Abstract: An active impedance line interface circuit for connection to a two-wire telephone line comprises tip and ring unity gain d.c. amplifiers having outputs d.c. coupled via feed resistors to tip and ring wires respectively. Loop current and common mode current conducted via the feed resistors are monitored individually and converted to digital signals supplied to a digital control circuit, which can be programmed to provide desired operating conditions. Digital control signals produced by the control circuit are converted into analog signals and used to control currents passed by current sources through resistors at inputs of the d.c. amplifiers, thereby determining voltages at the d.c. amplifier inputs and hence on the tip and ring wires. These d.c. voltages are smoothed by capacitors used for coupling an a.c. signal to the d.c. amplifiers.

Abstract: A two-wire telephone line is supplied with d.c. feed from a drive circuit including two amplifiers via which a transmission signal is supplied to the line. Output potentials of the two amplifiers determine a loop current on the line, which is monitored and used to control a supply voltage for the drive circuit, the amplifier output potentials being offset from the supply potentials by headroom voltages to accommodate the transmission signal. The resulting current limiting voltage control acts iteratively to converge the supply voltage and a target voltage which is inversely dependent on an amount by which the loop current exceeds a predetermined value, the rate of convergence being fast for high loop currents for rapid response and slow for small differences to provide stable operation. The headroom voltages can be increased to nearly the supply voltage to reverse the polarity of the d.c. feed while maintaining current limiting.

Abstract: An automatic protection and recovery method for a telephone line interface circuit is described. The interface circuit monitors current on the line, and in the event of a fault producing an excessive current a protection relay interrupts a connection of the interface circuit to the line, and the interface circuit monitors common mode voltage on the line. The interface circuit is reconnected in response to the monitored voltage being below a relatively high threshold. If a second interruption results, the interface circuit is then reconnected in response to the monitored voltage being below a lower threshold. If a further interruption results, the interface circuit enters a ground fault recovery process. The method facilitates protection and rapid recovery from arbitrary fault conditions on the telephone line.

Abstract: A two-wire telephone line interface circuit comprises a driver circuit for supplying direct current on the line, a controlled voltage generator, a sensing circuit for monitoring the line current, a control circuit for controlling an output voltage of the voltage generator, and a switching circuit. The control circuit controls the switching circuit to supply, as a supply voltage for the driver circuit, a battery voltage in an on-hook state of the line or the controlled output voltage of the voltage generator in an off-hook state of the line, this output voltage being controlled to provide off-hook current limiting. The control circuit also controls the switching circuit to selectively supply the controlled output voltage of the voltage generator as a signalling voltage to at least one wire of the line for high voltage signalling, e.g. ringing, on the line. Desirable forms of the switching circuit are described.

Abstract: An active impedance line interface circuit for connection to a two-wire telephone line comprises tip and ring unity gain d.c. amplifiers having outputs d.c. coupled via feed resistors to tip and ring wires respectively. Loop current and common mode current conducted via the feed resistors are monitored individually and converted to digital signals supplied to a digital control circuit, which can be programmed to provide desired operating conditions. Digital control signals produced by the control circuit are converted into analog signals and used to control currents passed by current sources through resistors at inputs of the d.c. amplifiers, thereby determining voltages at the d.c. amplifier inputs and hence on the tip and ring wires. These d.c. voltages are smoothed by capacitors used for coupling an a.c. signal to the d.c. amplifiers.

Abstract: An impedance generator, especially suitable for determining a terminating impedance of an active impedance telephone line interface circuit, comprises a subtracting circuit for forming a difference between an input voltage multiplied by a multiplier and a high pass filtered version of the difference multiplied by another multiplier, and a summing circuit for combining the input voltage multiplied by a further multiplier with a low pass filtered version of said difference to produce an output voltage. In one arrangement, the input voltage multiplied by the further multiplier constitutes a third input to the subtracting circuit. The high pass filtered version of the difference is produced by subtracting the output of a low pass filter from the difference. The multipliers are constituted by controllable gain elements, enabling the generated impedance to be easily programmed under digital control.

Abstract: A telephone subscriber line is connected to a SLIC via contacts of an isolation relay. Line voltage on the line side of the relay contacts, and line current on the SLIC side of the relay contacts, are detected during opposite polarity half-cycles of an a.c. fault on the line, and the contacts are opened to protect the SLIC if the voltage and/or current is excessive. The interruption of current causes the line voltage to rise to maintain the protection for the duration of a fault.

Abstract: A lines status sensor provides for monitoring of the operating status of one or more telephone lines at an associated telephone station apparatus. The lines status sensor includes a translation circuit being responsive to potential differences appearing across the telephone line, for generating a voltage signal of corresponding polarity in relation to a reference voltage. A voltage divider circuit provides the reference voltage in addition to a first limit voltage being of a more negative potential than the reference voltage, and a second limit voltage being more positive than the reference voltage.