Siemens SIMATIC S7 System Manual page 355

Example: STOP, END, and WDR (Watchdog reset) instructions
LAD
Note
If you expect your scan time to exceed 500 ms, or if you expect a burst of interrupt activity
that prevents returning to the main scan for more than 500 ms, you should use the watchdog
reset instruction to retrigger the watchdog timer.
Use the watchdog reset instruction carefully. If program execution loops prevent scan
completion or excessively delay the completion of the scan, then the following processes are
inhibited until the scan cycle is completed.
• Communications (except Freeport mode)
• I/O updating (except Immediate I/O)
• Forced values updating
• SM bit updating (SM0, SM5 to SM29 are not updated)
• RUN-time diagnostics
• STOP instruction, when used in an interrupt routine
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
When an I/O error is detected,
force the transition to STOP
mode.
When M5.6 is ON, execute the
watchdog reset instruction to
extend the allowed scan time by
500 milliseconds.
When I0.0 is ON, terminate the
current scan.
Program instructions
7.13 Program control
STL
Network 1
LD SM5.0
STOP
Network 1
LD SM5.6
WDR
Network 1
LD I.0
END
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