A surgical robot arm extending between a base and a terminal link, the terminal link connected to an adjacent link in the surgical robot arm by a joint which permits the terminal link to rotate about a longitudinal axis of the terminal link, the terminal link comprising: a drive assembly having a first drive assembly interface element for driving a first instrument interface element of a robotic surgical instrument when the surgical robot arm engages the robotic surgical instrument, the first drive assembly interface element engageable with the first instrument interface element on the longitudinal axis of the terminal link, and the first drive assembly interface element being linearly displaceable along the longitudinal axis of the terminal link so as to drive the first instrument interface element along the longitudinal axis of the terminal link.