L35E 4-20 mA signals raw values bouncing

Hi,


I have a compactlogix L35E and my analog input signals are bouncing all over. It's a 1769-IF8 card. We measured the signal at the wire going into the card with a fluke clamp on meter and the 4-20 mA signal is steady as can be. We have changed the card and the same issue exists.


Any ideas would be much appreciated.


Thanks!

Austin,
Try sourcing a 4-20 mA signal with test source. Your Fluke may be able to source. The raw values should smooth. Also try with all card inputs removed, this should validate your card and noise free PLC power.

Electrical noise is a big problem with analog inputs, check wire shielding is grounded only at one end (the PLC panel end).

Any new VFDs nearby? These are the biggest noise generators.

May need to install 4-20 mA signal isolators. I have fixed several noisy input problems by installing these. They regenerate the 4-20 mA from local power.

Good luck

Paul

Have you tried this..?

http://www.plctalk.net/qanda/showpost.php?p=361883&postcount=8

Set up a dedicated power supply to test it out. Noisy field devices can cause these issues if they are on the same power source

Thanks for the feedback. We had a 24V relay where a 120V was needed,somehow that compressor was still starting!


Scott

Thanks for the follow-up. I'll add that tidbit to the knowledge bank.