Abstract: A subscriber line interface circuit apparatus includes a linefeed circuit and a subscriber line control circuit (SLCC). In an embodiment, the linefeed circuit includes a signal conversion circuit which provides a differential mode signal and a common mode signal in response to at least the tip signal and the ring signal from the subscriber loop. The linefeed circuit includes a tip control circuit and a ring control circuit. In an embodiment, the SLCC is provided in a single integrated circuit chip and is coupled to the linefeed circuit which isolates the SLCC from the tip or ring signals. The SLCC includes a first and a second differential mode inputs for receiving the differential mode signal, and a common-mode input for receiving the common-mode signal. In an embodiment, the SLCC also provides various tip and ring control signals to the tip control circuit and the ring control circuit, respectively.

Abstract: A method and apparatus is provided for performing an active impedance feedback. A signal is received. An active feedback synthesis is performed based upon the signal. Performing the active feedback synthesis includes using an active feedback network for conditioning an output signal and performing a signal summation of at least two components of the conditioned output signal for feedback.

Abstract: In one aspect, a phone-interface device determines whether a problem has occurred at a control panel by determining whether a signal from the control panel has been received, and when a signal has not been received within a period of time, the phone-interface device transmits an error message to a monitoring station. In another aspect, the control panel determines whether a problem has occurred at the phone-interface device by determining whether a signal from the phone-interface device has been received, and when a signal has not been received within a period of time, the control panel transmits an error message to a user. In another aspect, the phone-interface device contains memory to buffer data transferred between the control panel and a monitoring station. In this way, a high-speed connection between the phone-interface and the monitoring station can be accommodated while using a lower-speed connection between the phone-interface device and the control panel.

Abstract: A system for determining the transmit power of a communication device operating on a twisted pair subscriber line includes a communication server coupled to a first subscriber line and a second subscriber line. The communication server comprises a communication device that communicates a signal using the first subscriber line. A memory coupled to the communication server stores noise information and cross-channel-coupling information for the first subscriber line and the second subscriber line. A processor coupled to the memory determines the transmit power of the communication device based upon the noise information and the cross-channel-coupling information.

Abstract: A method and apparatus is provided. The method includes transmitting a signal having an AC component to a subscriber line and receiving at least a portion of the transmitted signal from the subscriber line. The method includes determining at least a portion of a period of the AC component of the received signal, and performing a function of a line card in response to determining at least the portion of the period of the AC component. The apparatus includes circuitry that is capable of transmitting a signal having at least one of an AC component and a DC component to a subscriber line, and receiving at least a portion of the transmitted signal from the subscriber line. The apparatus includes a filter and computation logic. The filter is capable of filtering the DC component from the received signal. The computation logic is capable of determining a value proportional to a power of the AC component of the received signal over at least a portion of a period of the AC component.

Abstract: A method of automatically resolving a Digital Subscriber Line failure can include detecting a failure of the Digital Subscriber Line and establishing a call over a public switched telephone network with an administrative system for the Digital Subscriber Line. The method further can include notifying the administrative system for the Digital Subscriber Line of the failure over the established call. The administrative system then can cause the Digital Subscriber Line to be reset.

Abstract: A system for extending the effective distance of digital subscriber line service is provided. The system includes a central office terminal having a data interface and a plurality of line units. The system further includes at least one communication link, coupled to one of the plurality of line units, that carries signals using digital subscriber line service and at least one remote access multiplexer, coupled to the at least one communication link. The at least one remote access multiplexer includes a plurality of ports that are adapted to provide digital subscriber line service. The remote access multiplexer is adapted to multiplex signals between the plurality of ports and the at least one communication link. Further, the at least one remote access multiplexer is located at a distance from the central office terminal so as to provide digital subscriber line service to user terminals that are located more than 12 kilofeet from the central office terminal.

Abstract: A variable power supply is controlled to supply power at approximately a first supply level for an electronic device. The variable power supply is controlled to control a transition of the power from approximately the first supply level toward a second supply level prior to controlling the variable power supply to supply power at approximately the second supply level for the electronic device.

