Siemens Simatic S7-200 System Manual page 181

Example: Stop, End, and Watchdog Reset Instructions
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
S7-200 Instruction Set Chapter 6
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