Monitoring system and technique

Claim 1

In apparatus for monitoring the states of a plurality of condition responsive sensors, each sensor being identified by a unique address, the monitoring apparatus including an evaluation circuit which generates binary coded sensor address signals and binary coded test command signals, said evaluation circuit also evaluating binary signals commensurate with output signals provided by an addressed sensor in response to stimulation, the improvement comprising: dedicated signal processor means connected to each sensor to be monitored, said processor means each being responsive to received binary address signals commensurate with the identification of the associated sensor for applying a stimulating signal commensurate with a received test command signal to the associated sensor, said processor means also detecting the state of the sensor upon application of said stimulating signal thereto and providing binary status signals commensurate with sensor state; a two conductor bus for connecting the evaluation circuit to said signal processor means; a source of direct current connected across the conductors of said bus, said source providing a direct current operating voltage for said signal processor means; and interface means for connecting the evaluation circuit across said bus conductors and for connecting each of said signal processor means across said bus conductors, said interface means each including: transmitter means responsive to binary coded signals provided thereto by the evaluation circuit or to binary status signals provided thereto by a said signal processor means for modulating the voltage measured between said bus conductors between a first level commensurate with a first binary state and a second level commensurate with the second binary state whereby sensor identification and test commands or the response of a sensor to a stimulating signal commensurate with a test command may be caused to appear as variations in the direct current bus voltage; receiver means responsive to variations in the direct current voltage measured between said bus conductors for providing an information bearing pulse train to an associated signal processor means or the evaluation circuit; and buffer means for maintaining an operating voltage at the associated signal processor means or the evaluation circuit.