Error in reading voltage using DAQmx with PXI-6259

Question

Hi,
We are trying to acquire analogue voltage from 5 channels using PXI-6259. We trigger the data sampling with clk 0 (i.e. using PFI 12).
The problem we had is:
1)When we tried to get voltage from 5 channels, we get 2 data at the same after twice the period. For example, If we set the 
  sampling rate to be of period 1 second, we get 1 signal at 2 second and around (2+t), where t is a random time less than the execution period of the loop.
  Then the next cycles would be 4 second and (4+t).
2) This timing problem did not occur if we only acquire voltage from 2 or less channels.
3) We tried to run exactly the same program with NI USB-6210 and everything worked just fine.
4) We replaced the DAQmx read with DAQ assistant and still encountered the same problem.
Attached is the a simple program that showed the error. Is this possibly a bug in PXI or DAQmx?
Please help us
Arul (CLAD)
Attachments:
test rig.vi ‏74 KB

Answer 1

Good afternoon Nitin,
Thanks for contacting National Instruments with your issue, we'll try our best to resolve it for you as quickly and efficiently as possible.
As the error message stated:
"Analog input virtual channels cannot be created out of order with respect to their physical channel numbers for the type of analog device you are using. For example, a virtual channel using physical channel ai0 must be created before a virtual channel with physical channel ai1."
If you need to randomly switch which channels you want to either control IEPE excitation or coupling on, a better approach rather than having all of these channels in one task as you've done in your attached VI, is to parse your program down to multiple tasks that can run simultaneously. 
I have attached a screenshot of this approach, which hopefully is not a compromise to your application.
Best of luck with your project.
Sincerely,
Minh Tran
Applications Engineering
National Instruments
Attachments:
ParallelTasks.JPG ‏52 KB