Abstract: Current to a relay causes a telephone to be connected to a SLIC. Following LOOP, a sleep switch opens to interrupt current to the power supply converter after a first predetermined period expires. If the telephone is on-hook, loss of current to the relay causes disconnection from the SLIC, but causes connection to a battery. Current sensed from the battery through an off-hook hook switch generates a current signal. The current signal closes a current switch, which is in parallel with the open sleep switch, thereby providing a current path to the converter. The sleep switch then makes and then the current switch breaks. After awakening, a ring-back signal informs a user that telephony service has been restored and a message may indicate that telephony service is being sought. When the telephone is placed back on-hook, counting of a second predetermined period starts, after which the sleep switch opens again.

Abstract: A faster modem apparatus communicates with customer premise equipment according to a variety of communication protocols. The faster modem apparatus communicates with customer premise equipment comprising an analog soft-modem comprising a codec. A method utilizes an unused portion of the codec operating range to code and decode data signals having frequencies above 4 kHz. The faster modem apparatus also codes and decodes data signals above 4 kHz. The analog soft-modem and the faster modem apparatus transmit and receive coded analog signals at a signal level of greater than ?9 dBm. In one embodiment, the faster modem apparatus is also operative to communicate with additional modems such as dial-up modems and DSL modems. In one embodiment, the faster modem apparatus supports simultaneous voice and data communications.

Abstract: A facsimile apparatus can properly detect whether or not the facsimile apparatus is connected to a line. The facsimile apparatus comprises a network control device formed of a semiconductor DAA device having a primary line control part driven by a local power supply and a secondary serial interface circuit communicating with said primary line control part through a photocoupler and a local power supply detecting part detecting a local power supply to said primary line control part. As a result, it is possible to detect whether or not the facsimile apparatus is connected to the line.

Abstract: A telephone line interface and associated circuitry that may be implemented to simultaneously couple DC loop voltage, ring bursts, and caller-ID data to the phone line side of a DAA circuit through a single two wire resistively coupled interface. The interface and circuitry may be implemented to combine processing of the DC loop voltage and ring bursts into a single circuit, with processing of caller-ID data being performed separately.

Abstract: A novel method of configuring a physical layer transceiver for providing data communications via residential wiring. A transmit section of the transceiver produces a pulse signal having selected amplitude. This pulse signal received by an input circuit in a receiver section of the transceiver is used for adjusting the gain of the input circuit to a fixed optimum level. In particular, the gain of the input circuit may be adjusted in response to at least one pulse. A calibration circuit of the transceiver includes a comparator for comparing the receive signal produced at the output of the input circuit, with a threshold level, and controller that supplies the input circuit with a gain control value, and sets the threshold level. The controller reduce the gain control value to decrease the gain of the input circuit when the receive signal exceeds the threshold level. The gain control value is increased to raise the gain of the input circuit when the receive signal is less than the threshold level.

Abstract: A passive echo cancellation circuit for reducing echoes on a subscriber line includes an output driver adapted to drive a downstream signal on the subscriber line. First sensing circuitry is coupled to the output driver and adapted to sense a current on the subscriber line to generate a first voltage. A balancing network is adapted to receive at least a portion of the downstream signal. Second sensing circuitry is adapted to sense a current in the portion of the downstream signal passing through the balancing network to generate a second voltage. An upstream driver is adapted to subtract the second voltage from the first voltage to generate an upstream signal.

Abstract: A method is provided for introducing analog loss to an information-bearing signal being transmitted to or from end-user terminal equipment over a communication system such as a cable network The method begins by establishing a communication path to the terminal equipment over the communication system and then introducing a level of loss into the communication path to reduce echo. The level of loss that is introduced is based on the information content of the information-bearing signal.

Abstract: The object of the present invention is to remove the dead region of a diode bridge and maintain stable operations in a telephone terminal equipment interface circuit. In order to realize this object, a telephone terminal equipment interface circuit, according to the present invention comprises a diode bridge for rectifying line current supplied from a subscriber line and supplying the current to a call transmission/reception circuit, and a forward biasing circuit that supplies forward biased voltage to the diode bridge. Supplying forward biased current to the diode bridge makes the diode bridge operate in an activated state, thereby removing the dead region of the diode bridge and enabling more stable operations to be maintained in the interface circuit.

Abstract: A wireless local loop (WLL) system capable of interfacing with an analog communication device is provided. The WLL system comprises: a SLIC (SLT interface Circuit) capable of matching analog data line characteristics of an analog communication device; and a MODEM for at least (1) receiving an analog data signal provided by the analog communication device through the SLIC, and converting the analog data signal into a class ‘0’ digital data, or (2) receiving a class ‘0’ digital data provided by an MSM, converting the class ‘0’ digital data into an analog data signal, and providing the analog data signal to the analog communication device through the SLIC.

