PLC5 analog input vibration value

[Hani ahmed]

we have a problem with redundant processor PLC5 where there is analog input value be stable with the primary processor.
and at fault when working with the secondary processor the analog input value vibrate around it's real value where this vibration shown at HMI screen.
So we had to use rslogix5 compare tool to compare between the two application programs where the result indicated that the ladders are the same as shown in the attachment.
so could any please advise us why this vibration happens?

 

[Lancie1]

I would guess that it could be a problem with isolation or common grounding between the two analog input modules.
Are the Analgog Input modules the Isolated Analog Input Modules?

If not, then any specific analog input can only be "common" to one of the two analog moudles, and the connection to the other one could be unstable.

 

[Hani ahmed]

thanks Lanciel1 for your interest but the control system is controller redundant only and not I/O's redundancy.

 

[Lancie1]

Hmmm....yes, that makes a difference. If the SAME input is showing up differently on two processors, it could be a timing issue - perhaps the secondary processor does not receive the signal at the same time as the primary processor.