| 摘要 |
In a data transmission system with such a construction that a plurality of terminal units are connected to a first and a second loop transmission line for transmitting messages in a first direction and in a second direction which is opposite to said first direction, respectively, through a pair of transmission control units (the one unit being connected to said first loop transmission line with the other unit being connected to said second loop transmission line), each transmission control unit sends a check message sequentially in order to check the state of a minor loop which connects two adjacent pairs of transmission control units. This check message has a data field, in which an initial value is set at the sending time. The value of this data field is incremented by one in each transmission control unit through which the message passes to make a round through the minor loop. Each transmission control unit decides as to whether the value of the data field of the received check message has a prescribed value or not, thereby to discriminate if the message generated by the transmission control unit itself has made a round through the minor loop and returned. If the sent out check message is not received within a prescribed period (after sending), the transmission control unit confirms the presence of an abnormality on a minor loop with the unit as a start point, and thereafter switches its operation to use the bypass transmission line for sending usual messages.
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