VISA close error

[Blokk]

Hello,

I use simultaneously the attached 3 VIs. Each of them is a PID loop, reading data from some Keithleys, and driving the loops using Keithley 2400 sourcemeters. So there are 3 2400 sourcemeters in my project. The data communication is via GPIB (NI HI-USB GPIB controller)

I use the official VISA drivers for the Keithley 2400s (some minor modifications in the "configure output" VI, I use double precision instead of single precision as in the official driver).

Everything works perfect, the three VIs run parallel, no communication error, so all is good.

But, randomly, lets say 50 percent of the time, when I want to shut down only one of the VIs, I get the VISA Close error in the error output. 

Here is a screenshot when I get this error: http://dl.dropbox.com/u/8148153/Untitled.jpg

What can cause this such a error? The same VISA drivers are not allowed to be used parallel?

Thanks for help!

[AndrewAlford]

under options:Environment make sure the Automatically close VISA sessions is unchecked. I would think this could cause your problem as you shut down one VI it is trying to close the VISA session but the other open VI's are using that resource creating the conflict.

[Blokk]

Hello,

The Automatically close VISA sessions is unchecked (i think it is also the default state). So this cannot cause the experienced problem. So far this problem is not that serious, since I am at just the prototyping phase in my project (that is why i run the 3 VIs separated), the final MAIN VI will close all the VISA resources at once at shut down.

But I like to learn, so i am still curious about what this strange problem with VISA...

Regards,