Apparatus for transmitting a clock and data from a first module to a second module connected by a single outward line and a single return line, comprising: means for transmitting a data pulse on the single outward line comprising means for asserting a first edge on said single outward line, said first edge representing a timing edge for the clock and means for asserting a second edge on the single outward line a selectable time period after said first edge, said selectable time period representing said data; and means for receiving a return pulse on said single return path comprising means for receiving a first edge and a second edge on the single return line, the first and second edges being separated by a first time period, said first time period representing an acknowledgement.