Difference in readings between DAQmx and Traditional with SCXI 1100 module

[CJW1800]

Hi,

My existing setup is a PCI 6036e connected to a SCXI 1001 chassis through slot one that contains a SCXI 1100. I'm using the traditional daq drivers.

I have come to change the PC and the setup has changed to a PCI-6221 connected to the chassis and I've changed to the DAQmx drivers.

Doing some checks between the old and new PCs has uncovered an issue with one of the channels on the SCXI 1100 module. The channel in question is +/-10v and reads about 0.2ish volts low on the new setup compared to the old setup.

If there is any more information you need let me know.

Any help would be much appreciated.

Thanks

Chris

[TSJim]

Hi,

I would check for grounding problems.  If a ground is daisy-chained on the DAQmx system to a power line this can easily generate 200mV on the ground line and make it seems like the measurement is low. 

Using a DMM with one connector on one ground and taking measurments of other ground points and Vin's can help narrow down the problem.

Thanks,
Jim

[CJW1800]

Hi Jim,

Thanks for the reply.

Non of the hardware, apart from the PC and DAQ cards have changed (that maybe wasn't clear from my explanation), so would i not expect to see the same issue on the traditional setup if it was something to do with grounding?

Cheers

Chris

[Joel_G]

Hi Chris,

Do you mean it reads 0.2V lower than it should? or does it read 0.2V at the lowest? If it's the latter then could it be that the range is set to be 0-10V instead of -10 - 10V. I only ask because typically if you specified that kind of range and gave it a value outside it, it would give a reading just below the spicified minimum.

Thanks,

[CJW1800]

Hi Joel,

Thanks for the reply.

The 0.2v is the error between the DAQmx and traditional, not the lowest value that I am seeing. Also, the error increases as the voltage increases.

I have made some headway:-

When I ran the test panels for the channel in Max for both DAQmx and Traditional, they read the same voltage. When I ran the code with the DAQmx configuration, I got the same voltage as I saw in Max, but when I ran the code with the traditional configuration the voltage was reading high.

The Traditional configuration is using a synchronised acquisition vi to acquire the data from the two devices. I quickly setup a vi to acquire just from the device connected to the chassis and the voltage came out the same as I had seen in Max.

So that leads me to believe that there is something wrong with the acquisition vi for the Traditional configuration (see attached), however I can't see anything obvious.

Any thoughts?

Thanks

Chris

[Joel_G]

Hi Chris,

Sorry for the delayed reply,

If the measurement appears to be correct with the DAQmx VIs, why don't you just continue to use these?

Now that you have the new system is there any reason to continue using traditional DAQ?

Kind regards,

[CJW1800]

Hi Joel,

We are planning to move forward with the DAQmx setup, but it would be good to know what the issue is as the traditional setup is common to other applications on site.

It would also be good to get to the bottom of it for peace of mind!

Thanks

Chris

[Joel_G]

Hi Chris,

Just to clarify, are you taking the two readings (DAQmx and traditional DAQ) through the two different hardware setups but of the same signal?

Or are you taking the two different readings with DAQmx and traditional DAQ through the same hardware?

I was wondering if it might be to do with the calibration of the DAQ cards, have you been doing self-clalibrations in MAX?

Thanks,

Kind regards,

[CJW1800]

Hi Joel,

Originally, the readings were being taken through the same SCXI chassis, but different DAQ cards in different computers. However, I did run both the DAQmx and Traditional setup on the original computer and I saw the same issue.

So, in summary I have the same DAQ cards, the same chassis, the same computer and I get a different reading between my DAQmx acquisition vis and my Traditional RTSI Synch.vi (attached in previous post). When I use a single card acquisition for my Traditional setup I get the correct reading.

Cheers

Chris