a serial printer wherein a print head is reversibly displaced transverse to a print medium by an open looped grooved rotating cylindrical cam is provided . a unidirectional drive shaft is operatively coupled to the cam by a drive gear engaged with planet gears mounted on a clutched rotary member for rotating the cam when the rotary member is stationary . an intermediate gear is intermittently engaged with the drive gear for changing the rotating direction of the cam upon engagement of the intermediate gear between the drive gear and planet gear . the rotary member includes geared regions adapted to be engageable with a print medium advancing assembly for advancing the recording medium during rotation of the rotary member when the drive of the print head is stopped and the direction of the cylindrical cam is about to be reversed . advancing the print medium during change of direction of the print head when the cam is not being driven reduces the load placed on the motor , thereby permitting reduction in power consumption .