An image forming apparatus comprising: a first mounting slot located in a first designated position for mounting a first developing unit; a second mounting slot located in a second designated position for mounting a second developing unit; a third mounting slot located in a third designated position for mounting a third developing unit; a fourth mounting slot located in a fourth designated position for mounting a fourth developing unit; wherein the image forming apparatus is adapted to operate correctly when a specified colour developing unit is installed in each developing unit; the image forming apparatus includes a control unit being adapted to select each of the mounting slots one at a time by transmitting a signal (400, 410, 420, 430) to one of the developing units, mounted to a selected one of the mounting slots, and to receive a signal from said one of the developing units, mounted to the selected one of the mounting slots, and to determine whether the one of the developing units, mounted to the selected one of the mounting slots, is the correct colour based on the signal from said one of the developing units, mounted to the selected one of the mounting slots; wherein the image forming apparatus further comprises: a switching unit; a first clock signal line connecting the switching unit and the first mounting slot; a second clock signal line connecting the switching unit and the second mounting slot; a third clock signal line connecting the switching unit and the third mounting slot; a fourth clock signal line connecting the switching unit and the fourth mounting slot; and a common data line that is separate to the clock signal lines and connects each mounting slot to the control unit; wherein the signal being transmitted to each mounting slot along the clock lines and the signal being received along the data line and the determination as to whether the correct colour has been mounted is based on whether the signal is received or not. a first mounting slot located in a first designated position for mounting a first developing unit; a second mounting slot located in a second designated position for mounting a second developing unit; a third mounting slot located in a third designated position for mounting a third developing unit; a fourth mounting slot located in a fourth designated position for mounting a fourth developing unit; wherein the image forming apparatus is adapted to operate correctly when a specified colour developing unit is installed in each developing unit; the image forming apparatus includes a control unit being adapted to select each of the mounting slots one at a time by transmitting a signal (400, 410, 420, 430) to one of the developing units, mounted to a selected one of the mounting slots, and to receive a signal from said one of the developing units, mounted to the selected one of the mounting slots, and to determine whether the one of the developing units, mounted to the selected one of the mounting slots, is the correct colour based on the signal from said one of the developing units, mounted to the selected one of the mounting slots; wherein the image forming apparatus further comprises: a switching unit; a first clock signal line connecting the switching unit and the first mounting slot; a second clock signal line connecting the switching unit and the second mounting slot; a third clock signal line connecting the switching unit and the third mounting slot; a fourth clock signal line connecting the switching unit and the fourth mounting slot; and a common data line that is separate to the clock signal lines and connects each mounting slot to the control unit; a switching unit; a first clock signal line connecting the switching unit and the first mounting slot; a second clock signal line connecting the switching unit and the second mounting slot; a third clock signal line connecting the switching unit and the third mounting slot; a fourth clock signal line connecting the switching unit and the fourth mounting slot; and a common data line that is separate to the clock signal lines and connects each mounting slot to the control unit; wherein the signal being transmitted to each mounting slot along the clock lines and the signal being received along the data line and the determination as to whether the correct colour has been mounted is based on whether the signal is received or not.