COMMUNICATION SYSTEM

PURPOSE:To identify whether it is natural or abnormal that one bus in a pair cannot be used by recognizing the connection state of couplers in whole stations based on a detection result obtained in a self communication station and a detection result broadcasted from the other station. CONSTITUTION:When only a coupler 31 or 32 on the side of one bus (bus 21 or 22) is connected to the self communication station 5 CPU 51 identifies whether it is caused by the setting of a single bus or the abnormality of one bus although double buses are set based on information stored in memory for live list 531 or a memory for TDR 532. When only the coupler (31 or 32) on the side of one bus is connected to the whole stations as the result of the identification, the setting of a single bus is judged. When the station to which only the coupler on the side of one bus is connected and the station to which the couplers of the buses 21 and 22 are connected exist, one bus-side is judged to be abnormal.