Abstract: A circuit for notifying a PBX that a data port in a telephone connected to the PBX is about to be used. The circuit comprises a current sensor for generating a signal that indicates a modem is drawing current through the data port. A large resistor limits the current being drawn through the data port to a level that does not indicate an off-hook condition to either the PBX or the modem. In response to the signal generated by the current sensor, a separate telephone circuit provides an off-hook condition that draws a first dial tone from the PBX. A microprocessor in the circuit generates a signal that causes a dialer in the telephone circuit to dial a number that notifies the PBX that the modem is attempting to access the PBX through the data port. After the PBX is notified of the imminent data port usage, it presents a second dial tone on line two which is the dial tone for an outside line.

Abstract: A line-powered circuit for connecting a telephone line to a telephone voice circuit comprised of a digital part and an analog part. The digital part is a logic circuit for generating a line connect signal that indicates a telephone line should be connected to the voice circuit. The power for operating the logic circuit is provided by the line voltage associated with the tip and ring leads of the telephone line. The analog part comprises a first switch means for connecting the voice circuit to the tip lead in response to the line connect signal, and a second switch means for connecting the voice circuit to a ring lead in response to the line connect signal.

Abstract: A message waiting circuit comprised of a pair of electrical leads which carry a message waiting signal, a capacitor for storing electrical energy extracted from the message waiting signal, a transistor for allowing the capacitor to charge when the message waiting signal is within a predetermined range; a Darlington transistor for causing the capacitor to discharge; and a light emitting diode (LED) which emits light when the capacitor discharges. The flashing LED indicates that a message is waiting for a user of the telephone set associated with the message waiting circuit. Approximately ten to twenty-five message waiting circuits and associated telephone sets can be connected to a single telephone line.