Abstract: An interface device and method for testing a telecommunication circuit by utilizing a test cord that has a first end that is integrated with the interface device and a second end that terminates with a test connector. Upon insertion of the test connector into a test port of a connectivity block, the interface device may be configured to allow for monitoring of the telecommunication circuit, without disrupting the circuit. Similarly, the interface device may be configured to disrupt the telecommunication circuit and allow a user to examine both sides of the circuit.

Abstract: Methods and devices of determining the interface class used on a communication pathway are disclosed. The method compares information derived from the communication pathway to information about known interface classes to determine whether the derived information is similar to the information corresponding to the known interface classes. Devices according to the present invention include hardware and computer readable storage mediums having encoded instructions for carrying out the method.

Abstract: A method and structure for cutting a ring current when a telephone enters an off-hook state. The method includes: (1) activating an off-hook detected signal when the telephone enters an off-hook state, (2) applying the off-hook detected signal to a selected input terminal of a coder/decoder (CODEC), (3) storing configuration information in the CODEC identifying the selected input terminal and a selected output terminal of the CODEC, (4) activating a ring cut control signal on the selected output terminal in response to the activated off-hook detected signal and the configuration information, and (5) cutting the ring current in response to the activated ring cut control signal. The CODEC includes a control register for storing the configuration information, and a hardware cut ring current (HCRC) circuit, which activates the ring cut control signal on the selected output terminal in response to the activated off-hook detected signal and the configuration information.

Abstract: In accordance with the present invention, a Digital Subscriber Line (DSL) network for improving the transmission of DSL signals over a local loop is disclosed. The DSL network includes a loop extender with communications, control, and diagnostic functionality, and a central office including a Digital Subscriber Line Access Multiplexer (DSLAM) and a central office controller coupled to the loop extender via the local loop for controlling the loop extender.

Abstract: A system and method for interfacing plain old telephone system (PATS) devices with a cellular network is presented. In a broad sense, the system includes an interface that bridges the POTS devices with the cellular network. The interface is configured to convert POTS compatible signals into cellular network compatible signals, and vice versa.

Abstract: An integrated digital subscriber line transceiver includes a single integrated circuit that has a digital signal processing engine, a time division multiplexing framer (for example, a T1/E1 framer), and a static random access memory. The integrated circuit also includes a microprocessor coupled to the digital signal processing engine, the time division multiplexing framer, and the static random access memory. The digital signal processing engine, in one embodiment, includes an HDSL2/G.SHDSL data pump and an HDSL2/G.SHDSL framer.

Abstract: A Digital Subscriber Line (DSL) network for improving the transmission of DSL signals includes a plurality of local loops for transmission of upstream and downstream DSL signals, control signals, and Direct Current (DC) power, a plurality of loop extenders with communications, control, and diagnostic functionality for amplifying the DSL signals, a loop extender communications/power supply for receiving the DC power and control signals, providing DC power to the plurality of loop extenders, and broadcasting the received control signals to the plurality of loop extenders, and a central office controller/power supply for controlling access to the plurality of local loops and controlling the plurality of loop extenders.

Abstract: A method and apparatus is provided for DC level control in a line card. The method includes receiving a digital input signal, determining a first DC component value of the digital input signal at a first preselected time, and determining a second DC component value of the digital input signal at a second preselected time. The method further includes determining a difference between the first DC component value and the second DC component value. The method includes providing the first DC component value to a digital-to-analog converter in response to determining that the difference is less than a first preselected value. The apparatus includes a digital-to-analog converter and logic. The logic is coupled to the digital-to-analog converter, wherein the logic is capable of receiving a digital input signal, determining a first DC component value of the digital input signal at a first preselected time, and determining a second DC component value of the digital input signal at a second preselected time.

Abstract: The present invention provides a facility management platform to monitor and view the status of a plurality of individually addressable downstream devices including, but not limited to, addressable terminals, IRG's, settops, cable modems, taps, nodes, and/or hubs at a network control center. The FMP may display problems at these downstream devices, for example, power loss, and/or may automatically notify the appropriate companies and/or personnel to correct the problem.

