PetroStreet

Is there any impact on the scan time of a PLC processor when used in a redundant configuration?
If so, can anyone explain why?

No, there shouldn't be an impact when you're using the PLC in FULL REDUNDANT configuration. Depends up to what level you are enforcing the redundancy. If you're using redundant I/O modules but a single processor, then obviously there's going to be an impact on the scan time. But if you use full redundant, meaning redundant I/O modules, plus redundant processor, there shouldn't be an impact at all.

Either way, the PLC vendor will be able to give you a better picture, because many vendors offer in-card redundancy, the scan time in which case will depend on their internal architecture.

My question was regarding a fully redundant configuration, i.e. redundant I/O cards plus redundant processor.
I agree that every PLC has a distinct approach of providing redundancy. In some cases, each processor communicates with the I/O cards independently for updation of data while in the rest, one of the processors (the Primary/Master) communicates with the I/O cards and the other (the Slave/Secondary/Backup) merely copies data from the Primary. There might be other approaches as well, depending from vendor to vendor, of course.

Well the 2 most widely used PLC's I can think of are Allen Bradley Controlgix and Siemens S7. So if somebody can answer with respect to these specific ones it should give a reasonable idea.

I think the best people to answer that would be AB or Siemens guys. Just give them a call, vendors are always more than happy to help.