PURPOSE: To vary mounting flexibly by using only a bus wiring pattern in common with a front printed board unit and a power supply wiring pattern as wirings on a backwire ring board and equalizing the conditions of connection to the front printed board unit and the backwire ring board.

CONSTITUTION: Only a bus wiring pattern in common with a front printed board unit 10 and a power supply wiring pattern are wired to a backwire ring board 1 on which a plurality of printed board units can be mounted. Even when the printed board units of different kinds are employed, a terminal A1 is brought to +5V and a terminal A2 is grounded and the positions of terminals used in common in a clock, etc., are fixed previously in a terminal A3. Accordingly, the conditions of connection of the backwire ring board of each front printed board unit 10 are equalized, and an exclusive wiring pattern between the front printed board unit 10 is led out through a rear connector 3 and a rear printed board unit 20, and connected to an external cable 4, thus freely mounting the unit 10 at any position of a connector 2.