An optical transmission device for interconnecting an optical full duplex transmission line and a half duplex transmission line. When a start bit of a data block of an electrical signal received from the half duplex transmission line is detected, a maintaining signal to maintain the same transmission direction is generated during which the data block can be transmitted. An optical signal is received from the optical full duplex transmission line and converted into an electrical signal. Similarly, a maintaining signal to maintain the same transmission direction is generated. In response to either one of the two maintaining signals, passage or stopping of signals by a driver for sending out an electrical signal to the half duplex transmission line and a receiver for receiving an electrical signal from the half duplex-transmission line are controlled. Possible interference of data is prevented and the transmission direction is changed over in this manner. Further, the level of an optical signal is monitored, and when the optical signal level becomes lower than a reference level, an abnormality warning data is transmitted succeeding to the data block.

A branching device to send out data received at an arbitrary one of three terminals thereof through the other two terminals. In the device, the terminals are coupled to each other by means of photo-couplers. The head of a data block received by each receiver connected with each terminal is detected, and after such detection of the head, a signal which specifies the length of the data block is generated. Thus, after communication data are received by way of an arbitrary one of the terminals, reception of communication data by way of the other two terminals is inhibited while transmission of the communication data is enabled in response to the signal specifying the data block detected. Since the transmission lines of the three directions are electrically isolated from each other by the photo-couplers, even when a failure of short-circuiting occurs at a main transmission line or a branch line or the like, communications are continued bypassing the location of the failure similarly as in the case of a failure of breaking of a wire.

A monitoring and control system includes a center main system (209) installed in a sales office and a substitute center system (208). The monitoring and control system further includes a different substation terminal interconnection function that, in the case where there occurs an abnormality to a communication channel provided for each substation terminal and connects the substation terminal to the center main system (or the substitute center system), instructs a different substation terminal to perform the interconnection and communication with the center main system (or the substitute center system) instead of the substation terminal. With this construction, through the different substation terminal that received the instruction, the center main system monitors and controls a switch to be monitored by the substation terminal connected to the communication channel to which the abnormality occurs.

A digital radio transmission system having a first terminal office (10), a second terminal office (20) and at least one drop/insertion (DI) office (30) located therebetween. Long distance lines between the first and second terminal offices are handled in the DI office together with first and short distance lines each being replaced by first and second protection lines, respectively. A protection line for the long distance lines is realized by commonly using the first and second protection lines inherently allotted to the first and second short distance lines. Further the line switching, due to a fault in the long distance line, from the faulty long distance line to the both the first and second short line side protection lines can be achieved by communicating a particular line switching control signal among all the offices.

A method and apparatus for communicating between a host station and a plurality of slave stations, each detachably connected to a junction box permanently affixed at a node in a loop-type network. Each junction box has a variable code input which may be used as the system address for any slave station connected at that particular node. The system address may be input by the junction box into a predetermined location in the system transmission frame for access by the attached slave station or may be input separately by the junction box into the attached slave station in order to permit the slave station to read information in a particular slot of the transmission frame or write information to such slot.