What happens when DAQmx - searches read more timed period loop?

Hi all!

I'm not sure how Labview manages the following situation:

I have a loop that is timed with a period of 1000 m, it is configured to ignore the lost iterations and keep the initial phase. I created a task that needs to be read continuously at a frequency of 1 kHz. In the timed loop, there are the DAQmx - read function (analog signal, 1 channel, N samples)

Now, if I put the number of samples per channel at 1500 for example (while it takes 1500 * 1 ms = 1500 ms > timed_loop_period to read), I noticed that the timed loop takes ~ 1500 ms to complete. The same phenomenon occurs with any other number greater than the period of the loop.

This means that the period of the loop passes to 1500 ms? Or something different?

Thank you!

Best wishes

Samuel Lopez Santamaria

You go winding with an iteration of the loop of 2000 msec.

An iteration of the loop will always be until he has to do.  If you tell him to keep the first phase and it works for a long time, then it will wait until the next 1000 msec happen before going to the next iteration of the loop.

Now with your situation, later iterations cannot take it any longer.  If you are permanently data acquisition, DAQ mx buffer fills while the loop is still pending.  So when he runs again, there may already be 500 msec value of data in the buffer, and he just needs to wait another 1000 ms up to what you have acquired the total of 1500 milliseconds of data you have requested.

The situation for the third iteration of the loop will depend on whether the other was done within the 1000 msec, or ran just a bit long.  It behaves more like the first iteration, or the 2nd iteration.

My question is why you use it timed while loop?  What characteristic of it you need that a regular while loop is not?  I think that mixing and matching the characteristics of a timed, while the loop with your DAQ acquisition will cause a continuously variable behavior time loop which is probably not what you want, or could cause you problems down the road if you're trying to do is to get the continuous acquisition of your DAQ hardware.

Tags: NI Software

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