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: A data transmission system including transmission stations for transmitting a plurality of different kinds of input data and receiving stations for receiving only one kind of the data transmitted from the transmission stations. A transmission station is actuated by the demand of the receiving station and the plural kinds of data are modulated at the modulation central frequency corresponding to the respective data. The demodulation central frequency of each one of the receiving units is set for only one kind of data and only one kind of data transmitted by the transmission units are demodulated in a receiving station.

Abstract: An apparatus for selectively granting priority use of telephone lines to a telephone or data transmitter each connected via the telephone lines to an exchange center. Access of the data transmitted to the exchange center is typically enabled by application of DC voltage of a predetermined polarity to the telephone lines by a control circuit. A current detection circuit is provided in series with the telephone for detecting when the telephone lines are providing current to the telephone during a time when the date transmitter is transmitting or receiving data via the telephone lines. In the event that the current detection circuit detects a current component on the telephone lines to the telephone, application of power to the data transmitter is stopped and priority use of the telephone lines is granted to the telephone.