| 摘要 |
PURPOSE:To prevent the relief of a network from being disabled because of disabled constitution of loopback due to a fault of a main transmitter (loopback) by providing plural transmitters outputting a check signal while changing each direction in the network through duplex transmission lines. CONSTITUTION:An active link 1 and a standby link 2 are set so that the progressing direction of a transmission signal is reversed as shown in the arrow. Signal selection switch circuits 3, 4 send some signals to the links 1, 2 switchingly. Separate selection command signals (a, b) are inputted to the link switches 3, 4 from a selection control circuit 7 and any of a transmission signal received from the active link 1, a transmission signal received from the standby link 2 or a test signal C generated from a test signal generating circuit 8 is selected by the link switches 3, 4.
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