Continuous vs Periodic Tasks in PLC

[JeremyM]

Task overlap is saying the code can not be executed within the time period. Which is bad!!!

Correct. Specifically, it means that a task is overlapping itself.

 

[JeremyM]

I would also add that in ControlLogix, the continuous task has the lowest priority of all. I/O is also a task with a priority of 6. People use the continuous task because that's what they're used to from PLC-5/SLC-500, because Studio5000 generates one by default, (unless you use a "P" model which generates the PlantPax tasking model) & because they are too lazy to read about the hardware they're programming. Case closed.
IMNSHO, Rockwell has done a poor job of communicating the advantages of this model, which is after all based on IEC 11631-3

Do you have a link to the documentation about the ControlLogix IO task?

I only ask because of the attached.

 

[Ron Beaufort]

Disclaimer: I retired about three years ago - so things MIGHT have changed since then ...

but ...

the "I/O Task" (priority of 6) being mentioned in this thread relates to the COMPACT-Logix platform ... the CONTROL-Logix platform does not have a separate task to manage its inputs and outputs ...

or at least that's the way it used to be back when I dealt with this stuff ... I have NOT been keeping up with the latest developments - so take this with a grain of salt ...

stay safe - stay well ...
__________________

 

[Aardwizz]

I kinda like using the most deterministic solution

You're valuing determinism in situations where it isn't required.

PIDs and Totalizers -- determinism is important.
Batch Sequencers -- not so much.

When everything is a priority, nothing is.

One-scan housekeeping logic doesn't deterministically work across multiple periodic tasks, because the "do this" bit set in one task may or may not be seen by another, depending on the respective periods and just when the interrupts happen.

Issues like this aren't showstoppers, but I've seen them bite even experienced programmers.

sing the continuous task is very bad for a big system with multiple controllers & HMI's unless you want to go in & crank up the system overhead time slice to 60% on every PLC

For L70's and lower -- yes. For L80's and going forward -- no. They have a communication co-processor that handles all the comms independently, so scan time is completely unaffected. There is no "system overhead time slice" any more.

It sounds like someone is doing "what they're used to" and " too lazy to read about the hardware they're programming".

 

[plvlce]

It sounds like someone is doing "what they're used to" and " too lazy to read about the hardware they're programming".

or just isn't working with the latest and greatest.