Program Control Instructions; Conditional End; Stop; Watchdog Reset - Siemens SIMATIC S7-200 System Manual

Program Control Instructions

Conditional End

The Conditional End instruction (END) terminates the
current scan based upon the condition of the preceding
logic. You can use the Conditional End instruction in the
main program, but you cannot use it in either subroutines or
interrupt routines.

Stop

The Stop instruction (STOP) terminates the execution of
your program by causing a transition of the S7-200 CPU
from RUN to STOP mode.
If the Stop instruction is executed in an interrupt routine, the
interrupt routine is terminated immediately, and all pending
interrupts are ignored. Remaining actions in the current
scan cycle are completed, including execution of the main
user program, and the transition from RUN to STOP mode is
made at the end of the current scan.

Watchdog Reset

The Watchdog Reset instruction (WDR) retriggers the system watchdog timer of the S7-200 CPU
to extend 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:
Communications (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
-
Expansion modules with discrete outputs also include a watchdog timer that turns off
-
outputs if the module is not written by the S7-200. Use an immediate write to each
expansion module with discrete outputs to keep the correct outputs on during extended
scan times. Refer to the example that follows this description.
S7-200 Instruction Set Chapter 6
167