PCI-6251 (or PCIe-, or USB-) acquires more than 4096 samples after digital trigger

[Vasilich2004]

Hi,

I need to acquire 50 kSamples on 1 MHz  rate ( = 50 ms) after digital trigger. Next measurement will be done after 1 sec.

There are PCI cards, for example PCI-6711, with onboard memory few MBs.

But what about PCI-6251 with 4096 kSamples onboard?

Let calculate PCI bus troughtput. 4 kSamples (=8kB would be measured 4 ms. PCI takes 8 kB / 100 MB/sec =  0.08 ms.

So there is not not problem for PCI bus if another task would not load it.

What about CPU? It depends how card works and I can't find any useful information.

In one way driver allocates memory for 50 kSamples and prepare ADC. ADC gets trigger and starts. When half onboard buffer has filled card initiates DMA transfer and writes into memory itself. When acqusition finished card sends interrupt to the program, In this case acquring doesn't depend from CPU.

In another way card sends interrupt each time when half onboard buffer has filled. Then the program initiates DMA transfer ...  In this case acquring depends from CPU and CPU must does small actions each 2 ms. This way is enough critical.

Which way does PCI-6251 use?

Maybe anyone has tested?

Thanks!

[tnek]

Hi Vasilich2004,

If you are just trying to acquire one channel of data, the PCI-6251 will be sufficient to acquire at 1MS/s in 50kS blocks. Since the PCI-6251 uses a multiplexer, the total sampling rate will be divided by the number of channels you want to sample which may cause a problem if you are using multiple channels. The NI-625x Specifications list that maximum sampling rate as 1.25 MS/s for a single channel and 1.00 MS/s for multi-channel acquisitions. The data transfer mechanism defaults to DMA but it can be changed in software. You can also select the condition for the data transfer such as transferring when there is any number of samples in the buffer or transferring when the buffer is half full.

Regards,
Kent
Applications Engineer