Dual clocked data bus

A synchronous parallel data bus particularly adapted for use in a data processing system where it is necessary to transfer data over long distances. The physical connection between communicating units includes a plurality of wires adapted to carry the parallel data signal and a wire which carries a clock signal to the remote unit. When data is transmitted from the remote unit to the base unit, the clock signal which originated at the base unit and was transmitted to the remote unit is "turned around" and transmitted back to the base unit for use in receiving the data from the remote unit.