Can you suggest a way to minimise USB latency using data acquisition tool box with NI6211.

I want to try a real time control idea using a NI 6211 DAQ analogue I/O unit through USB. That means single inputs and outputs from within a MatLab loop; I cannot send/receive streams of data. I know the USB will make it pretty slow, but I just want to try out the idea. Can you suggest the best way to minimise the slowness?

Answer 1

[Vinod]

Try using inputsinglescan and outputsinglescan inside your control loop.

[Jack]

We've tried that, but we have not got it working. We are using NI C-DAQ 9178 (input module 9234 and output module 9263). Matlab gives the warning: On this platform, notifications more frequent than 20 times per second may not be achievable. Error using Controller1test7 (line 93) The session contains channels that do not support on-demand operations using inputSingleScan and outputSingleScan. Only clocked operations using startForeground and startBackground can be done.

When I met Nick Gautier at Formula Student, he suggested that using the background to receive data and using a new data arrival to trigger a calculation and write the next data point. Attached is the code that fails to do that. Any suggestions?

Answer 2

[Lucien Robinault]

Hello, It will surely be too late for a solution, but have you tried this : https://fr.mathworks.com/help/daq/examples/software-analog-triggered-data-capture.html ?

And to control the data output flow to control in real time you can use the event 'DataRequired' who would trigger a function sending adequat output signal to control your device. https://fr.mathworks.com/help/daq/examples/generate-continuous-and-background-signals-using-ni-devices.html

You can easly see how it's done in the software analog triggered data capture example.

Answer 3

[Jens]

There is a toolbox that seems to replace the Mathworks interface (Matlab DAQ) to the NI driver (NI-DAQmx). Maybe it can help people with a similar problem:

https://monkeylogic.nimh.nih.gov/docs_NIMHDAQToolbox.html

And an illustration of how much faster it is:

https://forums.monkeylogic.org/post/update-on-usb-daq-cards-8630938#4

[Jack]

Thanks Jens for taking the trouble to answer my question.

I solved the problem long ago by using an Arduino as a real time controller with the Matlab program just sending it the controller coefficients to use. That worked pretty well up to about 20kHz.

[Jens]

I'm glad you were successful!

I also hear new Matlab versions perform much better for digital in/out over USB. Any experiences with that?

[Jack]

No. Since I got it working with the cheap and fast Arduino Due I've not tried anything else.