Execution rules
Executing a control application for a resource follows eight main steps within a loop. The duration of this loop is defined as the cycle timing for a resource.
- Scan input variables
- Consume bound variables
- Execute Program Organization Units (POU)
- Produce bound variables
- Update output variables
- Save retained values
- Process IXL messages
- Sleep until next cycle
Execution rules
In a case where bindings are defined, variables consumed by a resource are updated after the inputs are scanned and the variables produced for other resources are sent before updating outputs.
When a cycle timing is specified, a resource waits until this time has elapsed before starting executing a new cycle. The POU execution time varies depending on the number of active steps in sequential function chart (SFC) programs and instructions such as jumps, IFs, and returns. When a cycle exceeds the specified time, the loop continues to execute the cycle but sets an overrun flag. In such a case, the application no longer runs in real time.
When a cycle time is not specified, a resource performs all steps in the loop, and then restarts a new cycle immediately.
All digital output variables driven by the I/O scan are cleared on a power-up or when transitioning to Run.
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