PURPOSE: To attain effective information sending/receiving between the first and second units by mounting on the second unit a means to reset the first and second indication registers.

CONSTITUTION: After storing information in an information store register 3, the first unit 1 sets the first indication register 5 and waits for the register 5 to be reset. After recognizing that the register 5 has been set, the second unit 2 reads the information of a register 3 and resets the register 5 through a reset wire 7. Then, after recognizing that the register 5 is reset, the unit 1 stores the next information in the register 3 and sets the register 5. When information is sent from the unit 2 to the unit 1, the same sequency is followed using an information store register 4 and indication register 6. Thus, by so constituting the indication registers 5 and 6 as to be resetable through the reset wire 7, information can be sent/received between the units effectively.