DUPLEX SYSTEM LOOP FORM DATA TRANSMISSION

PURPOSE:To attain the instant system changeover processing by using a fault detection circuit to detect a fault of the system unique to each transmission station and using a system changeover control circuit so as to apply loopback control and to inform the fault of the system to the next station. CONSTITUTION:When transmission lines 1, 2 of systems 1, 2 between transmission stations 3c and 3d are cut off at a mark X, no carier is detected by a two-system fault detection circuit 20b, system changeover signals 22b, 23b are outputted through a system changeover control circuit 21b y the by the output of a fault signal 24b to close changeover circuits 5, 8 and the system changeover state of the system 1 loopback is caused. A carrier sending inhibition command 25b is sent to a carrier sending circuit 16b at the same time to stop the carrier transmission of the transmission line 1 of the system 1. Similarly, a fault signal 24a is outputted from a 1-system fault detection circuit 20a of a system 1 even in the transmission station 3d, switching signals 22a, 23a are outputted through a system changeover circuit 21a to close changeover circuits 6, 7 thereby causing the system changeover state of the system 2 loopback. A carrier transmission inhibition command 25a is sent to a carrier transmission circuit 16a at the same time to stop the carrier transmission of the line 2.