Specifications

SIMATIC Instructions
9-153
S7-200 Programmable Controller System Manual
C79000-G7076-C233-01
Sequence Control Relay
The Load Sequence Control Relay instruction marks
the beginning of an SCR segment. When the S bit is on,
power flow is enabled to the SCR segment. The SCR
segment must be terminated with an SCRE instruction.
The Sequence Control Relay Transition instruction
identifies the SCR bit to be enabled (the next S bit to be
set). When power flows to the coil or FBD box, the
referenced S bit is turned on and the S bit of the LSCR
instruction (that enabled this SCR segment) is turned
off.
The Sequence Control Relay End instruction marks the
end of an SCR segment.
Inputs/Outputs Operands Data Types
n S BOOL
Understanding SCR Instructions
In LAD and STL, 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 9-57 shows the S stack and
the logic stack and the effect of executing the LSCR instruction.
L
A
D
S
T
L
LSCR S bit
SCRT S bit
SCRE
SCRT
SCR
S bit
S bit
SCRE
F
B
D
SCRE
SCR
SCRT
S bit
222 224
333
221
S bit