Abstract: A control device for controlling the switching operation between a scanner having built-in printer port to which a printer is connected and the printer is disclosed. The control device has a computer connector for connection with the parallel port of the computer to receive control bus signals, status bus signals and data bus signals therefrom. The control bus and status bus signals are applied to the printer and the scanner through latches and tri-state buffers which are controlled respectively by the scanner and the printer. The two control units receive the data bus signals from the computer and perform decoding operation on the data received to enable/disable the latches and tri-state buffers so as to activate/de-activate the printer and scanner.