How do you tell VISA which instrument over which interface you want to connect to? All this information is coded into one string called Resource String that you pass to the VISA Open function. The USB class of devices natively supported by VISA are called USB-TMC (Test and Measurement Class). The last addition was support for the USB interface. Later, came its improved version called HiSLIP (High Speed LAN Instrument Protocol). For the LAN interface, the TCP/IP-based protocol was named VXI-11. The current maintananer of VISA is IVI Foundation. Out of the motivation to unify the software interface, VXIplug&play Alliance introduced VISA standard (Virtual Instrument Software Architecture). However, using a third software interface for the LAN-connected instruments would bring even more incopatibility for the measurement applications. Test & Measurement instrument vendors have realized the potential LAN interface can bring to the connectivity options for their instruments. The situation got even more complex as LAN interface has become the standard in the computer industry. Here too, applications communicated directly with RS232 hardware drivers. Later, the RS232 serial interface was introduced as a cheaper alternative to GPIB. At the time, where GPIB was the main physical communication interface, remote control applications communicated directly with GPIB hardware drivers.
