A coupling for making electrical connection of an avionics box (1) and for retaining it in a rack (2) has a rotatable jackscrew (11), by which cooperating parts (10 and 20) of an electrical connector on the box and the rack can be drawn together. The jackscrew (11) has a knob (16) at one end with an indicator button (48) which is pulled into the knob by a mechanical linkage (40) when the two parts of the connector are fully mated. The linkage (40) includes a sleeve (44) which embraces the jackscrew (11) and which is pushed towards the knob (16) against a spring (53) when the connector is mated. A ratchet mechanism (30) limits the torque applied by the jackscrew. Ball catches (61) engage asymmetric grooves (62) on the sleeve to make it more difficult to unfasten the jackscrew than to fasten it.
