I’d like to clarify the difference between these two terms as I have found that there is very often confusion between them. The distinction is entirely in the software model and the programmability of software-based analysis by the instrument user. A Virtual Instrument’s primary programming model is to present raw data to the user for customized measurements. A Traditional Instrument’s primary programming model is to present vendor-defined measurements to the user.
What about Standalone Instruments versus Modular Instruments? This is a question of form-factor, not software, and is therefore entirely orthogonal to whether the instrument is virtual or traditional. A standalone instrument can indeed be used as a virtual instrument. An example is a standalone oscilloscope that is automated to create custom measurements in software. Similarly, it is possible for a modular instrument to present only a traditional use model to the user; VXI instruments, for example, were most often vendor-defined instrument repackaged in a modular form factor.
While the definition of virtual instruments and modular instruments is orthogonal, it is true that many modular instrument standards lend themselves to building virtual instrumentation systems. In order to effectively perform user-defined analysis on a signal, the user must have access to the raw data from the instrument’s acquisition. For high-speed measurements, this requires transferring many megabytes of data from the instrument to a processor to be analyzed in software. High-speed interface buses such as PCI Express, which can transfer data at up to 4 Gigabytes/s, are well-suited to this application. Instrumentation standards such as PXI combine high-speed buses and upgradeable PC-based processors, making it an ideal platform for virtual instrumentation systems.