The Universal Serial Bus Test & Measurement Class (USBTMC) is a specific USB protocol, or "class," that is designed to allow USB-based instruments to be controlled in a manner very similar to traditional GPIB instruments. The goal of USBTMC is to allow programmers to use the same high-level software and APIs (like the VISA library) to control an instrument, regardless of whether it is connected via USB or GPIB.
When a USBTMC-compliant power supply is connected to a computer, it is recognized as a test and measurement device. The VISA software layer can then communicate with it directly, just as it would with a GPIB device. This means a programmer can send text-based command strings (like SCPI commands) and receive responses using standard VISA functions, without needing to worry about the low-level details of USB communication.
The USBTMC protocol defines the specific endpoints and transfer types that must be used to send commands, receive responses, and handle status information. This standard is highly advantageous because it combines the ease of use and high speed of USB with the well-established programming paradigm of GPIB/VISA, making it the preferred USB protocol for modern test and measurement instruments.