Abstract: A push-pull amplifier is used to drive a center tapped transformer in a modem/telephone line interface. The analog modem transmit signal is amplified and inverted, with the positive signal driving one end of the secondary and the inverted signal driving the second end and the center tap. This allows a lower voltage to be used to power the push-pull amplifier. A receive amplifier sums the signal at the second end of the secondary and the positive transmit signal to develop and amplify the analog signal received over the telephone line and transformed by the transformer. Zener diodes are placed in series in the telephone line to develop spikes on the secondary. If the magnitude of those spikes exceeds a predetermined level a ring indication signal is provided.

Abstract: A modem/telephone interconnect for sharing a telephone line or communication channel between a modem and a telephone or other communication device. The interconnect provides access to the telephone line by both devices but normally prevents each device from interrupting or interfering with the operation of the other. The telephone can be used to monitor data transfer by the modem without the possibility of interfering with the data signals or quiet times.