A multi-sensor security/fire alarm system comprises a master control unit which repeatedly interrogates, e.g., by a multiplexing scheme, the respective inputs and/or operating status of a plurality of remote sensor units (e.g., intrusion and fire sensors). In response to an alarm or other off-normal condition detected by any of the remote sensors, the master control notifies an alarm-monitoring service which either responds itself to the detected condition, or notifies the local police or fire department. To prevent one alarm-monitoring service from taking over, without authorization, the alarm-monitoring accounts of another service which may have expended considerable time and expense in installing the system, the master control unit is mated with its associated sensors by a unique code. According to a preferred embodiment, each sensor comprises an EEPROM which stores a unique portion (e.g., a single digital bit) of a multibit security code which is collectively defined by the unique code portions of all the individual sensors and stored in memory of the control unit. The control unit periodically addresses the remote sensors to compare the "distributed code" with the code stored by the control unit.