Abstract: A device and a method for detecting presence of service on a telephone line prior to and during activation of alternative service on the line. The method comprises performing a first voltage detection on a subscriber line (S20), activating a subscriber line interface circuit (SLIC) if no pre-existing service is detected (S30), the SLIC providing an interface between the subscriber line and a service provider, and performing a second voltage detection on the subscriber line after a usage-dependent time has elapsed (S60), the time being determined by heuristics which consider several factors, including usage history of the line and time of day.

Abstract: Subscriber line interface circuitry includes an integrated circuit having sense inputs for a sensed tip signal and a sensed ring signal of a subscriber loop. The integrated circuit generates a subscriber loop linefeed driver control signal in response to the sensed signals. The linefeed driver does not reside with the integrated circuit. In one embodiment, the linefeed driver includes power circuitry for providing battery feed to the ring and tip nodes of the subscriber loop in accordance with the linefeed control signal. The linefeed driver includes sense circuitry providing a sensed tip signal and a sensed ring signal. The sensed tip and ring signals correspond to a tip current and a ring current of the subscriber loop. In one embodiment, the integrated circuit is a complementary metal oxide semiconductor (CMOS) integrated circuit. In one embodiment, the linefeed driver comprises only discrete components.

Abstract: The connection architecture is configured to digitally monitor filters therein, directly connecting a test table to the intermediate distribution frame without requiring wiring to the DSLAM. Filter cards of the dominant operators and of second operators, at least one smart card containing the relays that are activated from the test table, for which a digital bus is used, allowing measurement of the subscriber loop for filters of second operators and the DSLAM signal as well for the dominant operator, are further provided with an additional card in charge of receiving and distributing the signal that arrives from the DSLAM, between the filters, and connectors to which are connected the filter cards so that when they are extracted, the input (voice) is connected to the output (voice +data) so that the voice signal is not disconnected.

Abstract: Methods for matching adaptive hybrids (15) to lines (16) for making reflectograms of the lines (16) measure (1) input impedances (Zmeas) of the lines (16), model (2) characteristic impedances of the lines (16) via physical models (Zphymod), estimate (3) parameters of the physical models (Zphymod) via cost functions which depend on the measured input impedances (Zmeas), convert (30) the modelled characteristic impedances into impedance values (13), and set (31) the adaptive hybrids (15) with the impedance values (13). Unwanted near-end reflections are minimised and wanted reflections can be measured. The physical models depend on frequencies s, Bessel functions Ji and parameters ai related to geometrical dimensions A, D and L and material constants ?, ? and ? of the lines (16).

Abstract: The operating characteristics of a network connection are modified to improve connection performance. Operating limits for the connection interface are identified. New gain setting are selected for the connection interface after the connection interface has finished a training session establishing a connection. The new gain setting are used by the connection interface to conduct a new training session. The new gain settings are selected for the connection interface when operating parameters for the connection are within the operating limits.

Abstract: A subscriber line interface circuit (SLIC) for a subscriber loop includes a pair of output amplifiers for connection to the subscriber loop and a transient output current limit circuit. The transient output current limit circuit may have at least one programmable output current limit and limit respective output currents from the pair of output amplifiers based thereon. The transient output current limit circuit may limit both source and sink currents for each of the pair of output amplifiers.

Abstract: An electric device (202, 301) for connecting an analogue data transfer device (202) by means of a control unit (203) to a digital transfer system comprises means for connecting to a data transfer device via a twin cable (204). In addition, it comprises a current amplifier arrangement (214, 314, 340) for feeding a certain current to the twin cable, a first current switching device (219) for switching a first control current to the current amplifier arrangement, and a second current switching device (220) for switching a second control current to the current amplifier arrangement. The current switching devices (219, 220) are preferably current mirrors.

Abstract: A method and apparatus for measuring signals conveyed by a varying the voltage polarity across the tip and ring terminals in a telephone system is disclosed. The invention provides a composite circuit including a first stage in the form of a differential amplifier, a second stage in the form of a low pass filter, and a third stage in the form of a Schmidt trigger. The invention is capable of discerning an intentional polarity reversal telephony signal by eliminating transmission noise, as well as false triggering due to a high frequency (relative to a defined threshold) ring signal.

Abstract: A system and method for delivering digital subscriber line (DSL) service to a subscriber from a remote terminal of a telephone network. Included is a first route for delivering a telephone signal to the subscriber, and a second route for passing the telephone signal through a DSL system before delivering the signal to the subscriber. A disruptor is then used to selectively activate either the first or second route.

