TASK¶

A task defines the scheduling of program execution within a resource.

IEC 61131-3	Section 2.7.2
Support	Supported

Syntax¶

TASK task_name ( INTERVAL := time_value , PRIORITY := integer_value ) ;

Parameters¶

Parameter	Type	Description
`INTERVAL`	`TIME`	Execution interval (cycle time)
`PRIORITY`	`INT`	Task priority (0 = highest)

Behavior¶

The runtime uses INTERVAL and PRIORITY to schedule task execution during the scan cycle:

Interval controls how often the task runs. On each scheduling round, the runtime checks whether the task’s interval has elapsed since its last execution. If it has, the task is ready to run. A shorter interval means the task runs more frequently but consumes more CPU time.
Priority controls execution order when multiple tasks are ready in the same round. Priority 0 is the highest. Higher-priority tasks always execute before lower-priority tasks. Tasks with equal priority run in declaration order.

If a task takes longer than its interval, the runtime skips the missed cycle and realigns forward. See Execution Cycle for details on overruns, watchdog timeouts, and the full scheduling model.

Example¶

RESOURCE DefaultResource ON PLC
    TASK MainTask(INTERVAL := T#20ms, PRIORITY := 1);
    TASK FastTask(INTERVAL := T#5ms, PRIORITY := 0);

    PROGRAM main WITH MainTask : MainProgram;
END_RESOURCE

Programs are associated with tasks using the WITH keyword. A task executes its associated programs at the specified interval.