No voltage reading, even though it is there

[Brian_Beal]

Here is the age old situation.  I can read 1.5 volts with a voltmeter at the analog input, but the device reads around zero(all 4 channels on 2 devices).  This is a USB-6210.  I am reading a 4-20mA signal across a dropping resistor.  The analog inputs are configured for differential mode.  Any ideas what I am missing?

[devchander]

Dear Brian,

does it happen along with the problem that you have posted here ??

Or does it happen even if you connect the DAQ cards to a system that has only MAX / LabVIEW to control your acquisition??

[Brian_Beal]

All of the issues that I posted last night are occurring on the system with the DSC application and global channels..  I guess I can pretty easily plug one of the 6210's into my laptop and see what happens.

[Sandra_T]

Hello Brian Beal,

It looks like you are trying to make a voltage measurement with your USB-6210.

I would first start with testing the hardware in Measurement and Automation Explorer. This will remove the programming from the picture. Start by reseting the device. You can do that by right clicking on the device and selecting Reset.

Now try opening a test panel, Analog Input. Make sure you have the signals connected correctly. You can reference the user manual for this information:

NI USB-621x User Manual

For instance, a differential measurement of AI0, connect + to ai0 and - to ai8. You can get the pinouts from the user manual. Configure the settings, making sure the range selected is correct for the value you are measuring. Now make a measurement.

I also noticed in your post, you have tried doing this already, but I restated these procedures to make sure we're on the same page. You are still measuring zero volts.

Try the following:

1) check to make sure the LED on the device is on and blinking. There is a list of LED Indicator codes.
2) Try measuring a battery or DC voltage.
3) These this hardware on different system.
4) can you use the AO, Digital I/O channels

Give these a try and let us know about the results of the troubleshooting.

Regards,
  Sandra T.

Applications Engineer | National Instruments

[Brian_Beal]

Sandra-

Everything is configured and wired like you are stating.  I cannot test in MAX until I get by this problem. The LED is blinking green.  The digital I/O is working fine.  I will try to plug it into a different system.

Brian

[Brian_Beal]

The card is working.  I checked the analog inputs with a steady 5V signal, but it only read correctly when I diconnected some of the other inputs.  I might have a common mode voltage issue.  Most of the analog inputs are set up for differential mode to read the voltage drop across a resistor.  The manual states that the AI- signal must be grounded, but doing that changes the current path that I am trying to measure.  The AI+ terminals are seeing 24V and the AI- terminals are seeing about 23 volts.  The differential across them is 1V, which I can read with a DMM, but that card cannot.  Looking at this KB, I believe that the shunt resistor is in the wrong spot.  I will change that and see what happens.

[MechMan]

Respected:

i m also facing the same problem ( almost ) , when i use DMM to measure the voltage of linear resistor of UTM machine , i found voltage of approx. 0.3xx mV and its change with the change of linear placement And on the other side i also found resistance change during linear placement , But didn't got any thing on my DAQ Card  ( NI - PCI 6601 - analog input - differential ).

plz help me out.

thanx

[Eric-J]

It looks like the PCI 6601 is a digital I/O card with no analog inputs.  Are you sure this is the card you are using?