Abstract: An exchange center is connected to telephone lines and a telephone and a data transmitter as terminal equipment are connected to the telephone lines. The data transmitter is actuated depending upon the polarity of DC voltage applied to the telephone lines. The telephone line is potentially opened by detecting the current of the telephone during the operation of the data transmitter. The impedance of the telephone line during the opening state is detected whereby the operation of the data transmitter is stopped by inverting the DC voltage applied to the telephone line and the telephone is predominantly communicated through the exchange center over the data transmitter.

Abstract: An automatic meter-reading system monitors broadly distributed utility meters over a communication link by indirectly measuring the resistance value representative of the digit shown on the meter from calculation using the equation derived from the theory of four-terminal network. Based on the similar principle, an automatic remote controlling system monitors the answer-back signal representative of the condition of the controlled equipment at a remote station. Similarly, an automatic meter-reading system monitors a telephone subscriber's telephone call meter centrally located in a telephone exchange office.