Redundancy In Plc

Hello Everyone. I Am Confused With The Terms "redundancy" Used For Plc. Can Anybody Explain?

Part of hardware identical with the same function operating in standby-alert. In the eminence of fails the redundant component assumes the control.

Some sentences:
1) Redundant is configured / enable in register of hardware.
2) Does not require, redundancy of software (allways?)
3) Exists bits, register of status of the operation of the redundant module.

Have the levels redundancy: Supply alimentation, processing and elements I/O. Trink is prioritary order
Also comunication include redundancy modules.

More questions:
A)Possible force operation redundant module via software?
B)Test standart in power-up check module redundant?
C)Have permanent check-list in module redundant it in standy-by? Or only is enable.

It's as simple as backup PLC just in case the primary fails.

In hardwire cat 4, the modules are redundant, because they check for the case using two seperate methods. This means it would be hard for a fluke to break both inspection routines...

Redundant, 2 ways to do a task, that check on each other.