PURPOSE: To make a complicated communication program unnecessary and to directly control a sequencer I/O, by providing a data processing means which performs conversion between a computer data format and an I/O interface unit data format in a transfer control circuit.

CONSTITUTION: In the transfer control circuit 9, the data processing means is provided which converts transferred parallel data from the computer data format to the I/O interface unit data format or a data format in an opposite direction based on a data processing command from a computer 1, and also, after storing converted data in a memory transiently, makes access to the memory based on a hand shake signal from the computer 1, and transfers the converted data to the computer 1 or an I/O interface unit 5 alternately. In such a way, it is possible to simplify a communication protocol, and to perform fast data transfer at speed suitable for a system.