Abstract: The data communication system includes an encoder to receive nonreturn-to-zero data signals and convert them to biphase signals, a current mode transmitter for transmitting the biphase signals, and a communication link connected to the transmitter. The encoder senses transitions in the nonreturn-to-zero data signals and controls the amplitude of the biphase signals in response to the sensed transitions. These transitions are sensed by inverting and delaying the nonreturn-to-zero data signals and exclusively ORing these delayed and inverted signals with the nonreturn-to-zero data signals.

Abstract: An automatic answering device controlled by attached data equipment and used with a modem powered from the telephone line. The modem and automatic answering device are automatically connected to and subsequently powered by the telephone line when a ring signal has occurred on the telephone line and the data equipment has indicated that it is ready to receive data. The energy needed to connect the modem to the telephone line is slowly accumulated from a data equipment over a period of time prior to the connection of the modem to the telephone line. The modem automatically hangs up when the local data equipment signals disconnect or the modem at the other end of the telephone line disconnects.

Abstract: A modem for coupling a data terminal or other apparatus to a telephone line and deriving its operating power for both transmitting and receiving operation from the telephone line, and providing electrical isolation between the telephone line and the data terminal.