The state LEDs situated on the processor enable the user to obtain information on operating mode of the PLC and on possible errors.
Errors detected by the PLC concern:
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circuits constituting the PLC and/or its modules: internal errors,
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the process controlled by the PLC or the cabling of the process: external errors,
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operation of the application executed by the PLC: internal or external errors.
Detecting errors is carried out during start-up (self-test) or during operation (this is the case for most hardware errors), during exchanges with modules or during the execution of a program instruction.
Certain "serious" errors require the PLC to be restarted, others are controlled by the user who decides on the behavior to adopt according to the desired level of application function.
There are three types of error:
