Continuous vs Periodic Tasks in PLC

[Dayvieboy]

What's with all the dots, I find them quite distressing.

I 100% agree
You cannot add indentions or dots to delineate text

It all looks like a run together sentence when left justified

Trying to figure this out

 

[Dayvieboy]

I wouldn't call an EtherCAT slave "... EtherCat on Allen Bradley"

It's a gateway.
You could possibly use an Ethernet/IP driver & not an Allen Bradley PLC on a PC?

This is new to me & I am trying to get them to change direction if possible
.

 

[rupej]

I've given this a lot of thought over the years. For me, it's continuous for everything unless there is a specific need, such as with PIDs or if I have such an enormous project that running a continuous task will result in too slow of a scan time for the time-sensitive logic. I've yet to run into that second issue, but that is a valid exception and something I would consider for your application.

Some of these have been mentioned, but below are my primary reasons for going continuous:

1. Continuous task has no effect on comms on the newer series controllers
2. I see no point in waiting for another scan when the processor is ready. This slows down system response time, however trivially
3. Asynchronous I/O scanning should be fixed by I/O mapping regardless of using periodic or continuous tasks. It's an unrelated issue.
4. Oneshots between periodic tasks can become nightmarish
5. If you have to add logic to a periodic task, you may have to increase the period to avoid task overlaps.
6. The only negative I can think of for continuous is a lack of consistency in scan time, which has never been an issue for me. What does it matter if one scan is 4ms and the next is 5ms? Still faster than 10/25/100ms or whatever I would have set an overall periodic task to. I suppose on massive plant-wide PLCs, there could be poorly debugged code that could cause unexpected issues if the timing changes, so that may be an application where this is important.


It all really comes down to a matter of opinion.

 

[AlfredoQuintero]

It's a gateway.
You could possibly use an Ethernet/IP driver & not an Allen Bradley PLC on a PC?

This is new to me & I am trying to get them to change direction if possible
.

There is this ControlLogix compute module which is is a PC on a ControlLogix chassis. I am thinking it would be possible to install a TwinCAT license and have an EtherCAT master and PLC application on that module. I do not know enough about the product, but the datasheet says the product offers ways to communicate the module to a Logix processor through the backplane bus. I would imagine these functions are provided in such a way that they should be relatively easy to integrate in TwinCAT. Just my two cents, and I admit it is a very rough proposal.

 

[DrakeTungsten]

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

 

[Aardwizz]

Case Open.

There are advantages to having a predominantly Continuous Task system.

There are advantages to having a multiple Periodic Task system.

An engineer designs a control system to meet specific needs, and take advantage of the available options, whether that is which languages to use when, or how tasks are organized.

 

[Jlandwerlen]

Case Open.

 

[Jim3846]

Continuous task = lowest priority
Periodic task = in middle priority but above continuous task
Event task = in middle priority but above periodic and continuous task
motion = highest priority

I think if you understand that above it keeps you out of trouble.

I think periodic task you can set the scan time and I always set that to 2.5 times the continuous scan time for that code. That way you don't get task overlap.

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

 

[DrakeTungsten]

"There are advantages to having a predominantly Continuous Task system"

Only if you consider real time control to be the least important thing your PLC does.
I kinda like using the most deterministic solution

 

[Jlandwerlen]

Continuous task = lowest priority
Periodic task = in middle priority but above continuous task
Event task = in middle priority but above periodic and continuous task
motion = highest priority

Maybe I'm confused, but periodic priority is whatever you have it set to.

 

[DrakeTungsten]

Maybe I'm confused, but periodic priority is whatever you have it set to.

Correct, from 1 (highest) to 16 (lowest) but all are higher priority than the continuous & will interrupt it. Also, using 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

 

[Jim3846]

Maybe I'm confused, but periodic priority is whatever you have it set to.

But i believe and there is a user manual that explains this, but that is the correct priority no matter what number you assign to the priority.

continous
periodic 1to10
event 1to 10
motion 1to 10

If you had multiple periodic , then you can set the priority of the task to the other periodic task.

But event driven and motion has priority over periodic.

 

[Jlandwerlen]

But i believe and there is a user manual that explains this, but that is the correct priority no matter what number you assign to the priority.

continous
periodic 1to10
event 1to 10
motion 1to 10

If you had multiple periodic , then you can set the priority of the task to the other periodic task.

But event driven and motion has priority over periodic.

Thanks for clarifying for me.

 

[Jim3846]

Rockwell Automation Publication 1756-RM094K-EN-P - October 2020

here is the user manual, that explains in detail task priority.

 

[JeremyM]

Correct, from 1 (highest) to 16 (lowest) but all are higher priority than the continuous & will interrupt it. Also, using 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

This isn't an issue with 5x80.