Abstract: An isolation system is provided that is suitable for use in telephony, medical instrumentation, industrial process control and other applications. Preferred embodiments of the invention comprise a capacitive isolation barrier across which a digital signal is communicated. The system provides a means of communication across the isolation barrier that is highly immune to amplitude and phase noise interference. Clock recovery circuitry may be employed on one side of the isolation barrier to extract timing information from the digital signal communicated across the barrier, and to filter the effects of phase noise introduced at the barrier. Delta-sigma converters may be disposed on both sides of the isolation barrier to convert signals between analog and digital domains. An isolated power supply may also be provided on the isolated side of the barrier, whereby direct current is generated in response to the digital data received across the isolation barrier.

Abstract: An electrical system is described comprising none or one or two function boards and at least one voltage converter. The function board/s and the voltage converter are connected such that a voltage generated by the voltage converter is supplied to the function board/s. The electrical system comprises an electrical circuit for checking whether two function boards are present and whether these two function boards are identical, and for disabling the voltage converter under the condition that two non-identical function boards are present.

Abstract: A digital signal line transmission system with reduced echo is provided. A communication line is between a carrier and a user terminal. A bridgetap line has a first end connected to the communication line. An adaptor having a capacitance is connected to the bridgetap line. The adaptor reduces the effect of echo from the bridgetap line on a rate of data transmission to the user terminal over the communication line.

Abstract: A line driver apparatus includes a peak detector operable to receive a data signal and output a first logic signal when the data signal is below a threshold value and output one of a plurality of alternate logic signals when the data signal exceeds the threshold value. The peak detector includes a signal magnitude calculator operable to calculate an amount by which the data signal exceeds the threshold value. The line driver apparatus also includes an amplifier operable to receive a filtered analog data signal and one of a time-delayed first logic signal and one of a time-delayed alternate logic signal, wherein the amplifier operates at a first voltage level upon receiving the time-delayed first logic signal, and the amplifier operates at a voltage level that is increasing toward one of an alternate voltage level upon receiving the time-delayed second logic signal.

Abstract: A voltage regulator circuit arrangement limits the DC voltage applied to a tip and ring amplifiers of a subscriber line interface circuit (SLIC), each of which has a first polarity input coupled to a first current flow path to which a DC input (battery) voltage is coupled. A first current source supplies a first current derived via a low pass filter path from that flowing through the first current flow path to a second polarity input node of the tip amplifier, while a second current source supplies a similarly low pass filter path-derived second current to a second polarity input node of the ring amplifier. A voltage regulator is coupled with the first current flow path and is operative to regulate the voltage at the first polarity inputs of the tip and ring amplifiers to a regulated voltage value Vreg, so that the magnitudes of the first and second currents are based upon the regulated voltage value Vreg.

Abstract: A synthetic impedance telecommunication line driver has no electrical energy-dissipating elements in series with its output, and synthesizes its output impedance in accordance with current fed back from an output current (mirror) sensing circuit. This allows the driver to realize substantially reduced power requirements for driving a telecommunication line, such as, but not limited to a DSX-1 line. The driver includes an operational amplifier having a first polarity input coupled through an input resistor to an input port, to which a signal voltage to applied to an output port is coupled. A second polarity input of the amplifier is coupled to a reference voltage. A feedback resistor is coupled between the amplifier output and its inverting input. An output current-dependent current source, such as a current mirror coupled in circuit with the output node, generates a current as a small fraction k of the output current.

Abstract: Systems and methods are provided for communicating data over a subscriber loop using a modem. One such method includes: transmitting data on the loop in a first frequency band; sensing an indication that simultaneous transmission on the loop in a POTS frequency band is imminent; and adjusting transmit power in the first band, in response to the sensing step. One such system includes an input/output connection in communication with the loop, a processor operable in at least two states, a means for sensing demand for simultaneous usage of the loop, and a means for controlling the state. The first state is characterized by a first transmit power spectrum in the first band. The second state is characterized by a second transmit power spectrum in the first band, reduced relative to the first spectrum. Upon sensing demand, the control means causes the processor to operate in the second state.

