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NI-VISA vs Agilent VISA and VEE error 814

Question asked by kdsassoc on Feb 18, 2010
Latest reply on Feb 23, 2010 by tsilchia001
Every time I think I understand when and why I need to have NI-VISA installed as the primary VISA and Agilent as the secondary I run across a case that contradicts what i know.  Recently, I upgraded to VEE9.0, and IO lib 15.5.  I had NI-VISA 4.1 installed as the primary VISA and agilent as the secondary.  I had a piece of equipment hang and used the task manager to shut down the vee process.  when i re-ran it i received error 814 "device locked... (by VISA)".  error trapping did nothing to help.  I could not clear the error until i shut down the machine.  Obviously I can't do that.   Using NI-spy i could see that visa does not unlock it when the vee process is killed.  i guess i have an orphan process.  In the past I did not have this problem.  so I've been trying different versions of NI-VISA and IO librarys.  After checking with a co-work, he showed me that he had the agilent visa installed as primary without the NI-visa and he was using the same NI-USB HS GPIB device i was.   He only instaled the NI-488.2 driver (which also offered support for labview).  I thought that i needed to install the NI-visa because I was using the NI USB gpib card. 
So I have a few questions"
The big question is:  When do i need to use the NI-VISA as primary and the agilent as secondary? when is it acceptable to just use the agilent visa?  will labview applications run without the ni-visa? with just the agilent visa? if only running vee is agilent visa acceptable.  What is realy happening when agilent visa instaled "side-by-side"?

is there another way to capture and clear error 814 other than shutting down?