SIMATIC Instructions
9-146
S7-200 Programmable Controller System Manual
A5E00066097-02
Watchdog Reset
The Watchdog Reset instruction allows the CPU system
watchdog timer to be retriggered. This extends the time
that the scan is allowed to take without getting a
watchdog error.
Operands: None
Considerations for Using the WDR Instruction to Reset the Watchdog Timer
You should use the Watchdog Reset instruction carefully. If you use looping
instructions either to prevent scan completion, or to delay excessively the
completion of the scan, the following processes are inhibited until the scan cycle is
completed.
• Communication (except Freeport Mode)
• I/O updating (except Immediate I/O)
• Force updating
• SM bit updating (SM0, SM5 to SM29 are not updated)
• Run-time diagnostics
• 10-ms and 100-ms timers will not properly accumulate time for scans exceeding
25 seconds
• STOP instruction, when used in an interrupt routine
Note
If you expect your scan time to exceed 300 ms, or if you expect a burst of interrupt
activity that may prevent returning to the main scan for more than 300 ms, you
should use the WDR instruction to re-trigger the watchdog timer.
Changing the switch to the STOP position causes the CPU to transition to STOP
mode within 1.4 seconds.
L
A
D
S
T
L
WDR
WDR
F
B
D
WDR
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