Abstract: Image forming apparatus having a surface potential control device incorporated therein, in which magnitude of a measured value of the surface potential sensor and an aimed potential value is discriminated, and an output from an output device such as an exposure device that affects the image formation is increased or decreased for a predetermined value by the discriminated output, and in which the measuring, discriminating, adding and subtracting operations are repeated to control the surface potential within a predetermined range in a definite number of times for the measurement by decreasing the adding and subtracting quantities with increase in the number of times for the repeated operations.

Abstract: Image forming apparatus includes a feeder unit feeding a recording member, processing devices forming an image on the recording member, an input and output device having a port selectively usable for data input and output, a selector device for determining whether the port of the input and output device is used for data input or output, an instruction device connected to the input and output device and supplying enable instructions to the feeder unit and processing devices, and a display connected to the port and interrupting a normal display to display indications corresponding to the signals appearing on the port in response to the selector device determining that the port is used for data input.