A method for transferring data over a half-duplex wired communications link, comprises, in each of a plurality of frames: transferring a synchronization data pattern in a first direction; transferring first payload data in the first direction; transferring second payload data in a second direction opposite to the first direction; and transferring control data, 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.