Analog inputs all show same value

[JTDWAB]

I have a VI that uses a DAQmx task made up of 12 analog inputs. In the VI and in the DAQmx test panel for the task all the channels show the same values (the value of channel 0).  If I go the hardware level and look at the voltage readouts (in the DAQ) of each of the channels in the SCXI chassis they are independant.  What can I do to seperate the channels so they read independantly in the task?

[Andy F.]

Are you reading the SCXI channels or the channels for the DAQ card.  When using SCXI in multiplex mode all channels are multiplexed to channel 0 of the DAQ card.  This might be what you are seeing.  I would check out an examples of using SCXI in NI Example Finder.  I searched for SCXI and found several hits for DAQmx for SCXI, depending on the application.  Hope this helps!

Andy F.

[JTDWAB]

Alas I have checked that. Within MAX the channels are each mapped to the SCXI module and connection point of the 1300 connection block. When read individually they indicate the proper values but when operated together as a task they all read similar to channel 1. There is variation between each single channel in the millivolt range. 

I tried using labview 7.1 and the DAQ asistant to create a simple VI in which I selected a couple of the channels one at a time assiging each to its terminal in the 1300 block.  When I ran the vi all the values followed the 1 channel just as in MAX.  I didn't expect anything different but since I was letting the software create the tasks outside of MAX I figured it was worth a shot.

[Andy F.]

Have you tried to use the physical channels in LabVIEW rather than the channels set up in MAX?  It might be helpful to attach a screen shot or your code (simplified to just show the problem code) and I may be able to help out a little more. 

Andy F.

[JTDWAB]

I was able to solve the problem, but it still doesn't make sense.  I was using a 4 bay SCXI chassic with an 1104 in, 2 was empty, 1102B in 3 and a1180 in 4. The 68 pin connector was plugged into the 1102B module since it and the 1180 were the only modules we were using on this project.  The system had worked fine in this configuration before. Just for grins I moved the connection of the 68 pin connector to the back of the 1104 module and everything worked out great. 

Anybody who can explain why it didn't work when put into the 1102 module, let me know I am quite curious.

Thanks for the help

jamie 

[Andy F.]

Jaime,

Have you recently gotten a longer cable or changed out your SCXI-1349?  I have recently seen an issue using a DAQ cable of 5 or 10 m, or a newer revision of the 1349 along with a SCXI-1102 and 1104.  It seems like you are encountering this same behavior.  The fix is, as you discovered, to move the controlling card to be the 1104 rather than the 1102.  These issues seem to stem from a specific hardware setup that you were able to replicate.  Hope this information helps!

Andy F.

[VIdesmos]

I understand this thread is extremely old, however, I came across the same problem myself. One may observe that all channels may show the same value when only one channel should read something. For e.g. you have 6 channels, and you are passing some non zero voltage to just one channel, you will observe that all channels show the same voltage as if all 6 channels are receiving the value. 

The reason this happens is that NI inherently does not tell what the channels should do when they are unconnected. So there is nothing wrong in the code, it is just that the device and NI utilizes the adjacent signals to give you a value. 
More info here: 
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA00Z000000P6TeSAK&l=en-US

In order to get a reality check, it is best to test it out with the real component, or by zeroing it with connection to ground (or changing your RSE/Differential mode).