[GreenKeys] HRS312 TTY Interface

[Jim Haynes]

On Tue, 10 Mar 2009, Jon Schlegel wrote:

> Also, are issues like this why a "holding" magnet selector is, I
> think, considered better than a "pulling" magnet selector?
>
Walt Zenner, who invented the holding magnet selector, told me that
the original motivation for it came from TWX switchboards.  He said
that the cord circuits in TWX switchboards had 120V 60MA loops, and
this exposed the switchboard operators to contact with lethal
voltages.  By going to the holding magnet selector they were able to
get the voltage down to 48V, which is the same voltage level used
on telephone switchboards and presumably a lot less dangerous.

Yet all the literature I've seen talks about the improved sensitivity
of the holding magnet selector and never mentions the issue of loop
voltage.

It's interesting that Frank Pearne, the inventor who was responsible
for Teletype getting started, came up with a solution to the switchboard
problem.  He got a patent on a plug with a mercury switch built into the
handle, so the tip was not connected to the cord until the plug was
horizontal.