Execution Rules

The execution of a control application is a synchronous system where a clock triggers all operations for a device. The cycle timing performs these steps:
  1. Scans input variables
  2. Processes program organization units (POUs)
  3. Processes
    Modbus®
    messages
  4. Updates output devices
  5. Saves retained values
  6. Sleeps until the next cycle
With a specified cycle time, a device waits until this time elapses before starting the execution of a new cycle. The POU's execution time varies depending on the size of the application. When a cycle exceeds the specified time, the loop continues to execute the cycle but sets an overrun flag. The application no longer runs in real time. Without a specified cycle time, a device performs all programs then restarts a new cycle without waiting.
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