short haul modems

[Al Kossow]

I picked up a bunch of Black Box "Short Haul Modems" yesterday.
I hadn't ever looked at such things closely before, but they
are just powered RS-232 to current loop converters.

Very handy for old CPUs that only have current-loop TTY
interfaces, like the Varian 620, or hooking a 33 to modern
gear (assuming it can talk 110 baud).

I'll get the circuit traced out in the next few days, and
check what current they are running through the loop.