Abstract: A transmission system comprising a plurality of stations, each having transmission and receiving circuits, that are connected in a ring configuration and use a HDLC-type protocol for transmission. The fault of a transmission circuit in a station can be detected in an idle state time by transmitting a fixed bit pattern during an idle state of transmission. Each station has a fixed bit pattern generator for outputting a fixed bit pattern during an idle time. Each station also has fault detection means which detects a fault when it receives a signal other than transmission data and the fixed bit pattern. Since the state of a line can always be monitored, a fault of the line state can be rapidly identified. Since transmission data is not encoded, the data transmission can be conducted at a high speed.

Abstract: A transmission system formed by a master station and a plurality of satellite stations which are connected by a transmission line into a loop, the data and command signals from the master station being passed sequentially from station to station around the loop. A failure in a satellite station that prevents further loop transmission is detected by watchdog timers in one or more successive stations. As a result, the stations detecting the failure switch themselves into a bus-mode of operation, the master station switches itself and the remaining stations into the bus-mode of operation. The switching back to the loop-mode of operation can be performed under control of the master station automatically, upon detection that the failure has been corrected. The switching also can be performed under external command.