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Old May 17th, 2014, 11:24 PM   #1
Iman
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Controller Task in Allen Bradley

Hey Guys,
I don't have any experience of Allen Bradley PLCs. I saw a subject in Allen Bradley CPU(Controllogix 1756-L6x) specification table,Controller Tasks, which has 3 parts:
1- 32 tasks
2-100 program/task
3-Event Tasks: all event triggers
I don't understand what the task is. Is it a kind of subroutine which is related to an interrupt or something else? Also in OMRON PLC I've found something similar , Number of tasks, which contains in that specific CPU 2 parts:
1- Cyclic tasks:32
2- Interrupt Tasks:256
Is it the same concept in Allen Bradley?
Thanks in advance for any response.
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Old June 8th, 2014, 12:51 AM   #2
Iman
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Hello Everybody,
I've found the differences and put it here for the others.
I think, Task concept in Allen Bradley is similar to OB block in Siemens or Sweep in GE. But I'm not sure, this is just my understanding from the documents of these companies
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Old June 8th, 2014, 03:39 AM   #3
daba
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I cannot comment on the similarity or otherwise with other controllers, but in the Logix5000 world there are 3 different types of tasks.

1. Continuous Task. This task runs all the time. When it has completed, it automatically restarts. This is the task most users put nearly all of their control code.

2. Periodic Task. This type of task is triggered at a timebase you configure. It will interrupt the processing of the Continuous Task, and any other tasks which have a lower priority.

3. Event Task. This type of task is triggered by a specific event, for example - change of state of input module data-points or status. It will interrupt the processing of the Continuous Task, and any other tasks which have a lower priority.

In ControlLogix - You can have a maximum of 1 Continuous Task, the remainder can be any mix of Periodic and/or Event tasks, up to a maximum of 32 tasks in total. Note that you are not obliged to have a Continuous Task at all.
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Old June 8th, 2014, 04:42 AM   #4
Iman
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Thank you for your response.
Do you agree with my comparison between Siemens, AB and GE?
And I think event task is something similar to Interrupt OB in Siemens.
Thanks
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Old June 8th, 2014, 04:56 AM   #5
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Either of the Event Task or the Periodic Task are "interrupt" type tasks.

They "interrupt" the processing of lower priority tasks. The Continuous Task has the lowest priority, and that can't be changed.

Note that Event or Periodic Tasks with the same priority will "time-slice", so they get as near as possible to equal priority.
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