Abstract: A ringback circuit in a security protection system, of the type which monitors a dc current, automatically reports to remote premises that the central station has, at an appropriate time, received an indication that the dc current has been changed to the normal current level for night operation. At the central station, a change in the current level from normal daytime to normal nighttime current is sensed, activating an oscillator. An electronic current limiter in the communications line responds to the oscillator through an isolation circuit, oscillating the current in the communications line, which activates a buzzer at the remote premises.

Abstract: Digital interface circuitry is utilized to analyze the condition of alarm detecting loops. The new circuitry is utilized as an alarm system of the type which has a detecting loop extending between a central station and a remote zone at which is provided a normally open detection switch in parallel with a resistor. At the central station a digital high voltage is supplied to the input loop lead and a resistor is connected to ground from a reference point on the return lead, to provide a voltage divider network with the remote resistor. Two voltage-sampling branches connect to the reference point; the first branch has a comparator or pull-up circuit to indicate digitally whether the voltage level is substantially zero or greater, while the second branch has a level detector and pull-up circuit or a comparator to indicate digitally whether the reference point voltae is greater than its normal level.