Abstract: A system is provided for the simultaneous transmission of two asynchronous base-band logic information signals in opposite directions between first and second information stations using only a single pair of transmission wires, while maintaining electrical isolation between said information stations. The first information station includes a voltage source for driving the transmission line. The source voltage is constant in magnitude but its polarity is determined by the state of the signal being applied to the logic input port of the first information station. The second information station includes circuitry that responds to the polarity of the transmission line voltage and controls the logic output port of the second information station through a photocoupler. The second information station also includes a current sink controlled through a photocoupler by the signal applied to the logic input port of the second information station.