PLCs (Input/Output Units), formally called "programmable logic controllers," are components used as an interface between devices and sequencers. They work by controlling the on/off switching of the output device using a preset program that operates in response to the on/off switching of switches and sensors. They were initially developed as an alternative to relay circuits and are used in controls for boilers, pumps, automated machinery that operate inside factories, and more. PLCs are broadly divided into 2 types, modular type and packaged type. Modular types require input components and standalone units to be combined and allow for more freedom of customization. Packaged types are totally pre-assembled to offer an inexpensive installation solution.