Specifications

SIMATIC Instructions

9-142

S7-200 Programmable Controller System Manual

C79000-G7076-C233-01

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 will cause the CPU to transition to

STOP mode within 1.4 seconds.

WDR

222 224

333

221