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scan time DCS

07/18/2009 1:13 PM

How to check the SCAN time of DCS ,n dont give therotical answers .How to check the SCAN time of DCS during FAT?

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#1

Re: scan time DCS

07/19/2009 1:20 AM

You are more likely to get an appropriate answer if you define your abbreviations! DCS and FAT mean different things to different people!

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#2

Re: scan time DCS

07/19/2009 2:32 AM

You may need to give more information. Scan time changes with the complexity of the program, so the manufacturer would have to specify the test and test conditions. If you want the scan time for a given DCS running a given program with a particular I/O configuration, then you could put an instruction in to alternately turn an output on and off and look at the resulting square wave with an oscilloscope.

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#4
In reply to #2

Re: scan time DCS

07/19/2009 9:56 PM

You're probably right, so DCS would mean Digital Control System in this case, and the scan time would refer to the execution time of one complete pass through the program. I still don't know how often the Forward Advance Transition occurs (or whatever else FAT may refer to).

Your idea of using the oscilloscope is a good one, as it would show changes in the scan time when branches to subroutines occur, if there are any, and they are sometimes invoked and sometimes not invoked during the observation time.

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#5
In reply to #4

Re: scan time DCS

07/19/2009 11:01 PM

FAT = Factory Acceptance Test

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#6
In reply to #5

Re: scan time DCS

07/20/2009 12:15 AM

Thanks. That does make sense, although I'm sure there are other possible interpretations...

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#3

Re: scan time DCS

07/19/2009 8:15 PM

"dont give therotical answers"

We don't allow therotic material on this forum.

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#7

Re: scan time DCS

07/20/2009 12:19 AM

Seems like a waste of time to try to read your mind.

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#8

Re: scan time DCS

07/20/2009 1:42 AM

Hi there,

If you are using a DCS (Distributed Control System), there should be a diagnostic utility that you can run (this is usually part of the system software). If your sotware scan is structured correctly i.e. all software is called in cyclic scans and not free scans, the utility should be able to tell you the loading of these scan cycles.

You should also be aware that you are doing this under FAT conditions. This would imply that most of your IO are not connected, which in turn would push up your scan times for all the diagnostic alarms comming in.

Regards,

Craig

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#9

Re: scan time DCS

07/20/2009 6:24 AM

Thank you guys and especially to dkwarner for giving "Forward Advance Transition"something new to me

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#10

Re: scan time DCS

07/20/2009 1:19 PM

rajkumar.renganathan,

most control systems keep track of the min & max scan times in registers. Check the system documentation.

Out of interest, may I ask why you are checking the scan time during FAT?

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#11

Re: scan time DCS

07/21/2009 11:25 PM

other way to know the effeciency of DCS is to track the SFC - "Sequencial flow Chart" i.e. your logic by SFC Visulalisation software

Take a Parrallel or alternate sequence SFC and give different Input and observe the changeover time from one sequence to other

This will not give you the exact Scan time of your DCS, but you can know the efficiency of the DCS.

I am refering to SIEMENS PCS7 DCS

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#12

Re: scan time DCS

01/24/2010 10:40 PM

what is normally scan time of DCS. Is 1 sec sufficient or it should be less.

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#13
In reply to #12

Re: scan time DCS

01/25/2010 12:40 AM

It depends on the complexity of the program and the number and kind of inputs being observed/evaluated.

For the programs I've written and used one second would be enough time for many cycles. Blink's answer in post 2 remains the best. attach an oscilloscope or counter and find out for sure!

On the other hand, if your program has conditional inputs, where the program executes a loop until one or more conditions are satisfied, then the cycle time never completes until all conditions are met, so the cycle time could be infinity.

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#14
In reply to #13

Re: scan time DCS

01/25/2010 10:24 PM

If we are purchasing a DCS system and the vendor says that its scan time is 1sec and can be configured to 500 msec and 300 msec for some particular I/Os. so then is it good to have 1sec system. i have observed that when i changed one I/O card to fast scan of 300msec the idle time of our system reduced from 52% to just 30%. how will you see that.

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#15
In reply to #14

Re: scan time DCS

01/25/2010 11:13 PM

My experience is with PLCs (Programmable Logic Controllers) and dedicated motion controllers (mostly Galil brand).

Are you purchasing just a controller which you or someone else at your location will program, or an entire system, already programmed? I suspect the latter, since the scan time depends on the complexity of the program.

It's also possible that we are talking about totally different things! I'm referring to scan time as the time required for one complete loop through the main program of a controller, with no delays caused by conditional inputs. In the simple machines I've built, I believe the time was around 0.1 second or less. there was no obvious delay between changing a parameter and the machine responding to that change. Unfortunately none of those machines are available to me now, so I can't verify anything.

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#16

Re: scan time DCS

08/22/2024 7:52 AM

One option would be to program an output to be on and then off on sequential program scan, and use a high speed camera to measure it.

Given that if something happens on a plant it takes place in more than a second (if it's real) question the need to do it at all, because it sounds like the sort of question some Consultant would ask to justify continuation and extension of their services contract.

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Blink (1); craigza (1); dkwarner (5); HoleInTheSnow (1); lyn (2); Macsimus (1); PWSlack (1); RAJ KUMAR (2); rajkumar.renganathan (1); The Prof (1)

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