ControlLogix L73 Scan Time - Purposely Delayed

MikeGuerard · Sep 28, 2016

Hi Guys,

Quick Question...

Is there a method or instruction to purposely delay the scan time of a ControlLogix processor?

We're looking at upgrading controllers / drives and were wondering what type of net effect on product cycle time a change in scan time would have.

Thanks!

Mike

Tharon · Sep 28, 2016

You can set up periodic tasks and define the frequency of the task operating. That may achieve what you want.

MikeGuerard · Sep 28, 2016

All right Tharon - that sounds like it just might give me the feedback I'd need!

Good suggestions - thanks.

We're seeing a reasonably variable scan rate currently (11-53ms) - I was hoping to have a set extension to that rate - i.e. add 10, 20 or 50ms to each scan.

Could that "add on" scan time be achieved by changing to an event-driven scan rate and triggering a delay timer at the end of each scan?

How would I trigger that at the ten

RA is saying they can cut our scan rate by ~30ms...mgmt. of course is trying to equate that to dollars and cents.

Thanks again!

Tharon · Sep 28, 2016

While it depends on your process, generally speaking scan time isn't improving cycle times. It's improving response and accuracy.

I'm not sure about using periodic tasks to add set amounts of scan time to a system. That may not be possible. The periodic tasks will ensure that your scans always happen at the same intervals though. Which can improve certain things.

keshik · Sep 28, 2016

I would try putting a FOR loop at the end of your scan, and have the FOR loop do a bunch of useless math or string processing. I would then play with the amount of stuff that it does until you get the time about right for what you're looking for.

That being said, what are you really trying to do? It sounds like you're trying to determine a financial cost per ms of scan time. I'm not sure that that an additional 30 ms is necessarily going to correlate directly to a reduction of 30 ms.

Dravik · Sep 28, 2016

If you want to dynamically change the Rate of a routine, you can use a SSV to write a new rate to it during run time.

MikeGuerard · Sep 28, 2016

All good ideas - thanks everyone - I'll give it a shot tomorrow.

Ron Beaufort · Sep 28, 2016

you might be interested in the "HEAVY LOAD - DELAY LOOP" figure shown near the end of the following post ... basically it lets you adjust the timer's PRESET value for whatever amount of "delay time" you need ...

http://www.plctalk.net/qanda/showthread.php?p=160378&postcount=3

CAUTION! ... naturally this shouldn't be added to a "working" system ... but I use this sort of thing extensively for training purposes ...

keshik · Sep 28, 2016

Ron - clever use of a jump! I never use jumps in my programs so that didn't even cross my mind.

MikeGuerard · Sep 29, 2016

Ron... that's one heck of a post! I'm going to print it off and furrow my brow for a bit.... thanks so much for the info!

Dravik · Sep 29, 2016

Ron, that's pretty neat, if horrible heh.
http://xkcd.com/292/

PCMusicGuy · Sep 29, 2016

I use something like this extensively for testing purposes:

LBL(WaitForTimer)NOP();
XIO(TestTimer1.DN) TON(TestTimer1,0,0);
XIO(TestTimer1.DN)JMP(WaitForTimer);

CageFreeBMW · May 14, 2020

I know this is an old thread, but hoping the OP is still around and can provide a follow up.

MikeGuerard did you ever upgrade your processors? I am looking at doing the same and going from L6 controller to L8's. Wondering what kind of increase you were able to achieve? If I am able to get 100ms increase on cycle time from the faster scan rate then the upgrade will pay for itself fairly quick including the cost of having to upgrade HMI's as well (running complex fully automated equipment with hundreds of air cylinders, tens of servos and thousand+ sensors per machine).

Any scan rate to cycle rate information you can share would be very helpful!

ControlLogix L73 Scan Time - Purposely Delayed

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