The operation of an automatic sheet feeder provided with a plurality of sheet hoppers which are used selectively to feed sheets to a printer is controlled. A control unit of the printer supplies a multiplex control signal including a hopper select signal and a sheet delivery signal to the automatic sheet feeder through a single signal line. The width of pulses of the hopper select signal is smaller than a predetermined time, while the width of the sheet delivery signal is greater than the predetermiend time. The pulses of the multiplex control signal are compared with the predetermined time to discriminate the hopper select signal and the sheet delivery signal from each other. That sheet hopper corresponding to the number of pulses of the hopper select signal is selected, and then the automatic feeding of sheets from the selected sheet hopper commences upon the reception of the sheet delivery signal.