In an image forming apparatus of the type including a plurality of functional modules each having particular functions proper to the unit, a control system having a master control unit and a plurality of subsidiary control units respectively in control of the individual functional modules wherein each of the plurality of subsidiary control units operates principally under the control of the master control unit through a communication network of the common interface bus type but is operable independently of the master control unit when required or preferred by the subsidiary control unit per se. At least one of the functional modules is provided with a standard mode of operation and a local mode of operation. The standard and local modes are incompatible with one another.