I just came across a similar issue to the user in this post:
http://forums.labjack.com/index.php?showtopic=4786&hl=%2Bmultiple+%2Bu3s
I'm attempting to connect to 2 U3 using the same machine. Using the LJControlPanel, I was able to read both devices and set the local ID. Using the ListAll VI, I was able to read both devices in LabView. Using the U3SingleAIN Example, the only way I was able to communicate with Device A was with the local ID (serial number did NOT work) while the only way I was able to communicate with Device B was with serial number (local ID did NOT work). Since I'm able to communicate to each device, I don't have a "problem" though it seems peculiar and it was frustrating working through that. Ideally I'd communicate to both devices the same way, but it looks like this should be stable.
Both devices are very new, hardware version 1.3, firmware version 5.31. The user in the linked thread was able to resolve his problem with a firmware update, but that doesn't seem to be the issue in my case. Is there a setting or configuration I can change or maybe fix that will allow me to communicate with both devices the same way, or is there a known scenario in which the U3 will behave this way?
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The most common problem is not setting FirstFound to False.
Try the following:
1. Use LJCP to set the LocalID of one U3 to 1 and the other to 2.
2. Close LJCP. Power cycle both U3s. Run LJCP and confirm they both show up as 1 and 2.
3. Close LJCP.
4. Run "Multiple U3 Efunction Loop Example.vi" and see if it works.