cRIO 9012 with 9205 not reading voltage consistently

[Sesmi]

Hello,

I'm fairly new to Labview (VERY new to cRIO). I have connected a circuit that should and does provide 5V across the terminals (verified by a voltmeter), but the 9205 I have connected seems to be reading the voltage almost sporadically. The voltage will read 5V in the Labview for a minute or 30 seconds and then will drop to 0.3V, even though the voltage is still reading 5V on the voltmeter! I've attached the vi I wrote along with a screenshot of the data collected. Is there something with the buffer? I've heard that this can be a problem, but I don't really know how to change that.

Thanks for the help,

Sesmi

PS - The screenshot will be reading a flatline of 0.23V. I originally was testing this with the 9205 set to read +/- 200 mV before changing it for further testing.

[Ever_Song]

Hi Sesmi,

I think there are two approaches we should investigate to test the voltage: 1) the hardware side and 2) the software side. First, let us eliminate the software side of things. Test setup by opening up a shipping example to read the voltages. The easiest way to do this is to in LabVIEW go to Help»Find Examples. This will open up the NI Example Finder. In the lower left corner, click the Hardware selector»Add device. Find the NI 9205 and add it.

Next, select the newly added NI 9205 cRIO option from the Hardware selector and make sure to check the Limit results to hardware box is checked. Browse to Hardware Input and Output»CompactRIO. Select an example depending if we want to use scan mode or the FPGA. Use the example to test the voltage.

On the hardware side of things, first check the connections since the voltage drop “width” does not appear consistent. Second, confirm the voltage from the circuit - if you have an O-scope that would be helpful. Also, if you have a power supply, try that instead of the circuit. If you have other modules to read the voltage, test them as well.

Let us know your findings.

Thanks,

Bill

[Sesmi]

Thanks Bill for replying to my post. I did check the voltage with an external power source and it did the same thing, however I found upon further inspection that our RIO wasn't properly grounded. After attaching to a real ground, the reading returned to the expected. I think there was some sort of power build up/ discharged that was occuring when not properly grounded. Could that be what we were experiencing?

Thanks again,

Sesmi

[Ever_Song]

Hi Sesmi,

I am glad to hear that you were able to resolve the issue. The grounding could definitely be the case since the modules will reference the case ground for their measurements and if this is not constant, it could lead to erratic measurements.

Best Regards,

Bill