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"Retriggerable" setting pauses clock?

Hi.  I'm using a DAQ board to acquire triggers using the retriggerable setting.  I'm acquiring waveforms and it is working great, except the times are incorrect.

 

If you take a look at the attached VI, the front panel controls the acquisition rate (default 250 kHz) and the number of samples to record after each trigger (default 10).  The DAQmx Read VI is called each time the 10 samples are read to read them into the computer as a waveform.

 

After outputting all of the waveforms as an array and looking at the t0 for each, the t0 values are not what they should be.  Instead, the first t0 value is correct, and the rest are separated by exactly 4x10^-5 seconds.  That would be correct if there was no pause between triggers (reading 10 samples at 250 kHz takes exactly 4x10^-5 seconds).  But when reading in triggers that occur 10 times a second, the t0 values should be separated by 0.1 seconds, right?

 

Is there any way to stop the DAQ board from pausing its internal clock while it waits for the next trigger?

 

Thanks!

Jeremy

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Can someone at least run this program on their computer so I know if it's just me with this problem?

 

Thanks,

Jeremy

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Take a look at the attached.  I think you were only looking at the waveforms from the sequential channels that were acquired each time.  I extended the time array to 2D to show all of them and they seem to look OK to me.
Message Edited by rpursley8 on 07-16-2009 05:14 PM
Randall Pursley
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Thanks for helping, although it seems to me like you've got the same problem.  You're down to 500 Hz, so the first timestamps are separated by exactly 0.02 seconds, no matter how far apart the triggers come in.

 

Jeremy

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Hi Jeremy,

 

I want to clarify for myself what your goal here is. Are you wanting to only timestamp when the triggers occur and have no timestamp associated with the individual samples?

 

Also, what data acquistion board are you using and what version of the DAQmx driver are you using?

 

Thanks!

 

 

Regards,
Margaret Barrett
National Instruments
Applications Engineer
Digital Multimeters and LCR Meters
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Your right.  I can't find any way to get the DAQmx waveform data to work the way you want.  This has to be considered a bug since the whole point of t0 would be to define when each waveform started.  It looks like you will have to keep track outside of the DAQmx functions which will not be as accurate.
Randall Pursley
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Thanks.  I guess I'll just go with Windows timestamps (although they're rounded to the nearest millisecond or so for me).

 

I'm using the NI DAQ-6251 USB with NI-DAQmx 8.8.

 

Jeremy

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Has somebody found a solution for that. I have very similar issue that is posted HERE.

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