Sequence Control Relay Program Control

Inputs/Outputs Operands Data Types
n S BOOL

Memory Ranges ENO Errors Instruction Support for S7-200 CPUs SIMATIC/International Mnemonics
The Load Sequence Control Relay instruction marks the beginning of an SCR segment. When n = 1, power flow is enabled to the SCR segment. The SCR segment must be terminated with an SCRE instruction.

The Sequence Control Relay Transition (SCRT) instruction identifies the SCR bit to be enabled (the next n bit to be set). When power flows to the coil or FBD box, the referenced n bit is turned on and the n bit of the LSCR instruction (that enabled this SCR segment) is turned off.

The Sequence Control Relay End (SCRE) instruction marks the end of an SCR segment.

Sequential Control Relays provide applications with a technique for organizing machine operations or steps into equivalent program segments. Segmenting areas of the user program allows easier editing and debugging.

The following is true of Sequence Control Relay instructions.

ˇAll logic between the LSCR and the SCRE instructions make up the SCR segment and are dependent upon the value of the S stack for its execution. Logic between the SCRE and the next LSCR instruction have no dependency upon the value of the S stack.

ˇThe SCRT instruction sets an S bit to enable the next SCR and also resets the S bit that was loaded to enable this section of the SCR segment.

Restrictions for using SCRs follow.

ˇYou 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 the subroutine.

ˇYou cannot use the JMP and LBL instructions in an SCR segment. This means that jumps into, within, or out of an SCR segment are not allowed. You can use jump and label instructions to jump around SCR segments.

ˇYou cannot use the FOR, NEXT, and END instructions in an SCR segment.

SCR Usage