PURPOSE: To vary the number of bits of words, by using a buffer register in which the data of a shift register is set in a receiving mode when a setting signal is produced and the own data is set to the shift register in a transmitting mode when the setting signal is produced.

CONSTITUTION: In a receiving mode, the serial input data are fetched successively into a shift register 1. The contents of the register 1 are shifted to buffer registers 2-0 and 2-1, and the contents of the registers 2-0 and 2-1 are read by a microprocessor 6. In a transmitting mode, the data are written into the registers 2-0 and 2-1 by the processor 6. Then the data of the registers 2-0 and 2-1 are set to the register 1, and the data of the register 1 are delivered successively by one bit.