PURPOSE: To take out the data without increasing a high order processing part load by holding converted data in a buffer memory for a constant time.

CONSTITUTION: An address counter 10 operates based on a control signal from a command control part 12 and supplies a synchronized address signal. A buffer memory 9 supplies cyclically the data supplied from a photocoupler 6 based on an address signal from the address counter 10 and a host computer and inputs the data through an address counter buffer 11 to a host computer. Further, the address counter 10 is cleared by an A/D starting instruction from the host computer, and the buffer memory 9 holds the data for a constant time.