub scheme would be a way to interconnect a station as well as
a way to organize the interior of a set.  The military folks came up
with the TEMPEST problem and its various solutions, and then EIA came
up with the low voltage polar interface as a way to standardize the
connection between a customer-premises modem and the data equipment.
This was a bending of the old telephone company "no foreign equipment"
policy because AT&T realized there was a market for data transmission
over the switched network and that they couldn't possibly provide all
the kinds of equipment that customers might want to connect.  Western
Union also developed a line of low-voltage equipment, probably in 
connection with DOD.

I didn't design the low voltage selector magnet driver as it turned
out to be - I had envisioned simply a saturated transistor, and someone
else came up with the constant-current circuit that was ultimately used.
Maybe that was helpful in going to 110 baud.  And it was I guess someone
at Bell Labs who decided the interface between the original TWX modem
and the Teletype set should be current loop rather than voltage.