AB PLC-5 1771-IR PV weirdness

These two RTD's are wired to channels 1 and 2 on a 1771-IR in a remote rack, communicating to a PLC-5/40 through an ASB. I believe that the RTD's are in bearings and go through a junction box before entering the I/O panel and landing on the 1771-IR.

What could cause these two values to exhibit this behavior?

WAG: VFD noise?

I believe that the RTD's are in bearings

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Are you trying to control the bearing temperatures? Because it looks like there is a response to the change in signal levels as if some sort of controller is working. As to the source of the jump, look for what else might be starting and stopping when the signal levels jump.

No VFD's in the area and no equipment is being started or stopped when the temperatures do this flipping.

The RTD's are not being controlled, they are being just being monitored for high temperature shutdown of the equipment.

The RTD's are not being controlled, they are being just being monitored for high temperature shutdown of the equipment

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Then I've got to ask if you're sure you're looking at the RTD signal. In the top trace, the trend shows a rising signal before the upset, then it shows a falling signal, exactly the reaction you'd expect from a sudden jump in the PV in a system controlled by a PID. Same thing in reverse for the bottom trace.
Are you looking at the raw signal from analog input channel or a scaled value?

we need a lot more information – but just as a GUESS ...

the two RTDs are working fine – but the graphing that you've provided has the polarity setting reversed for one of the signals ...

in simple terms, one of the traces is being graphed as a mirror-image of its actual self ... the signal is OK - but the picture is showing the data upside down ...

like I said – this is just a GUESS ...

good luck with it ...

stay safe – stay well ...

Interesting idea on the polarity, Ron. I guess I'll ask a follow-up that will likely betray my ignorance: if an RTD were to be wired to the module incorrectly, could the value still show what appears to be a valid number, but the value would decrease as the temperature rises and increase as the temperature falls? Or is it possible that the physical RTD itself is faulty and causes an increase in ohms when it should decrease and vice versa?

In my experience with RTD's, if they are not wired correct they read an off scale, not an apparently "valid" temp.

If you are suspecting some kind of issue with the software that shows the trends, I don't think that this is the issue but I will setup a live trend in RSLogix to see if the raw values in the block transfer data table match what the external trending software is showing.

Thanks to all for the suggestions!

If you are suspecting some kind of issue with the software that shows the trends,

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that's what I'd bet on - but I wouldn't bet more than pocket change ...