[GreenKeys] RS232 to Loop Converter

[Jack]

ZCZC

There's an item on Ebay (380084222466) that is made by Black Box.
It is an RS232-to-current-loop converter and the selling
price is $5.00 with 10 available. I "won" a few of these
last year and just got around to playing with them. (Since then,
I have also obtained a couple of related items: one unit has
16 converters in it and the other box has 8 in it...I'm set for
life!)

This gadget is capable of
converting RS232 signals to a 20 or 60 ma. loop, at up to
30 volts DC and vice versa. I have run one of my Model 28's directly
off this
interface...the 28 has a polar relay input. This converter will
not directly key TTY selector magnets reliably.

If a 12 VDC reed relay is wired to the loop output, its contacts
will easily key a 60 ma. HV loop. I suggest putting a resistor
and capacitor "snubber" across the relay's contacts. If you want
bidirectional RS232, you would put your keyboard into the
"send" loop connection which would be optioned for 60 ma. This
Requires the box to be optioned for four-wire loop control.
Or, you can option it for a two wire circuit. The reed
relay and keyboard would be wired in series across the loop input.
I used a relay out of my junk box.

For you telegraphers, this thing will also key a sounder;
you'd put your telegraph key in series with the sounder. If the
sounder doesn't work directly, then use the reed relay to key it.

You could interface one of these to a Bell 103-type modem and
put a TTY on a phone line. This would simulate the old
"D TWX" network.

By the way...making the output "solid state" with external parts and
a HV transistor would basically replicate the circuit you can use
WITHOUT this box to convert RS232 to key a loop. So, the reed relay
is the quickest/cheapest way to go. 

A final reminder....a WECO 255 polar relay, with its coils in series,
can also convert RS232 to key a loop. Just hook the relay to your
PC's COM port output and use the contacts to key your local loop
and TTY. Not exactly "mil spec", but I've been using this trick
for years with Heavymetal. After all...RS232 output is "polar".

73,

Jack K0TTY

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