Siemens SIMATIC S7 System Manual page 348

Program instructions
7.13 Program control
SCR operations
● SCR (Load SCR) marks the beginning of an SCR segment, and the SCRE (End SCR)
marks the end of an SCR program segment. All logic between the SCR and the SCRE
instructions is dependent upon the value of the S stack for its execution. Logic between
SCRE and the next SCR instruction is not dependent on the value of the S stack.
● SCRT (SCR Transition) transfers control from an active SCR segment to another SCR
segment.
Execution of the SCR transition instruction, when it has power flow, will reset the S bit of
the currently active SCR segment and will set the S bit of the referenced segment.
Resetting the S bit of the active segment does not affect the S stack at the time the SCR
Transition instruction executes. Consequently, the SCR segment remains energized until
it is exited.
● The STL only instruction CSRE (Conditional SCR End) exits an active SCR segment
without executing the instructions between the CSRE and the SCRE (SCR End)
instructions. The Conditional SCR End instruction does not affect any S bit nor does it
affect the S stack.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART