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

[drlegendre .]

Howdy, Jeff - and thanks for your reply. But I think I might have figured
this out in the meantime..

It turns out that the unit I was experimenting with (I have two identical
units) was defective - it had a bad 6N139 opto on the receive side. As
such, I may have figured most of this out in the meantime.. I now have
local loopback (echo) at baudrates up to 115,200. ;-)

Here's what else I found, though..

On Sat, Dec 27, 2014 at 8:17 PM, Jeffrey D Angus <jdangus at att.net> wrote:

> On 12/27/2014 8:03 PM, drlegendre . wrote:
>
>> 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+)?
>>
> Since they are marked, I think that would be + to + and - to -.
>
>> 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?
>>
> Actually, the 24v is for "long lines," the current limit of 55 mA is to
> protect
> the device in case of a shorted line.
> I'd suggest putting a 1200 ohm 1 watt resistor in series with each loop to
> limit the current to 20 mA.
>
>> 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.
>>
> This is fine as it is. See above about putting a series resistor in each
> loop.
>
> So connecting for loop back. Data TX => Loop TX ----- Loop RX => Data RX
> -----
> Data TX => Loop TX -----Loop RX => Data RX


You are correct, the Tx+ should be connected to Rx+ and so forth; but they
are not 'marked'. These units are almost totally undocumented, so I had to
make measurements and a few educated guesses about which pin was which on
the CL side. The only 'documentation' consists of some markings on the PCB
indicating how jumpers should be set for passive / active status on the T
and R circuits.

There are also three jumpers on each unit to hard-wire the CTS, DSR and
RLSD lines - I don't recognize the latter, though.. RLSD?

As for the current limit, you might have missed something. If a 24V circuit
is already limited to 55mA, that means the Zout is already ~436R. If I want
to further limit the current to 20mA, then an additional 764R - not 1200R -
is required.

This dissipates 300mW in the 764R, so a 1/2W part is OK here, as the duty
cycle is about 50%.. though 1W would not be a bad idea.

Once I have these things fully sussed-out, I'll document them as well as I
can, and offer the results for upload to any member who wishes to post them
on a site. Silly as it seems, I simply cannot find ANY docs for these
things out there - and this, despite the fact that they seem to be somewhat
common, and were clearly OEM'd for several companies.
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