Object: Task

Symbol:

You configure the task in the object. You can configure the priority, the type with time behavior, and a watchdog. You can also add PROGRAM calls.

The object is located in the device tree below the Task Configuration.

Tab: Configuration

Object: Task

Priority

Possible values: 0..31, where 0 is the highest priority

Task group

Assigned task group which can be assigned to specific processor cores in multicore

Example: IEC-Tasks

Note: The task group is shown in brackets after the task in the device tree.

Important

When using a runtime system in Linux, there exist fixed dependencies between task groups and priorities. For more information, see: Mapping of Task Priorities on a Linux System

Type

Type: Cyclic

Processing of the task is done cyclically.

Input field: Interval

Required

Time span after which the task is restarted (task cycle time)

As a time definition in the format TIME#
Example: t#200ms
As a number
Example: 200
Note: The number is displayed automatically in the format TIME# when the input field is in focus again.

Note: Deviations of the task from this desired task cycle time are displayed at runtime as periodic jitter on the Tab: Monitoring tab.

Time unit of the interval

If only a number and not a time definition is specified in the Interval input field, then the unit selected here determines the time dimensions.

Example: ms

Note: A task cycle time in µs is always displayed as a number.

Type: Event

Processing of the task starts event-triggered on the rising edge of the event variable.

Input field: Event

Global variable (Boolean type)

The task starts as soon as the variable value switches from 0 to 1.

Type: External

Processing of the task starts event-triggered on the rising edge of the event variable.

List box: Event

List with target system-dependent events (Boolean type)

Note: The target system determines which events are supported and offered in the list box.

Hint: Not to be confused with system events

Interval

Time definition in TIME# format or as a number with a time unit

Note: Only available when the event requires a time definition

Type: Freewheeling

Processing of the task restarts automatically in a continuous loop at program start and after the end of a run after a certain waiting time

Important: After completing a run, a certain amount of time is waited before the task is executed again. The duration is a percentage of the last cycle duration.

Note: You do not define a cycle time.

Type: Status

Processing of the task starts state-triggered by the event variables

Input field: Event

Global variable (Boolean type)

When the variable has the state TRUE, the task starts freewheeling. The task runs until the variable gets the value FALSE.

Note: The variable is typically reset in the task itself. In contrast to the event task, no event can be missed in this way. When an event occurs, the scheduler must save an old value, and this can change more often than it is checked. So if an event variable changes to TRUE only for a short time, the scheduler might not detect this change. This can be avoided with a status task. The status variable is set to TRUE by some other task and reset by the status task. This makes sure that the task is executed once each time it switches to TRUE.

Important

For fieldbuses, a fixed cycle matrix is necessary to assure a determined behavior. Therefore, you should not use the Freewheeling type for a bus cycle task.

Important

Note the following difference between the processing types Status and Event: if the given event yields TRUE, then the start condition of a task of type Status is fulfilled. In contrast, the start of a task of type Event requires a switch of the event from FALSE to TRUE. If the sampling rate of the task scheduler is too low, then the rising edge of the event can remain unnoticed.

Important

When setting the task cycle time, you need to identify which bus system is currently being used. For example, the task cycle time in a CAN bus system must match the currently set baud rate and the number of frames used in the bus. In addition, the times set for heartbeat, node guarding, and sync should always be a multiple of the task cycle time. If not, then CAN frames can be lost.

For more information, see: Tab: Monitoring

Watchdog

Defines the time monitoring for a task. If the target system supports an advanced watchdog configuration, then the following settings may be predefined in the device description.

Upper and lower limit
Default watchdog time
Time specified as percentage

The default watchdog settings depend on the device.

Enable

: The watchdog is active.

If the task exceeds the currently set time of the watchdog, then the task is halted with an error status (exception). The application in whose task the error occurred and its child applications are also halted. In this way, all tasks of the affected applications are also halted. Then the currently defined Sensitivity is also taken into account.

If you activate the option Update I/Os in the PLC Settings of the PLC, then CODESYS resets the outputs to the defined default values.

. Possible cases:

Multiple consecutive timeouts:
Sensitivity: 0, 1 – exception in cycle 1
Sensitivity: 2 – exception in cycle 2
Sensitivity: n – exception in cycle n
Single timeout: Exception if the cycle time of the current cycle is longer than (time * sensitivity). Example: Time=t#10ms, Sensitivity=5 (i.e., exception as soon as the one-time task runs longer than 50 ms).

Time (e.g. t#200ms)

Watchdog time

Defines (together with Sensitivity) the watchdog for a task; description as for Enable.

Depending on the target system, the monitoring time span is given as a percentage of the task interval if possible. In this case, the list box for the unit is disabled and displays %.

Sensitivity

Number

Defines (together with the watchdog) the watchdog for a task; description as for Enable.

Tip

Using the functions from the library CmpIecTask.library, you can deactivate a watchdog for specific PLC cycles. This is useful for cycles that demand more time due to initialization.

Example 539. Example

Deactivating/reactivating the watchdog:

VAR
hIecTask : RTS_IEC_HANDLE;
END_VAR

hIecTask := IecTaskGetCurrent(0);
IecTaskDisableWatchdog(hIecTask); //Watchdog disabled
...
IecTaskEnableWatchdog(hIecTask); //Watchdog enabled

The watchdog is deactivated before initialization with IecTaskDisableWatchDog for the rest of the cycle and is automatically reactivated at the next cycle.

The watchdog can be reactivated after initialization with IecTaskEnableWatchDog. The watchdog is then already reactivated for the rest of the cycle (the watchdog time window starts again from the beginning).

Initializations of function blocks which happen within the FB_Init method are not affected by this. But there was the limit of < 30 seconds because of the communication timeout. This time limit no longer exists since V3.5 SP18 because the online services are executed asynchronously.

Tip

The normal watchdog of an IEC task is triggered when the execution time of the IEC task exceeds the watchdog time.

The "Omitted Cycle" watchdog is triggered when the task does not start at all. This is the case when the task does not execute any cycle at all within the maximum of <Time * Sensitivity> or <2 * Interval>. The cause could be crowding by other tasks or a failure in the scheduler, which no longer enables the task.

Program calls

POUs (POU object with type `PROGRAM`) which are called by the task in succession The order in which the POUs are called corresponds to the call order configured here from top to bottom.
Add Call	Opens the Input Assistant. Select a `PROGRAM` POU. When you click OK to exit the Input Assistant, this is added to the list below as a new call.
Remove Call	Removes the call
Change Call	Opens the Input Assistant for the selected call. Select another `PROGRAM` POU. The call is changed when you click OK to exit the Input Assistant.
Move Up Move Down	Moves the position of the call up or down in the list. The call order changes accordingly at runtime.
Open POU	Opens the POU editor with the declaration and implementation of the `PROGRAM` POU