A method is used for transferring data over a half-duplex wired communications link, wherein the wired communications link comprises first and second wires. The method comprises, in each of a plurality of frames: transferring a clock signal on the first wire in a first direction; transferring first payload data on the second wire in the first direction; transferring second payload data on the second wire in a second direction opposite to the first direction; and transferring control data on the second wire, wherein the format of the frame is such that, irrespective of whether the control data is transferred in the first direction or in the second direction, there is only one pair of reversals of a direction of data transfer in each frame.