Chapter 6 Function of CPU Module
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Chapter 6 Functions of CPU Module
6.1 Self-diagnosis
(1) Self-diagnosis function means the function that CPU module diagnoses the error of PLC system itself.
(2) If the power of PLC system is applied or the error occurs during operation, it detects the error and prevents
the abnormal operation.
6.1.1 Scan Watchdog Timer
WDT (Watchdog Timer) is the function to detect the program congestion by the error of hardware and
software of PLC CPU module.
1) WDT is the timer used to detect the operation delay by user program error. The detection time of WDT
is set in Basic parameter of XG5000.
2) If WDT detects the excess of detection setting time while watching the elapsed time of scan during
operation, it stops the operation of PLC immediately and makes the output all off.
3) If the excess of Scan Watchdog Time is expected in the program processing of specific part while
performing the user program (FOR ~ NEXT instruction, CALL instruction), clear the timer by using ‘WDT’
instruction.
‘WDT’ instruction initializes the elapsed time of Scan Watchdog Timer and starts the time measurement
from 0 again.
(For further information of WDT instruction, please refer to Instruction.)
4) To clear the error state of watchdog, we can use the following method : power re-supply, manipulation
of manual reset switch, mode conversion to STOP mode.
Notes
1) The setting range of Watchdog Timer is 10 ~ 1000ms (1ms unit).
0 1 2 3 …..
…8 9
SCAN END
WDT Reset
WDT instruction
execution
0 1 2 …
WDT
count(ms)
0 1 2 … …6 7
SCAN END
0 1 2 …
