Siemens SIMATIC S7-200 System Manual page 187

Figure 6-31 shows the S stack and the logic stack and the effect of executing the Load SCR
instruction. The following is true of Sequence Control Relay instructions:
The Load SCR instruction (LSCR) marks the beginning of an SCR segment, and the SCR
-
End instruction (SCRE) marks the end of an SCR segment. All logic between the Load SCR
and the SCR End instructions are dependent upon the value of the S stack for its execution.
Logic between the SCR End and the next Load SCR instruction is not dependent on the
value of the S stack.
The SCR Transition instruction
-
(SCRT) provides the means to
transfer 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 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 C tl th SCR
remains energized until it is exited.
The Conditional SCR End instruction (CSCRE) provides a means to exit an active SCR
-
segment without executing the instructions between the Conditional SCR End and the
SCR End instructions. The Conditional SCR End instruction does not affect any S bit nor
does it affect the S stack.
In the following example, the first scan bit SM0.1 sets S0.1, which will be the active State 1 on the
first scan. After a 2-second delay, T37 causes a transition to State 2. This transition deactivates
the State 1 SCR (S0.1) segment and activates the State 2 SCR (S0.2) segment.
S7-200 Instruction Set
Load the value of Sx.y onto the SCR and logic stacks.
S stack
Logic stack
Before
t
Figure 6-31 Effect of LSCR on the Logic Stack
Chapter 6
ivS Sx.y
S bit
iv0 Sx.y
iv1
iv2
iv3
iv4
iv5
iv6
iv7
After
iv8
iv1
iv2
iv3
iv4
iv5
iv6
iv7
iv8
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