Watchdog Timer - Mitsubishi Q2ACPU User Manual

9. MAINTENANCE FUNCTION
9.2

Watchdog Timer

(1)
REMARK
(2)
REMARK
Watchdog timer (WDT)
The watchdog timer is an internal timer of PLC that detects PLC CPU hardware
errors and sequence program errors.
200ms is set as the default setting for this timer.
The time set for the watchdog timer can be changed using "WDT" in PC RAS setting
in the GPP function parameter mode.
The setting range is 10ms to 2000ms (in 10ms units).
Resetting the watchdog timer
The QnACPU resets the watchdog timer during END processing.
When the QnACPU is normally operating and executing the END instruction within
the setting value of the watchdog timer, the watchdog timer does not give time-out.
WDT times out when the END instruction is not executed within the value set for the
watchdog timer due to a QnACPU hardware error or an excessively long sequence
program scan time.
Scan time is the time taken for the execution of the sequence program, starting from
step 0 and ending at step 0.
The scan time is not the same in every scan: it differs according to the execution or
non-execution of the instructions used in the program. (Refer to Section 12.1.)
Scan execution
0
type program A
WDT reset
(Internal processing
of a PLC)
Sequence program Internal processing time
Scan execution
END
type program B
Scan time
Time measured by watchdog
timer
Fig. 9.1 Resetting the watchdog timer
9 - 2
Low-speed execution
type program C
0
Next scan time
WDT reset
(Internal processing of a PLC)