How can I collect both differential and single ended voltage inputs in labview or signal express

[jbh]

I am having difficulty collecting both single-ended (3 devices) and differential (1 device) analog inputs with labview.  I am using a USB 6211 device and have configured the input correctly (I hope!!).  Anyway, whenever I use labview or SignalExpress, the data collection crashes pretty quickly.  I believe it has something to do with the timing/buffer time of switching between the different inputs, but I haven't come up with any good solution.  Any suggestions?

Thanks!

-J

[tbob]

I use a PXI-6225, so I'm not sure about the 6211.  If I want to make a differential and single ended measurements, I need to put a 10mS delay between the two reads.  Read differential, delay 10mS, read single ended.  If I'm making just single ended measurements, I can perform multiple measurements in parallel.  If I'm making two or more differentials, I need to delay between each measurement.  I don't think there is a way to read differential and single at the same time.

[jbh]

THanks for the suggestion!  I have tried that (and even a longer delay time between the different channels), but the program still crashes.  I've now cut out all but one channel in SignalExpress and get this error message:

Logging was stopped because an error occurred with a Step in the Project.

If this error is because of a device buffer overflow, you can improve performance by turning off display updates using the "View>>Update Signal Views While Running" menu item.
You can also change the way signals are logged by changing the Logging Options in the Tools>>Options dialog.
Turning off "Prepare log data for viewing" will greatly improve performance.

I've taken the suggested steps, but still get this error.  I think I need some help with buffers in general, or my computer is not able to handle the info, but it meets/exceeds the stated requirements...

Thanks

J

[tbob]

It may be buffer overflow.  The error I got when trying to do both simultaneously was something about a resource problem.  But that was with Labview, not Signal Express.  Try cutting down the sample rate or number of samples.

[Matthew_Kelton]

Depending on how much data you're grabbing, you could also do everything in SE, and do the math yourself to change the needed signals to differential.

If you are running continuous data collection, you need to make sure you do not do too much data processing in your loop that causes the loop to not run fast enough to keep up with the DAQ card.  If you don't need continuous data collection, the do a fixed number of samples, and it won't matter if it takes you 10 minutes to read again, because their would be not data buffer to overflow.

[jbh]

Now I'm running into a problem of the error message telling me "that resource is not avialable."  This is all in SignalExpress, by the way.  I have successfully collected data from one channel (I've been working with the SE inputs).  But when I add the differential (or vise versa) channel to the mix, the program crashes.  I've tried to change the delay time and the sampling rate etc, but maybe I'm not doing it the correct way?  I can adjust the sampling period (which is correlated with the sampling rate, right?) and I have adjusted the steps so that there has been between a 125ms to 500ms delay between the two channels.  I have also attempted to collect the data 1 sample at a time (and assume the loop will allow me to keep collecting 1 sample at a time...it works for one channel, but not for 2).  But nothing seems to work.  Am I missing some other step or required input or something, like a defining a scanning order or something?

In terms of calculating things manually, in order to collect the data and calculate the differential, I would collect both the positive and negative inputs as single ended and then subtract the two (I'm an anthropologist, I don't really know much about electrical devices)?  I like that idea, if I can't get the programming to work.  But it really seems like the program should work!!

Thanks for your help,

J

[Matthew_Kelton]

I don't use SIgnal Express, so I cannot help you there, but yes, if you coolect your measurements in SE, then take the positive side-negative side and you will get the differential value.

[tbob]

Post your program here and we can look at it.  The 'resource' error indicates that the device is already in use (busy) and you can't start using it again.  It still sounds like a timing problem.