The status LEDs located on the processor inform the user of the PLC’s operating mode and any possible errors.
The errors detected by the PLC concern:
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circuits which constitute the PLC and/or its modules: internal errors
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the process driven by the PLC or the process wiring: external errors
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functioning of the application executed by the PLC: internal or external errors
Error detection is performed at startup (autotest) or during operation (which is the case for most equipment errors), during exchanges with the modules, or during execution of a program instruction.
Certain "serious" errors require the PLC to be restarted while others are left to the user to decide how to proceed depending on the level of application required.
There are three types of error:
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non-blocking
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blocking
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processor or system
