A surgical robot arm terminated at its distal end with a drive assembly 400 for driving a surgical instrument having an articulation 103 for articulation of an end effector. The articulation is driven by at least first and second rods E3,E4, each of which terminate in a sliding block 407,408 respectively. The longitudinal axes of the blocks are parallel to the longitudinal axes of the rods to which they are attached； linear movement of a block along its longitudinal axis causes its rod to be tensioned or compressed. The assembly comprises a first U shaped drive element 418, which engages with a first and a second coupling member 411,412 protruding in a direction transverse to its respective block. The drive element 418 engages both the first and second coupling members such that: when the drive element moves linearly, both first and second coupling member move in the same linear direction； and when the drive element rotates, the first and second coupling members move reciprocally in opposing linear directions, driving the first and second coupling members in opposing directions. The first and second coupling elements may be encompassed in a foam roller 416,417, each foam roller engaging with the drive element.
