Rockwell Automation Publication 1756-RM006K-EN-P - November 2018 475
Valid command states for the SCMD instruction
Use the SCMD instruction transitions to command an Equipment Sequence to
another state. The SCMD instruction commands may only be processed in certain
states. The following table lists the states in which commands are valid.
Command Valid in these states
START Valid in the IDLE state.
RESTART Valid in the HELD state.
HOLD Valid in the RUNNING and RESTARTING states.
STOP Valid in the RUNNING, HOLDING, RESTARTING, and HELD states.
ABORT Valid in the RUNNING, HOLDING, RESTARTING, STOPPING, and HELD states.
RESET Valid in the ABORTED, STOPPED, and COMPLETE states.
Arithmetic status flags and fault conditions
Arithmetic status flags are not affected by the SCMD instruction. The SCMD
instruction cannot trigger a fault, so there are no fault conditions for this
instruction.
Execution
Ladder Diagram
Condition Action Taken
Prescan The rung-condition-out is set to false.
Rung-condition-in is false The rung-condition-out is set to false.
Rung-condition-in is true The instruction executes.
The rung-condition-out is set to true.
Scan of structured text No action taken
Instruction execution The instruction tries to take ownership of the specified Equipment
Sequence.
Postscan The rung-condition-out is set to false.
Structured Text
Condition Action Taken
Prescan No action taken.
Rung-condition-in is false No action taken.
Rung-condition-in is true No action taken.
Scan of structured text In structured text, instructions execute each time they are scanned. To
limit the scan of an instruction, use a qualifier of an SFC action or a
structured text construct.
Instruction execution The instruction tries to take ownership of the specified Equipment
Sequence.
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