Virtual Instrument Software Architecture, commonly known as VISA, is a widely used I/O API in the test and measurement (T&M) industry for communicating with instruments from a PC. VISA is an industry standard implemented by several T&M companies, such as, Anritsu, Bustec, Keysight Technologies, Kikusui, National Instruments, Rohde & Schwarz, and Tektronix.
The VISA standard includes specifications for communication with resources (usually, but not always, instruments) over T&M-specific I/O interfaces such as GPIB and VXI. There are also some specifications for T&M-specific protocols over PC-standard I/O, such as HiSLIP or VXI-11 (over TCP/IP) and USBTMC (over USB).
The VISA library has standardized the presentation of its operations over several software reuse mechanisms, including through a C API exposed from Windows DLL, visa32.dll, over the Microsoft COM technology, and through a .NET API. Although there are several VISA vendors and implementations, applications written against VISA are (nominally) vendor-interchangeable thanks to the standardization of VISA's presentation and operations/capabilities. Implementations from specific vendors are also available for less common programming languages and software reuse technologies.
History
VISA was originally standardized through the VXIplug&play Alliance, a now-defunct T&M standards body. The current standard, "VISA Specification 5.0", is maintained by the IVI Foundation.