pxi-1409 timing- 30fps on average

[john_medberry]

I'm not particularly new to labview or image acquisition. I have a PXI-1409 in a PXI chassis running XP with LabVIEW loaded.

I have a very simple acquisition VI set up to configure the image acquisition off of CH0, using the BNC. This runs in a while loop, mainly for observation and as trimmed down as possible to get every frame with little to no overhead. It turns out, that I had to use it for some timing measurements and added millisecond clock acquistion for each frame. I got a surpise when the timing information was not uniform. I have 7 frames that are identical in time 31.25mS and the eighth frame is 46.875 mS. This averages out to 30 fps, but introduces complexity in my measurement. Is there anyway (and believe me, I've been over the manual and MAX like mad) to force it to run 30 fps all the time, or is the hiccup required due to the loosy-goosy dma's?

Is there a way to disable the other three channels?

Any thoughtful comments, esp experienced with this card using analog signals would be greatfully appreciated.

Thanks

[iZACHdx]

Hello,

Can you be a little more specific on how you have this coded? If you have your image acquisition setup as a snap within a loop, I would expect the results that you are seeing because your loop is nondeterministic. However, if you have your acquisition setup as a grab, I would expect a continuous frame rate with little or no fluctuation. Maybe a screenshot would help?

-Zach

[john_medberry]

I'll work on a screen shot, but it is a grab that is configured prior to entering the loop. The timing result is in a sequence, after the grab. It is exceedingly deterministic and just about everything in the loop is forced, by flow, to be so as well. Also, very precise on the timing that does occur.

Having discussed this with a couple of colleagues, it appears to be a delay and buffer for about 12 ms, then spend the next 7 frames catching up.

Since it is highly repeatable, I can live with it, but it is odd.