PURPOSE: To increase the line speed of data transmission by making the minimum number of flags up to the start flag of the next transmission frame from an end flag of one transmission frame proportional to the data transfer speed.

CONSTITUTION: The transmission data is set from a microcontroller to a transmission character buffer 31. The content of a transmission character buffer 231 is transmitted to a line via a series converting circuit 234 and a selector 235. A start flag and an end flag are transmitted from the selector 235 to a line. A speed detection circuit 236 receives a transmission clock from a transmission clock generating circuit 237 to detect the transmission speed. An inter-frame flag length is set in advance to the speed detection circuit 236, the inter-frame flag number is calculated from the detection speed to control that the flag number is proportional to the data transfer speed.