Sequence Control Relay (Scr) Instructions - Siemens Simatic S7-200 System Manual

Sequence Control Relay (SCR) Instructions

SCR instructions provide you with a simple yet powerful state control
programming technique that fits naturally into a LAD, FBD, or STL
program.
Whenever your application consists of a sequence of operations that
must be performed repetitively, SCRs can be used to structure your
program so that it corresponds directly to your application. As a
result, you can program and debug your application more quickly
and easily.
The Load SCR instruction (LSCR) loads the SCR and logic stacks
with the value of the S bit referenced by the instruction N.
The SCR segment is energized or de-energized by the resulting
value of the SCR stack. The value of the SCR stack is copied to the
top of the logic stack so that boxes or output coils can be tied directly
to the left power rail without an intervening contact.
Restrictions
When using SCRs, be aware of the following restrictions:
-
You cannot use the same S bit in more than one routine. For
example, if you use S0.1 in the main program, do not use it in
a subroutine.
-
You cannot jump into or out of an SCR segment; however, you
can use Jump and Label instructions to jump around SCR
segments or to jump within an SCR segment.
-
You cannot use the END instruction in an SCR segment.
Table 6-57 Valid Operands for the Sequence Control Relay Instructions
Inputs/Outputs Data Types
S_bit BOOL
Operands
S
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