Two-wire multi-zone alarm system

Claim 1

Apparatus for use in a multi-zone alarm system having a two-wire alarm loop, a plurality of alarm sensors in series with the loop, and a processor at a central location coupled to the loop and operative to indicate an alarm condition in response to alarm signals from any of said sensors, comprising: a constant current source serially connected to the alarm loop and operative to provide a predetermined current signal having a constant non-zero magnitude in the loop; a plurality of networks, each consisting of a resistor of selected different resistance value connected across a respective alarm sensor and operative in response to its sensor actuation and to the constant non-zero magnitude of said loop current signal to provide a signal pulse for transmission in the loop to the central location processor, the signal pulse having a selected different non-zero magnitude to denote the identity of the actuated sensor; and circit means at the central location processor capacitively coupled to said loop operative in response to the particular different non-zero magnitude of the signal pulses from any or more of the networks to provide a signal indication of the zone in which alarm actuation has occurred.