system control unit 200 are applied to a part of input ports of the cpu1 the display control circuit 500 is connected other ports of the cpu1 referring now to fig1 b the illustrated circuitry makes the control of the pressure on off controlling solenoid valves 132 138 134 and 140 as well as the pressure compensating solenoid valves 135 141 136 and 142 this entire circuitry is controlled by a microcomputer unit cpu2 energization of solenoids for the solenoid valves 132 138 134 and 140 are controlled by solid state relays ssr5 ssr6 ssr7 and ssr8 respectively which relays are controlled by the corresponding output ports or the microcomputer unit cpu2 driver circuits dv1 dv2 dv3 and dv4 are connected in parallel to those output ports of the cpu2 used for controlling the solid state relays ssr5 ssr6 ssr7 and ssr8 respectively the driver circuits dv1 dv2 dv3 and dv4 have all the same construction a part of signal lines from the control board 600 is connected to a connector j6 signals applied to the j6 are in turn applied to input ports or the microcomputer unit cpu2 through a buffer bf2 and a chattering removal circuit ch2 to other input ports of the cpu2 are connected the system control unit 200 and the display control unit 500 the driver circuit dv1 will be now described designated at tg is a trigger circuit for detecting the falling of an input signal.