Mitsubishi Q2ACPU, User Manual

Page: 158 / 570

9.
9 - 2
MAINTENANCE FUNCTION
9.2 Watchdog Timer
(1) 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.
REMARK
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).
(2) 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.
REMARK
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.)
Fig. 9.1 Resetting the watchdog timer
0
Scan execution
type program A Scan execution
type program B
Low-speed execution
type program C
END
Scan time
Time measured by watchdog
timer
WDT reset
(Internal processing of a PLC)
Sequence program Internal processing time
0
Next scan time
WDT reset
(Internal processing
of a PLC)