Why sample rate is different than I set up?

[pierluk]

Hi all,

I have set up an easy VI for understanding how nidaq-mx works. In the attachment you can find the VI. In particular the sample rate is set to 1Hz and I read 4 samples in 4 loops. So the acquisition should take 4 secs, isn't it? So why does it take just 0.03secs?

Thank you

[CTSFranklin]

Although your sample rate on your DAQ-MX may be set up to sample at 1 Hz, your loop can run much much faster. I would try adding the wait function into your loop to assure that the loop takes at least 1 second if that is your desire.

Hope this helps.

-Franklin

[altenbach]

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pierluk wrote:

I have set up an easy VI for understanding how nidaq-mx works. In the attachment you can find the VI. In particular the sample rate is set to 1Hz and I read 4 samples in 4 loops. So the acquisition should take 4 secs, isn't it? So why does it take just 0.03secs?

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It does not! I am currently on a laptop using a simulated device, but the VI takes over 4 seconds to complete. I suspect that you create an error in your initial DAQ setup, causing all IO to instantly error out and complete.

There are quite a few things a bit strange with your VI:

• If you only want to read one sample, why not use 1Chan1Samp?
• This will also eliminate the reshaping.
• The Graph 2 only shows the last point, replace it with a chart instead if you want real-time updates.

[altenbach]

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@CTSFranklin wrote:

Although your sample rate on your DAQ-MX may be set up to sample at 1 Hz, your loop can run much much faster. I would try adding the wait function into your loop to assure that the loop takes at least 1 second if that is your desire.

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This is incorrect. The read timeout is 10 seconds by default, meaning it will wait 10 seconds if no sample is available. This is sufficient to never timeout with a 1Hz sampling rate and the sampling rate will primarily determine the loop rate as it should be.

Adding an additional delay introduces a second timing dependency, possibly causing problems in the long run.

[pierluk]

Tanks for your reply. In the beginning I set up the NI-DAQMX Read in reading 10 samples, but the VI runs really fast. So I just tried to read only one sample for each iteration, but nothing changes. When I execute the VI I don't get any error and the right amount of samples. When in the waveform properties I get the dt is 1 for any sample rate I set up. I don't understand what's wrong on my VI

[Dennis_Knutson]

Post your new VI. In your original, you don't have a waveform data type anywhere at all and any dt that you write to the file is garbage/default value since you are not passing any dt value to the Write to Measurement File function.

[pierluk]

Thanks again for your reply, this is the new VI.

[Dennis_Knutson]

I never use dynamic data or the write to measurement file but your Convert to Dynamic Data looks wrong. You have a conversion dot on the input. Why did you select the input type as 1D Array of Scalars (which has no timestamp) instead of Single Waveform?

[pierluk]

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Dennis Knutson ha scritto:

I never use dynamic data or the write to measurement file but your Convert to Dynamic Data looks wrong. You have a conversion dot on the input. Why did you select the input type as 1D Array of Scalars (which has no timestamp) instead of Single Waveform?

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I already did it, but it doens't change anything. I don't understand what's wrong with that VI.

[Dennis_Knutson]

Another strange thing I noticed is your sample rate. You have it set to .5S/sec. With it set to something reasonable (i.e. 500 or 1000) and the correct Convert to Dynamic Data, the dt in the file is correct.