Siemens SIMATIC S7-200 System Manual page 182

S7-200 Programmable Controller System Manual
Tip
If you expect your scan time to exceed 500 ms, or if you expect a burst of interrupt activity that
could prevent returning to the main scan for more than 500 ms, you should use the Watchdog
Reset instruction to retrigger the watchdog timer.
Each time you use the Watchdog Reset instruction, you should also use an immediate write to
one output byte (QB) in each discrete expansion module to reset each expansion module
watchdog.
If you use the Watchdog Reset instruction to allow the execution of a program that requires a
long scan time, changing the mode switch to the STOP position causes the S7-200 to transition
to STOP mode within 1.4 seconds.
Example: Stop, End, and Watchdog Reset Instructions
168
Network 1 //When an I/O error is detected:
//Force the transition to STOP mode.
LD SM5.0
STOP
Network 2 //When M5.6 is on, allow the scan to
//be extended:
//1. Retrigger the Watchdog Reset for the S7-200.
//2. Retrigger the watchdog for the
first output module.
LD M5.6
WDR
BIW QB2, QB2
Network 3 //When I0.0 is on, terminate the current scan.
LD I0.0
END