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Analog Outputs on X-series DAQ based on Results Calculated in FlexRIO FPGA

I am trying to set up a simple control system using our existing PXI hardware.


I am using a PXIe-5122 High speed digitizer to sample to voltage signals at 20 MS/s, I am streaming the sampled data using peer to peer communications to our FlexRIO PXIe-7965R which processes the signals and will also have a control loop in it to determine a control voltage. I would like to then output the control voltage, which is an analog DC voltage, to an amplifier at a rate of 20 to 150 kS/s. We have a PXIe-6256 X-series DAQ card that I would like to use to output the control voltage. 


I have tried to use a Target to Host DMA writer to write the control values to the host computer from the FPGA and send the data to the X-series DAQ card using DAQmx, but this does not update the output of the daq card at the required rate. Is there a faster way to communicate the output voltages to the X-series DAQ card from the FPGA? 





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Hi smalltownkid64,


For clarification, you are using a PXIe-6356 (http://sine.ni.com/nips/cds/view/p/lang/en/nid/209215) is that correct?  In order to communicate to that card you would need to pass information from the FPGA to the controller, and then from the controller to the card.  Unfortunately, there will always be a bit of delay in doing this.  The best we can do is try to retrieve the data faster from the FPGA by removing more elements at faster rates (but be mindful of underflow conditions in the FIFO).  The limiting factor is really how well the controller can handle this throughput and how fast we can then get through the DAQmx driver to the 6356.


Best Regards,

Matt S.

Matt S.
Industrial Communications Product Support Engineer
National Instruments
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That is correct, I am using a PXIe-6356. I also have a PXIe-1082 chassis with a PXIe-8135 controller. 


Assuming that I had the data from the FPGA a high enough rate, how fast should I be able to update the analog output of the PXIe-6356, using the DAQmx write block in the "Analog DBL 1Chan 1 Sample" configuration? The PXIe-6356 analog output is rated at 3.33 MS/s but I believe that is for a periodic waveform that is loaded to the memory of the daq card. Is this correct






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