Abstract: An application module interface is disclosed that allows one or more modules to access voice or data channels in a private branch exchange environment that contains one or mole B channels (bearer channels) for transmitting voice or data signals, and one or more D channels (signaling channel) for transmitting data. The application module interface provides a control channel that allows a module to obtain and vary the status and configuration of a telephone terminal. The application module interface provides access to both directions of two B channels (B1 and B2) and one D channel. The application module interface is the only interface required to connect the two B channels (B1 and B2) and one D channel to or from the telephone terminal and the add-on module. A frame format includes a start bit (logic “1”), a number of data bits allocated to the D channels (D, D?) and B channels (B1, B1?, B2, B2?), and eight stop bits (logic “0”).

Abstract: A line circuit for serving a digital telephone set over a link of indeterminate length and attenuation characteristics includes a processor for issuing test messages over the link to the telephone set and which responds to the receipt of an acknowledgment message from the telephone set by inserting increasing amounts of finite attenuation into the link until the acknowledgment message fails to be received, at which point the value of inserted attenuation is a measure of the excess link margin.

Abstract: A full duplex, synchronous data set (10) includes primary signal processing circuitry which generates a modulated transmit data signal in response to serial data from a terminal interface (17). The modulated data signal is transmitted over a primary channel of a transmit line (11). The primary signal processing circuitry also receives modulated data signals from a primary channel of a receive line (12) and recovers therefrom a serial bit stream for presentation to the interface. The operating parameters of the primary signal processing circuitry are specified by a primary controller (30) over a plurality of buses (PA, PC, PD). The primary controller includes a microprocessor (310) and associated peripherals (315, 320, 325, 330, 335). The data set also includes secondary signal processing circuitry (40) which transmits and receives diagnostic and control information over respective secondary channels of the transmit line and receive lines.