I know it sounds like a silly question but do we?
Back in my early days (LabVIEW 5.1) I used to have a set of favorite VI's for my instruments that opened a VISA session, performed a task (like taking a measurement), then closed the VISA session. I used these a lot as I could drop them in and throw a program together without having to worry about wiring the VISA sessions. Fact this speed of test development really sold our company on LabVIEW...
Well later on I was having an issue with a larger program crashing. After spending some time on the phone with a NI Engineer troubleshooting he told me that the constant opening and closing VISA sessions was not a good idea and could be the cause of my issues. So I rewrote the program opening a VISA session once for each instrument and closing it at the end. This cured the crashing and forced me to abandon my library of drop-in VI's that had served me so well for so long, totally changing the way I was writing my programs.
Now fast forward to a couple years ago when my company made the jump from Labview 5.1 to 8.0 (that was a big step) now we are at LabVIEW 2009
Among the many other changes from 5.1 to 8.0 most notably VISA had changed, I/O wire types, VISA Open was gone, VISA Close was deprecated, all my old programs have the (old) tag on the VISA blocks... AND there was a Automatically close VISA sessions in the advanced options
The fact that VISA Close was deprecated kind of leads me to believe that we are being guided to used the auto-close option. It certainly makes programming faster not having to drag the VISA wires out and close them... But often I still do just out of habit.
This also has me wondering if we even still have to open a VISA session and wire it all the way through?
So take a look at the attached overly simplified examples and explain what is the "best method"?
I agree with nathand. Last year I was working on a VI that controlled a stepper motor using the VISA application. The first time i wrote the code my motor would rotate a certain amount of degrees and the program would stop running; however the motor kept rotating for the next two seconds or three seconds. I asked a coworker if he knew what the problem was and he told me that i needed to close my VISA in order to completely stop sending the signal. So I do think it is important to close VISA once the program is done running.