A system comprises a first module and a second module, connected by a transmission line comprising first and second wires. The first module includes common mode voltage circuitry, for imposing a common mode voltage onto the first and second wires. The first module includes signal generation circuitry, for generating a signal voltage in response to first data, and for imposing the signal voltage as a differential signal onto the first and second wires during periods when the first module has first data to transmit. The second module includes current generation circuitry, for generating a signal current in response to second data, and for injecting the signal current as a differential current onto the first and second wires during periods when the second module has second data to transmit. The first module includes respective resistances connected to the first and second wires. The first module includes a first detector for obtaining first output data based on voltages across the resistors resulting from the signal current injected by the current generation circuitry of the second module; and the second module includes a second detector for obtaining second output data based on differential signal imposed by the signal generation circuitry of the first module.