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

[Jeffrey D Angus]

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


-- 
Jeff-1.0
wa6fwi
http://www.foxsmercantile.com