PURPOSE: To set automatically a transfer speed and to implement the reception by applying bipolar/unipolar conversion to a data from a line, extracting a clock, and counting number of consecutive '1s' in a data to discriminate the data transfer speed.

CONSTITUTION: A bipolar/unipolar conversion circuit CONV 1 extracts a clock from an input of a line whose clock speed is 6.3MHz to generate a unipolar signal, and an FF 2 latches the signal to prevent malfunction. The clock is counted by a count circuit 3 and reset when an output of the FF 2 goes to an L level. A speed discrimination circuit 4 discriminates it that a data of 6.3Mbps is received when number of consecutive 1s in the data is 15 or below. The circuit 4 discriminates it that a data of 1.5Mbps is received when number of consecutive 1s in the data is 16 or over and 111 or below. A selector 5 selects a relevant clock by the discrimination of the circuit 4 and a data reception circuit 6 receives the data. The 1.5M clocks are missing clocks.