Mitsubishi Electronics 700 Series ユーザーズマニュアル

(1) Watchdog timer

A watchdog timer is the internal timer of the built-in PLC function designed to
detect hardware or sequence program faults.
Its default value is set to 200ms.

(2) Watchdog timer resetting

The built-in PLC function resets the watchdog timer before execution of step 0
(after execution of END processing).
When the built-in PLC function operates properly and the END instruction is
executed within the setting in the sequence program, the watchdog timer does
not time out.
If the hardware fault of the built-in PLC function occurs or the scan time of the
sequence program is too long to execute the END instruction within the setting,
the watchdog timer times out.

Fig 3.23 Watchdog Timer Resetting

(3) Processing performed when watchdog timer times out

If the scan time exceeds the watchdog timer setting, a watchdog timer error
occurs and:
1) The built-in PLC function turns off all outputs.
2) The P.RUN LED goes off or flickers.
3) M9008 turns on and the error code is stored into D9008.

The watchdog timer setting can be changed by built-in PLC function parameter setting of GX
Developer. (Refer to the GX Developer manual for details.)

Sequence program

END

0

0

WDT resetting

(Internal processing)

Internal 

processing time

Excess of scan time over setting
results in watchdog timer error.