Siemens Simatic S7-200 System Manual page 280

Instruction Set
Sequence Control Relay Instructions
L
A
D
S
LSCR
T
L
SCRT
SCRE
212 214
Understanding SCR Instructions
In ladder logic and statement list, Sequence Control Relays (SCRs) are used to organize
machine operations or steps into equivalent program segments. SCRs allow logical
segmentation of the control program.
The LSCR instruction loads the SCR and logic stacks with the value of the S-bit referenced
by the instruction. The SCR segment is energized or de-energized by the resulting value of
the SCR stack. The top of the logic stack is loaded to the value of the referenced S-bit so
that boxes or output coils can be tied directly to the left power rail without an intervening
contact. Figure 10-38 shows the S stack and the logic stack and the effect of executing the
LSCR instruction.
BEFORE
initial value
of s
Figure 10-38
10-92
The Load Sequence Control Relay instruction marks the
beginning of an SCR segment. When n = 1, power flow is
n
enabled to the SCR segment. The SCR segment must be
SCR
terminated with an SCRE instruction.
n
The Sequence Control Relay Transition instruction identifies
SCRT
the SCR bit to be enabled (the next S bit to be set). When power
flows to the coil, the referenced S bit is turned on and the S bit
of the LSCR instruction (that enabled this SCR segment) is
SCRE
turned off.
The Sequence Control Relay End instruction marks the end of
an SCR segment.
n
n
Operands:
215 216
Load the value of Sx.y onto the SCR and logic stacks.
S stack Logic stack
ivs iv0
iv1
iv2
iv3
iv4
iv5
iv6
iv7
iv8
Effect of LSCR on the Logic Stack
n: S
LSCR
AFTER
S stack
S-bit
Sx.y
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02
Logic stack
Sx.y
iv1
iv2
iv3
iv4
iv5
iv6
iv7
iv8