Telephone handset interface for automatic switching between voice and data communications

Abstract

A telephone handset interface is inserted between the telephone handset and base for automatic switching between voice and data modes of communication. Each station includes a telephone set with a base and handset, interface circuitry, and modem. The interface circuitry includes relay switches to couple the modem to the telephone base microphone and speaker lines in data mode and to couple the telephone handset to the telephone base in voice mode. The relay switches are controlled by a sensor that monitors the on-hook/off-hook status of the modem as an indication of when the modem is preparing to transmit or receive data. The interface circuitry also includes differential amplifier circuitry coupled to the telephone base speaker lines that enables the modem to continually monitor the telephone base speaker lines. In one embodiment, each modem includes circuitry for generating a start signal to cause a remote station at the other end of the communication link to enter data mode prior to transmitting data. Similarly, each modem includes a start signal detector that is coupled to the telephone base speaker lines through the differential amplifier circuitry to detect a start signal sent by the remote unit that causes the modem to switch into data mode in preparation for receiving data. The start signal can also include signals designating the rate at which data will be transmitted and its format.