[GreenKeys] Current-loop loopback connection question? - Camiacs RS-232 / CL Pt. II

[drlegendre .]

Howdy folks,

I've made solid progress on the 'mysterious' Camiacs RS-232 to CL
converters - it seems as if both devices are now up & running. As a final
test, I'm trying to set up a loopback connection, where the Tx of the
device feeds back into the Rx of the same device.

The idea here, is that if the converter is working correctly on both
receive and transmit, I should see my keystrokes echoed back to my terminal
screen - makes sense?

First question - how do I connect the Rx / Tx lines for loopback? Do I
connect Tx+ to Rx+ /  Tx- to Rx- or should the polarities be reversed (Tx+
to Rx- / Tx- to Rx+)?

Secondly, should I use a current limiting resistor in one or both loops, to
simulate long line resistance? Something like 470R to 1K?  The active power
on these units is 24V DC, with a 55mA short-circuit current limit. I would
have expected 20mA, but I guess this is to compensate for the resistance in
long lines?

Finally, the Camiacs converters can be configured (with jumpers) for active
or passive on both Tx and Rx. As it sits, the unit is configured for Active
Tx and Passive Rx - this seems sane to me, only one end of each loop must
be active, correct? So in this case, an active Tx output is connected to a
passive Rx input.

As ever, TIA for any help!
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