NI9205 - Voltage measurement is not working

[Leonardo_M]

Hello all!

->Set up:

     1) I am using the LabVIEW 2018 and running the example VI provided by LabVIEW to measure voltage: Under Help -> Find examples -> “Voltage – Continuous Input.vi” (Figure 1).

     2) For this, I connected the NI 9205 module to the NI CDAQ-9188 chassis.

-> Actual problem:

     1) When nothing is connected to the NI 9205 card. The voltage measurement reads 0 Volts. (as it should - See Figure 1)

     2) However, whenever I connect a harness to the module, the voltage jumps from 0 to roughly 10.7 Volts (Figure 2). The same behavior is verified when I try probing the pin I am measuring Voltage from (the moment I touch the pin, the voltage jumps from 0 Volts to 10.7 Volts while the multimeter continues to read 0 V)

    3) If I leave the harness or the multimeter probe connected, the voltage measurement remains constant at 10.7 V.

    4) However, if I disconnect them from the NI9205 module, the voltage displayed on the VI slowly decreases back to zero at a constant rate (Figure 3). 

This strange behavior prevents me from doing any measurement. The voltage measurement is not working.

Thus, my question is: Why is this happening? How can I fix it? 

I attach the VI I am using as well as some pictures that may clarify my problem.

Thank you very much

[Leonardo_M]

I should add that I have tried using a different NI 9205 module, a different NI CDAQ-9188 chassis, another computer, and even reading from another pin. The behavior was the same and none of these fixed the problem.

[RavensFan]

What do you mean by "harness"?

What have you physically connected?  What signal are you trying to measure?  What do you expect the voltage to me?

It seems to me that you might be trying to read an open circuit which could easily float to a positive or negative value if you have no real signal going into it.

[Leonardo_M]

By harness I mean an electric harness. I attach some pictures that may help me explain better.

When I have nothing connected to the pin I am reading, LabVIEW reads a constant value of 10.7 Volts. It does not float from one value to another (this is depicted in Figure 2 which I attached on the first post). This behavior itself I would qualify as strange and undesirable.

However, if I ignore that and connect everything, I still have a constant value of 10.7 V. By connecting everything I mean this:

       1) I built a simple circuit where a Power supply is supplying power to a resistor.

       2) I am measuring Voltage directly from the Power Supply (the signal pin is measuring voltage directly from the power supply)

       3) In this Power Supply, I can change the Voltage to whatever I want. For example, I tried setting the Voltage to 2 Volts and LabVIEW would still show those 10.7 Volts. I tried setting different voltages but LabVIEW would still read the same, 10.7 Volts).

Thank you very much!

[RavensFan]

Float doesn't necessarily mean that it floats from one value to another.  It means to goes to a value, somewhere different than what it should be, typically the high end or low end of the voltage range.  The fact that when you disconnect the cable it slowly floats back down to zero seems like it is like a capacitor slowly discharging.

It's possible the module is bad.  But if this is a new module, I think it is unlikely.

I'd suspect the cable.  Especially since introducing the cable is what you said causes things to act funny.

Does this do this for other channels as well, or just the AI4 you show in the pictures?

Who made this cable?  Why are there 3 heads on the one end?  Are there any electrical components built into this cable, or is it just wires and connectors?

Is it possible there is a short in the cable between some of the pins?  I would try ringing it out for continuity between the various pins making sure there are no shorts between any of them.

Can you try a different cable, one that may be simpler?  Something that you can inject the power to just the correct pins on the module without bringing this cable into play.

[deceased]

Need to know more about how you have this configured and wired.

These Analog Input modules can be set as either single ended or differential measurement. Which do you have?

Can you make a really simple schematic showing how you have your basic circuit wired, giving the pin numbers of the 9205 and what they are connected to?

Reason I ask, is that this is possibly a common mode voltage issue, or some incorrect grounding.

0xDEAD

[Leonardo_M · Accepted solution]

I figured out the problem. The terminal configuration was set to "Differential", which made no sense on my current setup. I changed it to "RSE" and it works fine now. 

I leave the NI9205 manual attached if anyone is interested in knowing more about these terminal configurations.

Thank you very much for all the help!

[Leonardo_M]

I figured out the problem. The terminal configuration was set to "Differential", which made no sense on my current setup. I changed it to "RSE" and it works fine now. 

I leave the NI9205 manual attached if anyone is interested in knowing more about these terminal configurations.

Thank you very much for all the help!

[RavensFan]

Great.  I'm glad you got it figured out.  So it essentially was an open circuit because the differential setting was causing it to read relative to a pin that wasn't connected.

It's interesting that it only seemed to manifest itself once you connected the cable.