Scan Time for PLC Processors

[ali.abbas]

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

[AbsarShah]

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.

[ali.abbas]

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.

[AbsarShah]

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.