scanning multiple daq devices

[smenon]

Hi,
I am acquiring data from two scxi devices with a single e-series board. In the labview program that i am running now i have a while loop in which i scan the two devices using ai config and ai start. I have set the scan rate of each to 80hz and the no of scans to 1000. So in theory it should take 12.5 seconds to complete the scan. But each time it take about 25 seconds to do the scan. I am unable to understand the way the scan works and why it takes the extra time. Isnt the system supposed to just trigger off the acquisition and then read the data from the buffer? Or does it complete the scan from one device and then do the same for the other device or is the extra time being used to do the processing that i also do in the while
loop on the acquired data. I am also attaching a part of the VI with the while loop.
Thanks in advance,
Shyam.

[Russell]

Hello smenon,

I see the problem. You have two separate acquisitions configured. As you have it, the board will acquire from one module and then begin the other. Since you are using a single e-series board, you can configure all channels in the same acquisition. (Have one AI Config, AI Start, and AI read with all channels from both modules in the channel list) If you do it this way, you'll get the approximate 12.5 seconds you calculated plus a little bit of software overhead time of course.

I hope this helps!

Russell
Applications Engineer
National Instruments
http://www.ni.com/support