Siemens Simatic S7 Series

Program instructions
7.13 Program control
S7-200 SMART
System Manual, 09/2015, A5E03822230-AC
303
7.13.3
SCR (sequence control relay)
SCR (Sequence Control Relay) instructions provide a simple yet powerful state control
programming technique for a LAD, FBD, or STL program. Whenever an application consists
of a sequence of operations that must be performed repetitively, you can use SCRs to
structure the program so that it corresponds directly to your application. As a result, you can
program and debug the application quickly and easily.
WARNING
S bit usage in POUs
Do not use the same S bit in more than one POU. For example, if you use S0.1 in the main
program, do not use it in a subroutine.
Multiple POUs accessing the same S bit could result in unexpected process operation,
possibly resulting in death or severe personal injury.
Check your program to ensure that multiple POUs do not access the same S bit.
Note
SCR programming restrictions
•
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.