Chapter 4 CPU modules MASTER-K
4-4
4.2.3 Scan time
The series of steps from step 0 to the next step 0 or from an END instruction to the next
END instruction is called a scan. The scan time is total time spent to execute a scan.
1) The calculation of scan time
The scan time is calculated as a total of the processing time of sequence program (step 0
to the END), interrupt routine, and internal processing of CPU.
Scan time = Sequence program processing time + Interrupt routine processing time
+ Internal processing time
① Sequence program processing time :
The total processing time to execute step 0 to END instruction
② Interrupt processing time :
The total processing time to execute interrupt routine during a scan
③ Internal processing time :
The total processing time to execute self-diagnosis, I/O refresh, timer/counter update,
and communication operation
2) The scan time varies with executing interrupt routine and communication operation or not.
3) The scan time of CPU module is stored in the following special relays (F area).
- F50 word : The maximum scan time (unit : ms)
- F51 word : The minimum scan time (unit : ms)
- F52 word : The current scan time (unit : ms)
4.2.4 Watchdog timer
1) The watchdog timer is an internal timer of the CPU to detect the error of hardware and
sequence program. The default value of watchdog timer is 200msec, and it can be
changed in parameter setting. (setting range : 10 ~ 6000msec, unit : 10msec)
2) When a scan is not completed before, the watchdog timer error occurs and the operation
of CPU is stopped. At this time, all outputs of I/O module are turned off.
3) The watchdog timer is reset before step 0 is executed (after the END processing is
finished) or the WDT instruction is executed. When write a sequence program contains
FOR ~ NEXT loop or a lot of subroutines, increase watchdog timer setting value or put
WDT instruction to avoid watchdog timer error. The setting range of watchdog timer is 10
~ 6000msec
4) When a watchdog timer error occurs, it can be cleared by power cycle, manual reset
switch (K1000S), or mode change.
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