Cyclic task

How it applies

• Engineering: Programs are assigned to tasks with an interval and a priority. Fast, time-critical logic goes into a short-cycle, high-priority task; slow logic such as data logging goes into a longer interval. Some controllers additionally have a free-running cyclic program that restarts as soon as it ends.
• Commissioning: Engineers check the actual execution time of each task against its interval. If a task regularly exceeds its interval, cycles are skipped or the Watchdog timer stops the controller.
• Operation: Execution time varies from cycle to cycle, and higher-priority tasks interrupt lower ones. The resulting Jitter matters for motion control and measurement.
• Documentation: The software documentation should list all tasks with interval, priority, watchdog setting and assigned programs. This task overview is one of the first things a service engineer needs when analyzing timing problems.

Cyclic task vs. event task

A cyclic task runs because time has passed. An Event task runs because a condition occurred, such as a rising edge of a variable. Cyclic tasks give predictable timing; event tasks reduce load when nothing happens.