Abstract: By the use of a feedback arrangement, deleterious circuit effects of an isolation transformer in a terminal device for a telephone line can be eliminated. By this arrangement, a smaller, cheaper transformer may be used. A terminal device employing such arrangement can present a balanced termination to a telephone line, exhibit a known and controlled impedance to that line and provide for signals to be coupled to and from that line.

Abstract: A telephone station subscriber line interface circuit for connecting a telephone subscriber station to a telecommunications transmission line comprises a loop current sensing circuit for providing an output signal proportionately representative of DC resistance in the telecommunications transmission line. A ring-side driving circuit is coupled to the loop current sensing circuit and is responsive to the output signal for providing an alternating current voltage source and a direct current voltage source to a transmit amplifier in periods of high constant DC resistance in the telecommunications transmission line, the ring-side driving circuit maintaining the output of the alternating current voltage source constant to the transmit amplifier and automatically transferring from a constant direct current voltage source to a constant direct current current source in the event of a decrease in the DC resistance in the telecommunications transmission line sensed by the loop curent sensing circuit.

Abstract: This invention relates to apparatus which can detect both interruptions and reductions in an alternating current input extremely quickly. This allows an orderly shut down of the equipment before the output of the equipment power supply decays. The device can be utilized in any application requiring detection of changes in an alternating current input.

Abstract: The invention relates to a circuit for switching between a ring signal and a message waiting signal in a telephone system, such as a PBX, without the occurrence of "bell tap". The instantaneous value of the ring signal is compared with a D-C value of the message waiting signal to generate a smooth transition between the ring and message waiting states of the composite RAC/MW signal. The circuit selectively suppresses the controlling edge of the ring synchronous signal whenever a particular subscriber station would be connected to the RAC/MW signal at an inappropriate time. Accordingly, the problem of "bell tap" is eliminated.

Abstract: A subscriber line interface circuit is disclosed for a 2-wire telecommunication subscriber line. The circuit includes a DC voltage source for supplying power to the subscriber line and an automatic power denial circuit for interrupting the power if an overload condition exists. The power denial circuit includes two resistors connected in series in one wire of the subscriber line and a transistor connected to respond to the voltage drop across these two resistors, thereby to sense an overload condition. A second transistor, controlled by the first transistor, is connected across one of the two resistors and is biased to be normally conducting when no overload condition exists. When excessive current flows through the two resistors, indicating an overload condition, the second transistor is cut off to deny power to the subscriber line.