DAQmx Read Property node affects sampling of data

[jjohannsen]

Has anyone had delayed data sampling due to repeated use of a DAQmx Read property node?

I have an externally clocked DAQmx task reading digital input ports from a PXI-6533 DIO card.  External clock is coming in at approx rate of 10 ms +/- 0.1 ms.  I am reading 10 samples from the buffer every 100 ms.  When I view the recorded data, occasionally there is an evident lag in the data sampled by the 6533.  The lag can be seen repeatable at every 10 samples when it is present.

I believe I am misusing the DAQmx Read property node, and want to know if relocating it to the Initialize portion of my Task will solve the issue.   I set the "Relative to" and "Offset" properties before every read (once every 100 ms), which is consistent with the period at which the lag occurs.  I was setting "Relative to" to 'Current read position' and "Offset" to '0'.

Basically, what I want to know is, can setting DAQmx read property node values "Relative to" and "Offset" have any effect on the 6533 card when it is attempting to place samples in the buffer?

I have a related question with other details listed in another thread here: http://forums.ni.com/ni/board/message?board.id=170&thread.id=291034

if anyone has any ideas.

Thanks for your help,

Jason

[jjohannsen]

After moving the Read Property Node to the initialize portion of the DAQmx task, the occasional sample lag is still occurring.  Does reading the DAQmx buffer with DAQmx read affect the sampling of the PXI-6533 card?  The task is set up for external clocking, with the clock signals coming in on RTSI (PXI bus) every 10 +/- 0.1 ms.  At some points in the acquisition, I am attempting to read samples that have not yet been aqcuired.  Example:  Attempting to read 10 samples from the 6533 buffer when only 9 are available.  I have the read timeout set to 30 ms, so the DAQmx read VI will wait for the 10th sample to be placed into the buffer.  Could this cause a delay in that sample point, or the next one?  I could setup the code to only read the available # of samples, so the DAQmx read VI will never have to wait for more samples to be placed into the buffer.

What I need to know is, could allowing the DAQmx Read VI to wait for more samples to become available affect the way the PXI-6533 card places samples into the buffer?  Is there anything else that could cause the PXI-6533 to occasionally delay placing samples into the buffer?

Thanks,

Jason

[J_Thomas]

This is a duplicate post. Please refer to this link for further information.

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