Abstract: A modular system for providing security to an office or residence is provided that includes a plurality of modular components that each connect to a common AC electrical power supply line and communicate digitally with each other by supplying and decoding a digitally encoded message via a carrier signal that is modulated over the AC line. At a minimum, a control panel and at least one sensor are required. A signaling device provides indication of an emergency situation. Easy installation is provided by installing each of the modular components and by powering the control panel lastly which, upon power-up, polls every possible digital address to determine which modular components are connected. Optional telephone connections to a central monitoring site are also provided as well as remote control of the system over the telephone line by the monitoring site or by the user.