A Modicon application can schedule several tasks with different priorities. The MAST task does the everyday work, sliced into ordered Sections. The FAST task cuts in for time-critical checks. An EVENT task responds to a configured event. The pumping-station simulation shows the tasks competing for CPU time and lets you trigger an event to inspect the order of execution.
The wet well fills with inflow. MAST runs the level control in three Sections every period; FAST checks the high level on a shorter periodic cycle, while the EVENT task responds to the simulated high-level float. Increase the inflow, then select 🔔 Trip high-level float to see the event task interrupt the lower-priority work shown in the scheduler. These are ordinary process-control examples; a safety function requires an appropriate safety relay or safety PLC.
The red EVENT bar appears on top of the green MAST bar because the event interrupts that simulated execution. MAST resumes afterward. FAST runs at its configured interval with priority over MAST, but all tasks still depend on the available CPU time and scheduling rules; a short period is not a guarantee of zero response delay.
There's also an AUX task for additional work on supported CPUs. Compare the supported task types and priorities before deciding where logic belongs. Higher-priority execution reduces the CPU time available to other work, and the task's watchdog provides execution-time monitoring. Check the configured limit and CPU response rather than assuming every missed period has the same effect.
A process-control high-level response must run sooner than the 50 ms MAST period. Which task suits that ordinary control check? If the response is safety-related, it belongs in an appropriate safety relay or safety PLC, not merely a higher-priority standard task.