A link with a transmission cable (14) between a driving pulley (15) and a driven pulley (22) causing a mobile unit (5) to rotate, has two strands (23, 24) connecting the driven pulley (22) with idle pulleys (17, 18) which are substantially aligned with an axis (Y) of rotation of an part (2) intermediate to the unit (5) and to the base (1) on which the motor (12) is secured. Thus, the motor (12) is positioned on optionally fixed parts and rotation of the support (2) has only very little effect on the tension of the cable (14), so that it may always work properly. The invention is especially applied to articulated robot arms.