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

S7-200 Programmable Controller 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 6-59 Valid Operands for the Sequence Control Relay Instructions
Inputs/Outputs
S_bit
172
Data Types Operands
BOOL S