Abstract: A subscriber line interface circuit using integrated semiconductor circuit is provided with improved immunity to overvoltage and overcurrent conditions by increasing value of the battery feed resistors in series circuit with the ring and tip lines to increase the impedance encountered by a lightning-induced voltage spike to increase the survivability of the semiconductor circuit components. The battery supply voltage is increased to effect a commensurate increase in loop current, and the loop impedance is re-adjusted by appropriately configured operational amplifiers. A thermally responsive sensor is mounted in a heat-conducting relationship with resistive elements in the subscriber loop with an increase in temperature representing an overcurrent situation, such as power line cross-over, and provides a control signal effective to disconnect the subscriber loop from interface circuit.

Abstract: An inexpensive universal telephone signalling tone receiver employs switched capacitor filters to provide selective tone reception in accordance with pre-selected clock signal frequencies. The clock signals, which in the preferred embodiment are provided from an EPROM, are employed to determine the pass band center frequency for respective filters. The selected signalling tones, when received, are rectified and passed to respective sample and hold circuits, the output signals of which are sequentially applied for processing to a common analog-to-digital converter through a multiplexer. Depending upon the clock pulse frequencies programmed for a particular application, the receiver can be used for various signalling protocols, including R1 and R2, in either direction, as well as other single or multi-tone signalling applications.

Abstract: An improved telephone system selector unit employing an electronic control arrangement is capable of directly replacing or being added on to existing electro-mechanical switching arrangements of the Strowger or other types which position themselves directly in response to subscriber dial pulses. The electronic circuitry establishes rapid outlet scanning via a matrix of twelve-pole relays, each of which permits selection of three four-wire lines or four three-wire lines, depending upon a pre-selected operating mode. A digit treatment circuit evaluates the counter digits, selects the appropriate outlet group, and precludes scanning of restricted lines or numbers not present in the numbering plan. During scanning each three or four-wire circuit of each relay is scanned in turn until an available circuit is found. The digit treatment arrangement permits additional outlet groups to be added without regard to the numbering sequence.

Abstract: An identifier circuit for connection between a telephone system party line and a party line subscriber to permit identification of a calling party line subscriber with automatic number identifying equipment at a central office.