JP2843496B

PURPOSE:To detect line abnormality immediately in an idle time without waiting the interval of transmission/reception time by providing an abnormality detection means detecting abnormality based on the consecutive reception of the same logical level bits not less than the number of the consecutive bits of the same logical level. CONSTITUTION:In a communication system by a base band transmission system, a station on a transmission side consecutively outputs the same logical level bits of a prescribed number not less than a first prescribed number and, after then, inverts the logical level. The system is provided with a fixed bit pattern output means 2202 which successively repeats consecutively outputting the same logical level bits of the prescribed number not less than the first prescribed number. Then, a switch means switches the system to transmit the output of the fixed bit pattern output means 2202 to 2204 in the idle time excepting for the time of transmitting transmission data. Then, a station on a reception side detects abnormality based on the consecutive reception of the consecutive bits of the same logical level, which are outputted by the fixed bit pattern output means 2202.