Data transmission circuit for establishing a bidirectional data path in a telephone system

A data transmission circuit for use in a telephone system. The telephone system includes a telephone line connected with tip and ring terminals and a subscriber telephone coupled across the tip and ring terminals. The on/hook condition of the telephone is determined by a telephone status monitor circuit which periodically examines the voltage across the tip and ring terminals and produces an output signal when the telephone is in an on/hook condition. Following this output signal, and when data is to be transmitted between a first terminal of the data transmission circuit and the tip and ring terminals, a continuous dc loop is established between the tip and ring terminals and, after a dialtone period, the loop is made and broken in accordance with dialing pulses representing a telephone number of the location to be called. At the termination of the dialing operation, the dc loop is maintained and an ac path is established between the first terminal and the tip and ring terminals for the transmission of data between the first terminal and the tip and ring terminals.