How to count time precisely?

I was trying to count time using a VI like the one attached.
My trouble is that this is not couting correctly. There is a possible reason I observed.
Maybe the computer is not able to process the VI as fast as necessary.
I thought about this because, when in edition mode the time counting was very wrong, and, when I created an aplication and installed it, it was running ok, as it got "light". But today, when I tryed to use this aplication with an old computer it didn't work. The clock is delaying.
Is it possible to count the time correctly?
I'm using a DAQ6009 to do my works, my VI is pretty big, and I need an iteration at each 1 ms (milisecond) as I'm using digital inputs to count 5 encoders events (I know that this is not the correct way to count encoders, but this is the only way I can do that, and it's working).
Thank you all in advace! hope you can help me!
hugs!
Solved!
Go to Solution.
Attachments:
teste.vi ‏6 KB

OK. Let me try to help you understand a few things.
The Tick Count and Wait (and Wait Until Next ms Multiple) functions all use a 1 kHz clock that is derived from the processor clock. The accuracy of these clocks is not great, although probably comparable to your stopwatch accuracy. Any timing generated by these functions will show some differences from the time of day clock as it is derived from a different oscillator.
The front panel updates are timed both by LabVIEW and the OS. Most screens are updated 60-80 times per second, maximum. Depending on what else the OS is doing, a screen update could be delayed 10s of milliseconds or longer. It is certain that not every value at the output of the Add function will be displayed.
The Wait Until Next ms Multiple will wait more than 1 ms if the other code in the loop takes longer than 1 ms. I am not sure, but I think the indicator is buffered so that any delays by the OS in updating the display should not slow the loop. I tried removing the indicator and it did not seem to make much difference.
I modified your VI to provide several indications of performance. First, I added the three-frame sequence structure to allow timing of the loop. Second, I changed to a for loop which will run a preset number of iterations. Third, I connected the output of the Wait Until Next ms Multiple timer to an autoindexing tunnel so that a record of the time of each iteration appears in the Ticks array. I calculated the differences between successive times and displayed that on a graph so that you can see the performance.
I did not mean to sound critical of your use of English. You write in English much better than I could in your language! The point I was trying to make is that general descriptions such as "is not counting correctly" without quantitative results or evidence to support the claim do not have enough information to help us help you.
Lynn
Attachments:
teste.2.vi ‏13 KB

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