Can I get 20MHz continuous acqusisition with 6541?

[Turing Machine]

I am using a PXI-541 HSDIO with 1Mbit/Chan digital I/O card.  I need to gather around 4-8Msamples of data at 20 MHz.  I can't seem to get enough speed to get continuous acquisition at this high rate.  Is there any way to buffer or optimize the card in software to get this type of performances.

[Kenn_N]

Turing Machine,

Thanks for contacting National Instruments support.  This is a high data rate and the speed at which you can acquire and generate is dependent on the system itself and the the width of your data, whether 8, 16, or 32-bit.  So if you have a sampling rate of 20MHz and a data width of 8-bit, that's 20M*8/8bits/byte=
20MB, however if you have a data width of 16-bits then that is 40MB/s attempting to be transferred over the PCI bus.  This is theoretically possible it is not always practically possible depending on the system.

We do have some examples that specify how to stream data to disk.  They are currently located in C:\Program Files\National Instruments\LabVIEW 8.2\examples\instr\niHSDIO\Dynamic Acquisition.llb.  There is one example in that llb called Continuous Acquisition - Stream to Disk.vi.  The main point to notice about this vi is the use of niHSDIO Fetch Waveform as opposed to niHSDIO Read Waveform in the while loop.

Let me know if you have further questions.

Regards,