Some Basic Facts About Iec Tasks; Flowchart Of An Iec Task - WAGO 750-872/020-000 Manual

84 • Fieldbus Controller
Fieldbus Controller 750-872/020-000

3.1.8.3 Some Basic Facts about IEC Tasks

Attention
Consider please with the programming of your IEC tasks the following facts.
• All IEC tasks must have a different priority level. If two tasks have the
• A running task can be interrupted by a task of higher priority. The
• If several IEC tasks use the same input/output variables from the process
• After every task cycle, freewheeling tasks are stopped for half of the time
•
• Only for cyclic tasks the sensitivity value is used. Sensitivity values of 0
• To cyclic tasks with activated watchdog applies:

3.1.8.3.1 Flowchart of an IEC Task

• Get system time (tStart).
• If the last I/O bus cycle is not complete.
• Read the inputs and the outputs from the process image.
• If the user application program is running.
• Write the outputs into the process image.
• Get system time (tEnd).
same level, an error message is displayed when the program is compiled.
interrupted task will resume execution after all higher priority tasks are
completed.
image, the values of the input /output variables can change during the
execution of each IEC task, causing contention in the application program.
needed by the task itself (min. 1 ms). After that, a new task is started.
Example: First task 4 ms
Second task 2 ms
If no task is applied in the task configuration, a freewheeling default task is
applied when the program is compiled. The watchdog of this task is
deactivated. This task, named "DefaultTask", is internaly identified with
this name, so don't use this name for your own tasks.
and 1 are synonymic. A value of 0 or 1 means that the watchdog triggers on
a single cycle time overrun. With a sensitivity value of 2 for example, the
watchdog triggers on two consecutive cycle time overruns.
- Is the adjusted maximum runtime less than the sampling rate, the
watchdog will also trigger if the runtime exceeds the sampling rate,
irrespective of the value that has been entered for the sensitivity.
- Is the adjusted maximum runtime greater then the sampling rate, the
watchdog will trigger if the maximum runtime is exceeded, irrespective of
the value that has been entered for the sensitivity.
-> Wait for the end of the next I/O bus cycle.
-> Execute the program code of this task.
-> tEnd - tStart = run-time of the IEC task.
--> Waiting time 2 ms
--> Waiting time 1 ms
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