Rapidity of background transfers with USB bus using Matlab DAQ toolbox

[KNY]

Hi,

I currently use an X-series PCIe-6323 multifunction card running on a Windows 7 PC. For one of our projects, we are controlling the card from Matlab (DAQ toolbox). There are very noticeable latencies (e.g. 4-8 ms for a single digital or analogue read/write). Because we need an accurate record of data (although we need only fairly low sampling frequencies; ~1000 Hz) and also need to have rapid access to that data, we have been using continuous background aquisition with the "listener" (Matlab's version of a callback function) set to access data every 20 ms or so. Although Matlab warns that this rate of transfer might not be possible, with some experimentation and tweaking (of sampling rate etc.) we can get it to work in a fairly stable way.

My question is, what kind of performance can I expect from a multifunction USB device when doing something like this (again using Matlab under Windows 7) and will this aspect of the performance be affected much by whether the card is budget, M series or X series, given that I don't need very high sampling rates? (I ask because I'm thinking of getting a portable setup running from a laptop).

Thanks in advance for any pointers.

[JakeA]

Hello KNY,

To my knowledge the difference between a USB 2.0 connection and a PCIe interface is relatively small compared to the latency you're seeing. The 4-8 ms you cited does not seem right given the spec of the 6323, but it could be the processing time involved in your overall system. If this is the case then I don't think you'd see a major difference when switching to a USB DAQ device.

We do have a white paper which deals with the differences between different instrument buses, it gives some of the latency statistics so it's worth a look: http://www.ni.com/white-paper/12117/en/

To be honest, from the sounds of it you would really benefit from speaking with our technical sales team. What they do is advise you on which one of our products best fits your needs after getting to know your requirements and overall setup. I personally think that if you're considering buiding a portable setup similar to your current system that speaking with a sales engineer is the best way to find you the right device.

Regards,