rslogic500 /controllogic 5500

[sportster]

Alan,



The way I understood the manual, Comms is part of Overhead. Overhead does not interrupt a Periodic task. Overhead will run when there are no other tasks to run (in a program with no continuous task). In a program with a continuous task, Overhead will run based on Time Slice Percentage. Please correct me if I'm wrong. I'm working a project right now where I can't decide if I should use the continuous task or setup all periodic tasks. So finding out exactly when this stuff runs would help a lot.



Patrick

 

[RSL]

The overhead time slice only applies to the continuous task. The Continuous task only runs when periodic and motion task are satisfied. You are correct, Communications will not interupt a periodic task. Keep in mind we ARE NOT talking about prouced/consumed tags or I/O here. They are another topic all together.

So, if you set a 20% timeslice you are allowing 20% of the time available to run the continuous task to be used for overhead work. Keep in mind that if you set periodic tasks to execute so frequently that there is no time left over, then you are allowing 20% of nothing to be your overhead time slice.

If you use all periodic tasks then I believe you should set the time slice to 100%? I have never done an application this way before though.

I usually put the important stuff in periodic tasks. I set the period to a REASONABLE value. If it does not need to execute every 2ms, then don't ask it to. If you use reasonable values for your periods you will find TONS of processor power available. Then I put anything trivial in the continuous task. Things that can execute at their leasure. If you do this then the default settings for overhead time slice will work fine. If you have a lot of MSG's and or HMI traffic you can raise the time slice up. I have never ran it higher than 30% myself.

It is very important to check for overlaps when using periodic tasks. You should look at all the tasks scantimes and interval times. If you write them all down and do some math you can see where all of your processor's time is being spent.

The safe bet if you don't have time to figure it all out is put it all in the continuous task. Most applications work fine this way. Yes, this wastes some of the best features of CLX but it will run.

Here is a document regarding timeslice. http://domino.automation.rockwell.com/applications/kb/RAKB.nsf/0/48434E1880AEFEAB85256D7B006FEBB0?OpenDocument

As stated above, "read the manual"

RSL

 

[RSL]

I just read a little more. The overhead time slice has NO effect when there is not a continuous task defined. So you do not need to set it at 100%.

Here is a better document detailing the time slice funcionality.

http://domino.automation.rockwell.com/applications/kb/RAKB.nsf/0/0991CCA00E882F9D85256AFB005A85B9?OpenDocument

RSL

 

[DDOBSON]

I learned this the hard way so you don't have to. Your communication from the laptop to the processor also occurs during the overhead timeslice. When tasks are improperly scheduled and/or do not have time to complete your overhead timeslice will be next to nil. Go to program mode and check your minor faults for a watchdog fault. find the overlapping task and change its frequency. The problem was I had my tasks schedule to run again before it was able to finish running. In properties for a task you can see the max scan times and should always set your yours not to be scheduled before it can complete. That was awhile ago. I teach a class now using 4 of AB's publications. Definately a must read. Getting Started, Getting Results, User manual and Programming manual. They are a little redundant but are kind of incremental in their description of how the CLX works and how to harness it. I love CLX.

 

[Alan Case]

Sportser, You are right, I stand corrected on the comms.
The online books also have some good info on timslice and task scheduling.
Regards Alan

 

[DDOBSON]

Sportster the easiest explaination of this is in the Common procedures manual Glossary 32-34/pdf page 446-448. If that helps. By the Way Produced/Consumed tags actually use a connection but still take up band with on your network. I Would love questions on this; I have built P/C networx on EtherNet IP and ControlNet.
I have used the ControlNet with a 5ms NUT to control 18 75-150 horsepower motors. Sent the drives their speed reference with accel rates and did the S-curve in the proceesor instead of the drive. using the Function Block Diagram in a. It worked pretty smooth once I got the bugs worked out with the Reliance comm cards. No analog...just numbers.

 

[moltra]

The easiest way to see how your tasks are running is to use the Logix5000 Task Monitor. It shows alot of information about each task.
Run time of each task
Maximum run time of the task
Overlaps of each task
CPU utilization of each task

 

[sportster]

Thanks for the info and links guys! That helps a lot.
I have another question now: What is the advantange to using all periodic tasks in a program? Knowing exactly when certain logic will execute? What is the advantage to using the continuous task? Say I have a program with three periodic tasks; one at 5 ms, one at 30 ms, and one at 100 ms. If my calculations are correct; I end up with a total time of 360ms of actual logic scan time per 1000ms. That leaves 640ms for running overhead tasks. Now if I take that same program and combine the 30ms and 100ms tasks into one continuous task (the 5ms task stays as that controls servos); with a time slice of 20%, there is less time for overhead roughly 200ms. Maybe I'm banging my head against the wall for nothing. In this program there isn't much overhead; just HMI comms to a PV+ and a little data logging to RSView32 (ten words maybe). All data transfer between processors is handled by a main "Gateway" CLX. Maybe a better question would be: How do you guys determine which task to use?

Thanks,
Patrick

 

[Alan Case]

Personally I would make sure that the 5ms task executes quick enough to allow the continuous task time to run then if it is only a small program I would put the rest in the continuous task. (Without knowing the process then this may not be the way to go)
As a bit of info, there is a good chance from version 17 that comms will interrupt certain priorities of periodic tasks.

Regards Alan Case