redundant system bus

What exactly is a system bus in the context of PLCs?Is it the internal bus within the CPU or the communication bus from PLC?If it is the internal CPU bus how do I ensure it is redundant?

It's pretty much the same (not to say exactly the same) as a systembus in the context of a PC: the connection between the CPU and the peripherals . After all: a PLC is only a computer especially designed to control industrial equipment.

To ensure it is redundant, you have to buy a PLC with a redundant systembus. Read the specifications.

Kind regards,

I may be wrong here but it sounds like you are looking for a redundant system in case you PLC, or a peripheral, dies. If that is the case there are several options from different suppliers. AB, Omron, and Seimens all have a completely bumpless redundant PLC backups. They all have a fiber optic link (bus) between two identical PLC/racks. If one dies the other takes over without affecting the process.