A ribbon (40) is driven past the printing station of a printer (12) only when a reciprocating print head (22) is moved in the right-to-left direction across the printer (12). An endless timing belt (46), which reciprocates together with the print head (22), causes rotation of a gear pulley (48) that is in mesh with an idler gear (66). The idler gear (66) is supported on a pair of projecting fingers (80, 82) of a plate (68) that is swingable on the spindle (56) supporting the gear pulley (48). The idler gear (66), being maintained in mesh with the gear pulley (48), is responsive to the direction of rotation of the gear pulley (48) through a drag effect of the ends (84, 86) of the projecting fingers (80, 82) on the bottom surface of the idler gear (66), to cause swinging movement of the plate (68) in accordance with the direction of rotation of the gear pulley (48), such that the idler gear (66) is in mesh with a ribbon drive gear (88) only when the print head (22) moves in a right-to-left direction across the printer (12).