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Accuracy of millisecond timer on cFP-2020

What is the resolution of the millisecond timer on the cFP-2020 and 2000 controllers?
What is the long-term accuracy?
What is the relationship between the millisecond timer value and the millisecond value of the real-time clock?
What are the resolution and accuracy of the real-time clock?

(for code running on the controller)



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Message 1 of 7
I don't have specific answers to your questions but here are a couple of threads to check out
that might get you headed in the proper direction.

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Message 2 of 7
Thank you, Matt.  There is indeed some good, detailed technical information hidden in an attachment there...

Unfortunately my timestamp observations are still partly unexplained:

In a loop with only an FP-read on LED-A and a wait-on-next-millisecond-multiple running on the cFP-2020 I would expect the difference between consecutive timestamps (deltaT) to be very close to the wait time.  My results:

wait 1000 ms: deltaT= 0 or 1000 or 2000
wait 5000 ms, deltaT= 4000 or 5000 or 6000
wait 333 ms, deltaT= 0 or 333 or 1333

(there's also a random variation of a few milliseconds on top of this, which I'm not concerned about)

How can this be?
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Message 3 of 7
That is interesting.  The wait-on-mS-multiple can have a short first iteration;
are any of your short time numbers from the first iteration only?

What is the source of your time for your measurement?

Can you post an example?


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Message 4 of 7
The short time intervals recur frequently.

The source of the time stamps is the FP-read.

I'll post it tomorrow.

Thanks for your interest!


PS.  I say that I'm still using Labview RT 6.1.1.

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Message 5 of 7

Here is a VI that produces my puzzling results.  I would like to take the advice given by NI and others to use the FP Read timestamps, but this VI appears to indicate that Get-Date/Time-in-Seconds provides more consistent time.

I am now using LV-RT 6.1.3.

I'd love to have an explanation!

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Message 6 of 7
Hi Anton,

This is most likely expected behaviour. A FieldPoint Read, when set up as event-driven, will only update the timestamp when the data point changes. Viewing the block diagram of the FieldPoint Read VI shows a case structure that only updates the timestamp in the true case (new data).

In this case you are monitoring value changes of an indicator on the controller. Because the value returned from the LED isn't continually changing from true to false, and vice-versa, between loop iterations you are only getting updated timestamps intermittently.


| Michael K | Project Manager | LabVIEW R&D | National Instruments |

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Message 7 of 7