Abstract: A security system including a plurality of sensor loops, each having a plurality of detectors and associated detector identification devices which upon actuation produce analog detector signals of different amplitude for each detector in such loop to identify the actuated detector. The sensor loops each have a common loop output which is connected to a different one of a plurality of input terminals of an analog to digital converter and multiplexer circuit which includes a multiplexer switching circuit for selectively switching between such inputs and which converts the selected analog detector signal to a digital detector output signal. The outputs of the analog to digital converter are connected to a digital data processor circuit, such as a microprocessor computer, for processing the digital detector signals to determine which detector produced such signals and to generate a corresponding output signal that may operate an alarm means such as a bell or indicator light.