A support assembly for a barrier movable between open and closed positions over an opening comprises a track. The track is attached to the wall for guiding movement of the barrier between substantially vertical closed position and a substantially horizontal open position. A jack shaft assembly is attached to the track and to the barrier for movement thereof. A mounting assembly is attached to the track for receiving a drive unit comprising a motor for supplying power to the jack shaft assembly to move the barrier and for limiting rotational movement of the drive unit.