Channel cross-talk on Analog i/p card

hi,

i m using auotmation direct plc and facing problem on my analog i/p card F408AD from automation direct.

i m using 7 channels of that card which are all connected for
2-wire transmitter, when i m testing each channel individually by disconnecting all channels than i m getting constant reading for that channel register in my plc....but when i m connected to more than 1 channel than my register value is continuously changing between 0-4095 instead of giving me the constant reading.

Also i would like to mention that for 1 channel it is working fine but when i see the register value for unsed channel on my plc( ie the wires not connected on analog i/p) i m still not getting 0 value ...instead i m getting the value same as channel 1 (which is connected to transmitter) on all other disconnected channel register also.

i tried changing the analog card...tried different slots ....chkd the wiring ....but still having the same problem....also chk the jumpers behing the analog card ..they are set for 4-20 mA ....which is correct ...

if anyone has any idea abt it ..plz do reply.

thanks,
ak

hello anks;

Sounds like an analog grounding problem. We often find them on Siemens analog input modules (we often have to check channel wiring).I am not familiar with AD analog modules, but most manufactures have provisions in their documentation on how to connect the unused channels so cross-talk is prevented. Maybe someone more familiar with this material can give you specific answers.
Hope this helps,
Daniel Chartier

hi daniel,

thanks for the reply, i guess it was a grounding issue, was able to get that after installing isolators.

anks