Abstract: A subscriber circuit, provided with a feeding circuit for feeding the current of a call to a terminal through a subscriber line and a switching circuit group for connecting the feeding circuit to the subscriber line and releasing the feeding circuit from the subscriber line, for controlling feeding to the terminal, comprising: said feeding circuit monitoring the state of a loop of the subscriber line, converting a two-wire signal sent from the terminal into a signal predetermined coefficient-fold, and supplying the same; a level converter, connected to the subscriber line through said switching circuit group, for converting a two-wire signal sent from the terminal into a signal any coefficient-fold and supplying the same, separately from said feeding circuit; and a signal monitor for monitoring a signal, using one of an output signal of said feeding circuit and an output signal of said level converter, according to upper control information and the output of monitoring the loop of said feeding circuit, and su

Abstract: A hybrid circuit that couples a broadband modem to a POTS or ISDN telecommunication line (TL) and includes a transformer bridge circuit (TBC) that includes a bridge circuit (BC). The bridge circuit includes four legs formed by impedance matching networks and parts of the transformer, and a differential drive amplifier (DRV) that performs impedance synthesis. A secondary winding (TS) of the transformer is magnetically coupled to the first and the second primary windings and is electrically coupled to the telecommunication line. First and second impedance dividers (Zhin1, Zhout1 and Zhin2, Zhout2) are connected to the various nodes to provide outputs to a receive amplifier (RCV). The impedance matching networks (Zm1; Zm2) include various passive and reactive components that together provide an impedance value selected based upon the line impedance reflected into the primary windings (TP1; TP2) of the transformer.

Abstract: In a line driver an input current (IINN) feeds a node (K1) which is connected to an input on an amplifier (OTA1). A further input on the amplifier (OTA1) has a reference voltage (VSGND) applied to it. The amplifier (OTA1) controls a current source (MN1) which outputs an output current (IOUTN). A current/voltage converter (R1) is connected between the node (K1) and the current source (MN1). A voltage/current converter (R2) is connected between the current source (MN1) and a ground (VSS).

Abstract: A communication device (1) is described, comprising a powered transceiver (2), a line transceiver (3), an isolation circuit (4) comprising three capacitors Cb1, Cb2 and Cb3, which are coupled between the powered transceiver (2) and the line transceiver (3), and a control circuit (5) arranged for controlling the transport of power from the powered transceiver (2) to the line transceiver (3) and for accomplishing a bi-directional communication across the isolation circuit (4). The control circuit is a two-mode controlled circuit (5), arranged such that during a first mode said power is transported and a communication in one direction is performed, and that during the second mode the communication in the other direction is accomplished.

Abstract: A linefeed on a subscriber loop is generated to generate a current on the subscriber loop. Whether a relationship between a voltage on the subscriber loop and a threshold voltage for an identified range of voltages satisfies a condition is identified. The linefeed on the subscriber loop is controlled to modify the current on the subscriber loop in response to identifying that the relationship between the voltage on the subscriber loop and the threshold voltage for the identified range of voltages satisfies the condition to help generate the voltage on the subscriber loop in the identified range of voltages. The identifying and controlling are continued to help generate and maintain the voltage on the subscriber loop in the identified range of voltages.

Abstract: An architecture for stacking DSLAMs employs a master DSLAM having a control circuit and an uplink communication circuit to a central office and two LVDS circuits for communication with two DSLAM boxes in adjacent circuit interconnection to the master DSLAM. Multiple slave DSLAMs each having a programmable control circuit and two LVDS circuits for communication with two DSLAM boxes immediately adjacent on either side in the circuit are connected in a ring or split stack arrangement. A control circuit in the master selectively programs the control circuits in each slave DSLAM to control the direction of communication in the LVDS circuits as round robin in a first selected mode and a split in a second selected mode to accommodate the desired structure.

Abstract: A network interface device for Ethernet networks using unshielded twisted pair cables comprising an overvoltage protection module and an interconnection module connected to the overvoltage protection module, both modules being connected in series in the Ethernet network.

Abstract: The telephone line coupler circuit has a single transmit opto-coupler whose output includes a DC bias component connected to a gate of a line seize switch for connecting a DC line seize load across the ring and tip contacts of the telephone line. The line seize switch is saturated by the transmit opto-coupler bias output and the AC component of the transmit opto-coupler output is sent over the telephone lines. The receive opto-coupler is used both for receiving communications signal and for detecting the ring signal. The band pass filter connected to the output of the receive opto-coupler may be switched to pass a ring signal frequency band or a communications frequency band. The circuit operates using two opto-coupler devices while conventional circuits require four opto-coupler devices.