PCI6251: How to reduce widely AO size FIFO or Disable AO FIFO in AO HW timed DMA operation?

I took aoex5 as a basis for the generation of signals timing hw AO0 DMA and it works well in my setup. At the same time two signals which depend on AO0 are measured to AI0/AI1 by hw-timed-DMA mode maximum rate (500 ksps / s) and it works as well.

My goal is to apply a positive feedback mathematically AI1 to AO0 nationwide, and the easy way would be to change the values in the channel DMA AO AO rate on the fly, by reading the latest channel DMA AI values. The processor speed and the DMA space would be quite sufficient. The problem is that the CED are charged by means of a FIFO buffer that has a size of 8192 samples for my Board, and so to change a value in the buffer DMA would have no effect on the output DAC.

I tried to simply disable the buffer FIFO of AO by setting AO_FIFO_Enable to 0, but it disabled the generation of signals AO together. Is there a way to disable AO FIFO and leave the DAC values directly from the buffer DMA? Or can I reduce the size of the FIFO AO programmatically to a minimum number of samples (2-16)? I could survive with a little latency induced by a small FIFO buffer, because I can compensate for this by changing the rate AO on the fly as well.

I know, I use timed by the sw operations to achieve the goal, but I assume that AO DMA without FIFO operation would give me a lower latency of feedback.

Thank you very much in advance for any helpful ideas.

Best regards

Rolf

Hello Joe!

I decided to start my experiments with the generation of signals AO0 clocked by sw.

In order to test I put AI_Bypass_Cal_Sel_Pos-7 and AO_Bypass_AO_Cal_Sel to 0, while the generated signal is directly available on AI0. I was programmed to the maximum rate of dual channel (500 ksps / s) in a linear DMA buffer of 128 MB. To AO, I programmed a triangle (ramp-up/down) for a period of 2 V which is represented by 12975 AO samples.

A tight loop software added an average across the last 4 Readings found in the DMA buffer to the AO value at the next exit, and once the LSB of the gross sample of AO changes, it is written to the DAC by Board-> DAC_Direct_Data [0] .writeRegister (Uc).

I did tests with different speeds of scanning voltage from 0,1 up to 250 V/s. The AO to 250 V/s sampling rate was 1.6 MECH. / s. The deviation of the curve at the higher rate to the lowest rate is only 0.1%.

Attached to this message you will find a diagram showing the magnification of the curves around the tip of the triangle. The Green curves taken at 100 and 250 V/s. Red to yellow to 0.1, 1 and 10 V/s. There are very low phase shift of 0.1% to scan rate of very hight perhaps due to the latency of your comments.

Honestly 1.6 MECH. / s mode AO NI by sw including the algorithm of feedback far exceeds the expectations and I must admit, I am perfectly satisfied with this result. Sorry, Joe, but with that, I lost the inspiration to try out your suggestions, reducing the size of AO DMA buffer and change the FIFO AO mode. Anyway, thanks a lot for your answers.

Best regards

Rolf

Tags: NI Products

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