A data communication control system for an apparatus which may be selectively equipped with two or more functional modules each having predetermined functions, the control system having a plurality of subsidiary control units respectively in control of the functional modules, a main control unit operative to communicate with each of the subsidiary control units and a data transmission network interconnecting the main control unit and each of the subsidiary control units to allow full-duplex communication therebetween, each subsidiary control unit being operative to generate mode data designating any of the functions assigned to the particular subsidiary control unit and transmit the mode data to the main control unit, wherein, when mode data designating any function is generated by one of the subsidiary control units and is sent to the main control unit, the main control unit transmits to each subsidiary control unit data designating the function designated by the mode data if the main control unit determines that the execution of the particular function is allowable, or the function designated by the mode data received prior to the currently received mode data if the main control unit determines that the execution of the function designated by the currently received mode data is not allowable.