What exactly is VISA?

[RedAG]

Hi guys,

I am relative newcomer to the community and LabVIEW. I have come across the term VISA a lot during this time. currently I am working on a project which concerns VISA.

From what I understand, reading online, here in the community and through NI, it is some sort of communication to test if devices, which need to be communicated with, are working?

Thanks for any answer, Anoop

[Wolfgang]

http://www.ni.com/visa/

[Dennis_Knutson]

No. What sort of search did you do? Here is the main page.

Quite briefy, VISA is an instrument communication api that abstracts the hardware details of protocols such as GPIB, VXI, RS-232, USB, Ethernet, PXI. For example, there is the low level GPIB functions such as Write and Read. You could use a GPIB Write to send the *IDN? query and then a GPIB Read to get the results. The GPIB specific functions would work as long as you are only going to use a GPIB connection and use an NI GPIB controller. With VISA, you would use a VISA Write and a VISA Read. The VISA api will determine the actual physical interface based on the VISA Resource Name used. The code does not change if the physical interface is GPIB. VXI, RS-232, etc. The same applies to different GPIB controllers. As long as the vendor provides a VISA interface to their hardware, the VISA application will work with GPIB controllers from NI, Agilent, etc.

[RedAG]

Thanks man, I had already read this. I was trying to get a little bit more info on it. For example, can a VISA session be opened to a set of inputs acquired through an FPGA? 

I understand that the VISA session is an interface between the programming environment and the hardware. The link also states that it can be used for configuring purposes. Configuring the communication between the hardware and the software?

Thanks, Anoop

[Dennis_Knutson]

You use VISA to configure the hardware - i.e. VISA Configure Serial Port.

NI FPGA products are not valid VISA resources.

[RedAG]

Thanks Dennis,

So by using VISA, one can side-step going deep into serial/parallel communication, especially the configuring & communication aspects involved? What I understand from your reply is that it can be viewd as a generic device for a number of communication protocols.

[RedAG]

OK, so once I confugure the hardware, I would need it unless I need to re-configure? I mean, I can use MAX and while programming, can choose from an installed hardware for any communication for example. 

[Dennis_Knutson]

You still have to provide some details such as baud rate, parity, etc. for serial communication and you do have the option of setting other lower level parameters but typically, the defaults will work.

[RedAG]

Would I need to open a VISA session everytime a new hardware is installed? I can give you an example: I just installed a PXI-7813R which I have managaed to locate and communicate through MAX. I have actually used a program to read in digital input of the PXI. But I have NOT done any VISA related communication nor have I across a situation to make me think so.

If I were to install another NI hardware in, would I need to open VISA to this particular device, atleast once to configure it?

[Dennis_Knutson]

To repeat, programming the FPGA is not done with VISA. NI DAQ card programming is not done with VISA. If your program needed to communicate to a DMM for example, then the program would tyically open a session to the DMM at the beginning of the program and close the session at the end. You would use the same reference to the session throughout the program whenever you needed to communicate to it.