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6
FUNCTIONS
6.15 Watchdog Timer (WDT)
(1) Definition of Watchdog Timer (WDT)
The watchdog timer is an internal sequence timer to detect CPU module hardware
and sequence program error.
(2) Watchdog Timer Setting and Reset
(a) Watchdog timer setting
(b) Watchdog timer resetting
(3) When watchdog timer expires
When the watchdog timer expires, a watchdog timer error occurs.
The CPU module responds to the watchdog timer error as follows:
The watchdog timer setting can be changed at the "PLC RAS" tab screen in the
"(PLC) Parameter" dialog box.
The default value of the watchdog timer is 200 ms.
The setting range is 10 to 2000 ms (in 10ms units).
CPU module resets the watchdog timer during the END processing.
• When the END instruction is executed within the set value of the watchdog
timer in the sequence program and the CPU module is operating correctly,
the watchdog timer does not time out.
• When the scan time of a sequence program is extended due to the CPU
module hardware error, and END instruction cannot be executed within the
set watchdog timer value, the watchdog timer times out.
1) The CPU module turns off all outputs.
2) The front-mounted "RUN" LED turned off, and the "ERR." LED starts flicking.
3) SM0, SM1 turns ON and the error code 5001 ("WDT ERROR") is stored into
SD0.
6.15 Watchdog Timer (WDT)
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