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3.16
Watchdog Timer
This function serves as a CPU module internal timer to detect errors of CPU module hardware and sequence
programs.
(1) Setting and resetting
(a) Setting
The watchdog timer setting can be changed in the PLC RAS setting of PLC parameter.
The default is set to 200ms.
The setting range is 10 to 2000ms (in increments of 10ms).
(b) Reset
The CPU module resets the watchdog timer during END processing.
• The watchdog timer does not time up when the CPU module operates normally and the END/FEND
instruction is executed within the setting value of watchdog timer.
• The watchdog timer times up when the scan time of the sequence program is extended and the
END/FEND instruction could not be executed within the setting value of watchdog timer due to the
hardware failure of the CPU module or execution of an interrupt program/fixed scan execution type
program.
(2)
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 on the front of the CPU module turns off and the ERR. LED starts flashing.
• SM1 turns on and the error code 5000 or 5001 is stored in SD0.
(3)
Precautions
(a) Watchdog timer error
An error is observed within the range of 0 to 10ms.
Set a watchdog timer while considering such an error.
(b) Resetting a watchdog timer when a program is repeatedly executed between the
FOR and NEXT instructions
The watchdog timer can be reset by executing the WDT instruction in the sequence 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.
MO
(WDT)
FOR K1000
Program repeatedly executed
WDT
NEXT
Repeated in
1,000 times.
WDT reset
CHAPTER 3 FUNCTIONS
193
3
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