A Programmable Logic Controller consists of various input and output terminals. Using the input terminals, it interprets the logical states from switches and sensors. There are two logical states, also known as high (or 1) and low (or 0). The output from the output terminals is used as a signal for various devices, providing them on/off controls. The electrical engineers understand the ladder logic diagrams, and so it was the inspiration behind the programming language used to program it.