Labview multiple channel measurement malfunction: NI USB-6259

[kb5644]

Hi there,

      I am currently working on a project which requires measuring analog inputs off of 29 different channels in LabView on an NI USB-6259 DAQ. In my LabView code, I am using the DAQmx read node to take in data instead of the DAQ assistant so that I have more control over the timing properties. I have written code to rotate through 29 of the DAQ analog input channels, measuring off of them one by one. A screenshot of this code is attached below for reference. However, when I run this code only half of the channels provide me with the correct amplitude measurements. Channels 2,3,4,5,6,7,16,17,18,19,20,21,22 read very small values close to zero when they should be reading a 1 volt sine wave.

      To try to troubleshoot this, I wrote another short section of code to measure off of one channel at a time. A screenshot of this code is also attached below. When I run this code, changing channels one by one, (with all 29 channels still physically attached to the DAQ screw terminals) I get the same results of only about half of the channels functioning.

      I should also note that disconnecting a consecutive channel impacts the channel in question. For instance, if I am attempting to measure off of channel ai2 and I have the next channel on the DAQ (ai10) hooked up to my input as well, ai2 reads an amplitude of close to zero volts centered about 0.0005V. However, if I attempt to read off of channel ai2 with channel ai10 disconnected (screw terminal left open) then ai2 reads a strange signal which starts around zero but decreases over a few seconds to be a signal of zero amplitude centered about -5.5V. I understand the concept of ghosting between consecutive channels, but this seems extreme.

      I also have an oscilloscope as part of my setup which I have used to check the inputs of the channels which are not functioning. The oscilloscope displays the expected 1 volt sine wave signal going into the screw terminals in question.

      I figured I would post here to see if anyone happens to notice anything going wrong in my code or my setup that I am missing. There is of course a chance that my hardware is damaged and thus is resulting in these issues, but I find it unlikely that so many channels would cease to function at the same time like this.

      Thank you all so much for your time and your help!

[KRojas]

Hello there!

Thanks for sharing your troubleshooting steps, that really helps to narrow down the issue.

I'd eliminate the LabVIEW code from the possible reasons, I believe it could be related mainly to hardware.

Could you try a Self-Calibrate on NI MAX? Also, try generating a signal from the USB-6259 and read it through different channels, what do you see?

Katherine Rojas

Applications Engineering

[Kevin_Price · Accepted solution]

You mentioned an issue with ch2 readings depending on whether ch10 was wired or not.  Are you feeding the same signal to all your channels?

In the code screenshots, you haven't wired the "terminal configuration" input when creating your virtual channel.  I don't know your device's default, but if the default is differential mode then some things make more sense.

In differential mode, ch10 is used as the negative terminal for readings from ch2.  The measurement will show the *difference* between the signals at the ch2 terminal and the ch10 terminal.  That might explain the ~0 V readings you get if both are wired with the same signal.  As to what you see with ch10 unwired, well, unwired inputs often show unhelpful behaviors that may include elements of ghosting, drifting, bias toward a voltage rail, etc.

Make a control for setting the terminal configuration and try different settings.  Also, read this guide to making the right choice of terminal configuration settings in software and physical terminal wiring.

-Kevin P

[kb5644]

Thank you guys so much for your generous help! It did turn out that the default terminal configuration was set to differential so resetting that to RSE solved it!