Precautions - Mitsubishi Electric MELSEC iQ-R Series User Manual

Precautions

The following lists the precautions on the scan monitoring function.
Measurement error of watchdog timer
Since the watchdog timer produces an error within the range of 0 to 10ms, take this into consideration when setting the scan
time monitoring time. For example, if the scan time monitoring time is set to 100ms, an error will occur when the scan time
falls within the range 100ms < t < 110ms.
Resetting the watchdog timer when repeatedly executing a program
The watchdog timer can be reset by executing the WDT reset instruction from the program. If the watchdog timer counts up
while a program is being repeatedly executed by the FOR instruction and NEXT instruction, use the WDT reset instruction to
reset the watchdog timer.
Program which processes data repeatedly
M0
Scan time when the WDT reset instruction is used
Even when the watchdog timer is reset by the WDT reset instruction, the scan time is not reset. The scan time is accumulated
until the END instruction is executed.
Internal processing time
END
Watchdog timer reset
(internal processing by the CPU module)
In redundant mode
• During system switching, scan time monitoring with the watchdog timer is interrupted. Thus, no error is detected even if the
scan time monitoring time has elapsed. Therefore, a time taken for system switching does not need to be considered in the
scan time monitoring time setting.
• Scan time monitoring with the watchdog timer is interrupted while the standby system is waiting for tracking data to be
received. Thus, no error is detected even if the scan time monitoring time has elapsed while the standby system is waiting
for tracking data to be received. Therefore, a time taken for tracking data to be received does not need to be considered in
the scan time monitoring time setting.
FOR K1000
Reset the watchdog
WDT
timer.
NEXT
Program
Scan execution
program A
0
Scan time
Watchdog timer measurement time
1000 times
of repeats
Internal processing time
Scan execution
program B
END
Watchdog timer reset
(internal processing by the CPU module)
Scan execution
program A
0
Next scan time
15 RAS FUNCTIONS
15.1 Scan Monitoring Function
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