The basic function of the fire alarm control unit (FACU) is to serve as the central processing and decision-making component of a fire alarm system. The FACU receives input signals from initiating devices such as smoke detectors, heat detectors, and manual pull stations. It evaluates these signals and, when alarm conditions are met, activates notification appliances including horns, strobes, and voice evacuation systems to alert occupants. The FACU may also transmit alarm, supervisory, and trouble signals to a supervising station or fire department.

In addition, it controls system outputs such as releasing magnetic door holders, shutting down HVAC systems, recalling elevators, and activating fire suppression or smoke control systems when required. By monitoring system integrity and coordinating detection, notification, and control functions, the FACU ensures timely warning, safe occupant response, and proper system operation during fire or emergency events.