Chapter 5 CPU Module
5-
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5.5 Functions
5.5.1 Self-diagnosis
1) Functions
(1) The self-diagnosis function permits the CPU module to detect its own errors.
(2) Self-diagnosis is carried out when the PLC power supply is turned on and when an error occurs the PLC is in the RUN
state. If an error is detected, the system stops operation to prevent faulty PLC operation.
2) WDT (Watch dog timer) function
The watch dog timer is an internal timer of a PLC to detect the error of hardware and a sequence program. The default
value is set as 200msec, and it is changeable with parameter setting. Refer the MASTER-K programming manual for
details on the parameter setting.
The CPU resets the watch dog timer before step 0 is executed (after the END processing is finished). When the END
instruction has not been executed within the set value due to an error occurred in the PLC or the long scan time of a
sequence program, the watch dog timer will times out. When a watch dog timer error is occurred, all outputs of the PLC are
turned OFF, and the ERR LED of the CPU will flashes. (RUN LED will be turned OFF) Therefore, when use FOR ~ NEXT
or CALL instruction, insert WDT instruction to reset the watch dog timer.
3) Battery check function
When the voltage of the battery for back-up the memory IC of CPU are lower than the minimum back-up voltage, the BAT
LED of CPU module will be turned on.
