PURPOSE: To decrease the quantity of electric wires required for the wiring and to improve the reliability of inputted/outputted data by connecting the main body of a controller and input/output units with a few signal lines in controlling lots of input means and output means.

CONSTITUTION: A control unit 100 consists of a microcomputer, in this example. In using a system bus of a main control section, the control unit 100 controls an output port P12 to open a gate of an output buffer 110 and address information, a read strobe RD and a write strobe WR or the like are outputted from an output port P13. A port P14 of the control unit 100 is connected to various control lines of a system bus of the main control section, and the main control section references and rewrites the content of an internal memory of an optional address of the control unit 100 via the port.