Strange Micrologix Issue

[esamuelson]

Yesterday I went up to replace what I thought was a bad IF2OF2 card. Any 4-20mA device attached to the first input port would show an approximate value (tank level) but jump around wildly (varying by about 3-5%) leaving a big fuzzy line on the trends.

Switching transducers would make the one attached to the first port read strangely (eliminates the idea of a bad transducer). When I check the current with a multimeter or panel meter through the input card, it's rock solid right where I would expect it to be.

I put in the new card and the problem still persists. I checked the circuits, everything is +24V -> transducer -> input card -> -24V.

It may be related to a problem with this PLC I've also been having. I can't do any online edits, it tries and then says "Online Edits Failed" and the spinning ladder logo goes into hyperdrive and I have to go offline and download program changes.

Are we looking at a bad PLC or what?

 

[Mickey]

It might help a little if you tell us which Micrologix you have.

Are there unused channels on your analog card? It sometimes helps to short out unused channel.

 

[esamuelson]

My apologies, its an ML1100. The communication problem is over Ethernet.

There are 2 transducers, and an old IF2OF2 and a new one. I've tried all combinations, but it seems that only one of the ports (out of 4) is reporting accurate values.

 

[Mickey]

There is an issue with online edits with your ML1100 see PDF below.

As far as your analog problem goes , check and double check grounding.

 

[blamb]

If the signal is rock solid with a multimeter and you have ruled out a bad transducer as well as a bad module, I'd say the problem is most likely in the wiring. If this is a new problem has anything recently changed or been added? Are you using twisted shielded pair with proper grounding?

 

[daba]

Just seen a similar issue myself with a controllogix combi card (which is a "High-Speed" card).

I was getting the "Rate-of-Change" alarm, even for slow signal changes.

I got a scope on it and traced high-frequency noise back to the 24V power-supply.

This PS is a switched-mode model, changing it to a linear PS would be the answer, or I can add some additional smoothing etc.