Allenbradely PLC analog inputr module 1756-IF8

Hi all,
iam facing problem in allenbradely analogue input module 1756-IF8, i.e one of the channel continuously fluctuating (CH NO-4). Iam not able to find out root cause. Kindly help me in this matter. I have checked field side and module replaced but problem not yet solved.

Move the signal to one of the other channels and see what it does. Simple process of elimination.

1. "Continuous flucutations" is a common description for what people use when a PID loop is not turned properly and the process cycles. Fluctuations due to incorrect PID tuning are not solved by blaming the analog input.

Is continuous fluctuations "noise"? What are the characteristics of the fluctuations? Using numbers, describe what is happening.

2. Check the output of the field device to see if it is behaving exactly as expected with a separate calibrator/DVM/ whatever (check it at the PLC using the wiring to the PLC).

Substitution of a field device does not neccessarily catch a problem that is affecting the operation a field device that is merely a reflection of a systematic problem outside of the device, for instance, a DP transmitter's impulse tubing is leaking or is blocked. A working, functional replacement will indicate the same problem as the functional device being replaced.

3. Check the AI channel with a simulated input (4-20mA?) to see if it is stable.

yes also check the earthing, the screen of the cable must be properly connected in one side only to instrument earth, otherwise you will create current loops.

Also check that noise isn't being introduced on that channel's cable run by being in close proximity to cables carrying large currents, VFD motor cables, etc.

If the instruments connected to this analog input are loop powered, then this fluctuation of analog input signal may have to do with loop resistance. i.e @ close to 100% process measurement current loop will be close to 20 mA and the loop resistance will drop the voltage across the transmitter to below what it requires causing it to shut down (if it is of smart type) the current of the loop then decreases increasing the voltage across the transmitter which will cause it to boot-up, then the transmitter trying to match the process measurement of nearly 100% will try to inject close to 20 mA again in the loop and the voltage drop will occur again resulting in it shut-down. and it goes again and again which causes your fluctuation.
Hope you understand me, you may want to check it out.