Watchdog Timer (Wdt) - Mitsubishi Electric MELSEC-Q Series User Manual

7 FUNCTION

7.11 Watchdog Timer (WDT)

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(1) What is Watchdog Timer (WDT)?
(a) The watchdog timer is an internal sequence timer to detect Basic model
QCPU hardware and/or sequence program error.
(b) When the watchdog timer expires, a watchdog timer error occurs.
The Basic model QCPU responds to the watchdog timer error in the
following way.
1) The Basic model QCPU turns off all outputs.
2) The front-mounted RUN LED goes off, and the ERR. LED starts
flashing.
3) SM1 turns ON and the error code is stored in SD0.
(c) The default value of the watchdog timer is 200 ms.
The setting range is 10 to 2000 ms (10ms units).
(2) Watchdog Timer Setting and Reset
(a) The PLC RAS setting of the PLC parameter resets the watchdog timer.
(b) Basic model QCPU resets the watchdog timer when the END instruction is
executed.
1) When the END/FEND instruction is executed within the set value of the
watchdog timer in the sequence program and the Basic model QCPU
is operating correctly, the watchdog timer does not time out.
2) When there is a Basic model QCPU hardware failure or the sequence
program scan time is too long, and the END/FEND instruction could
not be executed within the set watchdog timer value, the watchdog
timer times out.
(3) Precautions
(a) An error of 0 to 10 ms occurs in the measurement time of the watchdog
timer. Set the watchdog timer for a desired value by taking such an error
into account.
(b) The watchdog timer is reset with the WDT instruction in the sequence
program. If the watchdog timer expires while the FOR and NEXT
instructions are repetitiously executed, reset the watchdog time with the
WDT instruction.
Program for repetition processing
M0
FOR K1000
WDT reset
WDT
NEXT
MELSEC-Q
Repetition
(1000 times)
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