Multiplexing involves the sequential or simultaneous transmission of multiple pieces of information over a common communication link. The most popular technique, time division multiplexing, is based on a time-sharing operation wherein different signals are sequentially transmitted over a common communication line. This method can transmit large quantities of information over a single signal path, at very high speed, with relative immunity to signal compromise. An example is given of how time-division multiplexed monitoring systems are connected using two twisted pairs of wire as the signal path. Fiber optic cable and other means can also be used. Advantages of this system include high-speed signal transmission, use of a common line to monitor multiple sensors, resistance to compromise, and transmission of large quantities of information. Using a multiplexed system, a single communication line can link literally hundreds of alarm sensors. Diagrams are provided.