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3.3 Watchdog Timer (WDT)
WDT is an internal timer of the CPU module that detects CPU module hardware failures and program errors.
(1) Setting
Open the tab for watchdog timer setting. The setting range is 10 to 2000ms (in increments of 10ms).
200ms is set by default.
Project window [Parameter] [PLC Parameter] [PLC RAS]
A precision error is observed within the range of 0 to 10ms.
(2) Reset
WDT is reset in END processing. The watchdog timer times up when the scan time is extended due to a factor
such as an interrupt, and the END/FEND instruction were not executed within the setting value of watchdog timer.
When the watchdog timer times up, "WDT ERROR" is detected and the following status occurs.
• The CPU module turns off all outputs.
• The RUN LED turns off and the ERR.LED starts flashing.
• SM1 turns on and the error codes 5000 and 5001 are stored in SD0.
The watchdog timer can be reset by executing the WDT instruction in a program. To avoid the time up of
watchdog timer while a program is repeatedly executed between the FOR and NEXT instructions, reset the
watchdog timer by the WDT instruction.
Note that the scan time value is not reset even after the watchdog timer is reset. The scan time is measured up to
the END instruction.
Enter a value.
Repeatedly executed program
WDT reset
Repeated by
1,000 times.
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