Data Acquisition (daqmx create virtual channel, timing, read)

[NikolaiLe]

Hello everyone.

I read a lot of materials, but I'm still confused about some things.

I try to make a data acquisition program. I have few separate tasks. Each task can unite different ADC boards. And also, there is possibility that channels of the same ADC board will be used in deferent tasks. Currently, I have 6133, 6358, 6368, 6363 boards.

1) As I know, we can not set up deferent sampling rate for different channels in one task (it's possible if an ADC board has multiple cloak engines). So, I choose the lowest sampling rate and set up it as maximum sampling rate in a task. And I make sure that tasks are set up with the same sampling rate if same ADC board is used. Am I doing right?

2) If I'm doing right, should I consider the same approach for number of samples per channel and maximum and minimum value of voltage of expected signal?

3) I'm planning to read raw data. Again, as I know, samples can be united in one integer or one sample can be splitted into several integers if we do not use native size. I think to set up to read 1d array of I32 for entire task. But I'm worried if it affects my data anyhow. Should I go consider different approach?  

Thank you a lot for any info.

[crossrulz · Accepted solution]

A task will run all channels at the same rate and will read the same number of samples for all of the channels.  You will not be able to have multiple tasks on a single card.  And I think those boards are M-series boards, which means you will need a different task for each board.  Each channel can have its own set maximum and minimum input levels (ie range).

Now, if you want different rates for several channels on a single task, you will want to sample at the maximum rate and then do some type of filtering and decimation (simple averages are common) to lower the sample rate.

I don't understand why you are reading the "raw" values.  Why not just use the actual voltage read?

[Kevin_Price]

In addition to everything crossrulz said (which I agree with), I'm getting the impression you might want to *synchronize* data coming from more than one card at a time.

To do that well, you'll need to share timing signals among the boards.  I'm an advocate of simplicity here, and do this solely with a shared sample clock whenever feasible.  Solely sharing a start trigger may not be sufficient.

It will also matter whether you're dealing with desktop (PCI, PCIe) boards or a PXI chassis.  PXI provides a timing backplane to enable the sharing of timing signals.  It also has a reference clock which many boards can align their internal timebases to, which also helps to manage multi-board sync.   With desktop boards, you'll want to look into a multi-tap RTSI cable, or plan to do some external physical wiring of the needed timing signals.

-Kevin P

[NikolaiLe]

Thanks. Now, I'm clear.

No, they are S- and X-series boards. Actually, I already tried to unite in one task, and it worked.

I want to read raw samples to save disk space. I plan to store kinda a lot...

[NikolaiLe]

Yes, you're right! Thanks for your suggestion.