Glossary · PLC and control programming
Event task
Also known as: Event-triggered task, Single-shot task
German: Ereignistask
In IEC 61131-3 programming, an event task is a task that executes its assigned programs once each time a specified trigger occurs, typically the rising edge of a Boolean variable, rather than at a fixed interval.
- PLC programming
- Standards
In one sentence
An event task runs its assigned PLC program once each time a trigger, such as a rising edge of a variable, occurs.
Example
Each time the variable NewBatch changes from FALSE to TRUE, an event task runs a program that copies the new recipe into the working data.
How it applies
- Engineering: Event tasks suit work that only needs to happen occasionally, such as initializing data after a recipe change. They reduce controller load compared with checking the condition in every cycle.
- Timing: How fast an event task reacts depends on how often the controller evaluates its trigger and on the priorities of other tasks. It is not automatically faster than a short Cyclic task.
- Commissioning: Test what happens when the trigger fires again while the task is still running, and whether events can be lost.
- Documentation: In the task overview, list each event task with its trigger variable, priority and purpose. Explain in the software reference which function depends on the event, so that nobody removes the trigger by accident.
Event task vs. interrupt task
The term event task in IEC 61131-3 usually refers to a software trigger such as a variable edge. An Interrupt task is started by a hardware or system event, such as an input change on an I/O module, a time-of-day event or a diagnostic alarm. Vendors use both words with different meanings, so check the tool's own definition.