[GreenKeys] TTY loop interface

[Nick England]

FWIW Here’s the M28 SMD that runs off 48v
https://www.navy-radio.com/manuals/tty/wd/323810WD-SMD.pdf



On Thu, Aug 12, 2021 at 8:55 AM Jim Haynes <jhhaynes at earthlink.net> wrote:

> 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.
>
-- 
Nick England K4NYW
www.navy-radio.com
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