that extends across the rear interior of the module 12 see fig4 a motor 76 coupled by gears 77 to one end of the screw gear 74 see fig1 rotates the screw gear 74 in either clockwise or counterclockwise directions the actuator mechanism 22 also includes an actuator carrier 78 that travels upon the screw gear 74 behind the panels 32 a fixed nut 80 carried by the lower region of the carrier 78 engages the threads of the screw gear 74 rotation of the screw gear 74 advances the nut 80 along the gear 74 causing linear movement of the carrier 78 as the arrows in fig1 show the upper and lower regions of the carrier 78 slide along guide tubes 82 and 83 which extend above and below and parallel to the screw gear 74 the carrier 78 travels either left or right along the screw gear 74 depending upon the direction the screw gear 74 rotates the actuator carrier 78 supports a cassette drive gear 84 and associated drive motor 86 shown in fig1 as the carrier 78 travels along the screw gear 74 the cassette drive gear 84 moves in succession into and out of engagement with individual cassette wheel gears 48 the cassette drive gear 84 can engage only one cassette wheel gear 48 at a time when the cassette drive wheel 84 engages a cassette wheel gear 48 as fig4 and 10 show actuation of the drive motor 86 imparts rotation to the medication carrier.