Watchdog Reset Program Control

Operands Data Types
None None

Memory Ranges ENO Errors Instruction Support for S7-200 CPUs SIMATIC/International Mnemonics
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.

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 assume the STOP mode within 1.4 seconds.