The Application Programming Interface (API) is a set of definitions, protocols, and tools for building software and applications. It acts as a contract or an intermediary that allows two separate software components to communicate with each other. Instead of needing to understand the complex internal workings of a device or service, a programmer can use the API's clearly defined functions and commands to interact with it. In the context of programmable power supplies, an API abstracts the low-level communication details. For example, a manufacturer might provide a driver library (which is a form of API) with functions like SetVoltage(5.0) or MeasureCurrent().
When a programmer calls these functions in their code (written in a language like Python, C#, or LabVIEW), the API translates these simple commands into the specific SCPI command strings or binary data that the instrument understands and sends them over the appropriate communication interface (like USB, Ethernet, or GPIB). This greatly simplifies development, as the programmer can focus on the logic of their test or automation sequence rather than on the intricacies of communication protocols.
Web-based APIs (like REST APIs) are also becoming common, allowing instruments to be controlled over a network using standard HTTP requests, further integrating them into modern IT infrastructure and IoT systems. Essentially, the API is the bridge that connects user-created control software to the hardware's functionality.