Patent ID: 6996113

Claim:
A multiplex communication system comprising: a plurality of networks each of which comprises a communication line and a plurality of nodes connected to the communication line, the nodes communicating each other by exchanging data frames, a predetermined portion of each of the data frames being assigned to an identifier; a data relay unit for relaying data frames among the networks, the data relay unit comprising: a plurality of send/receive means for sending and receiving data frames, the communication lines of the networks being connected to the respective send/receive means, each of the data frames which is sent by a first node belonging to a first network to a second node belonging to a second network being received by the send/receive means connected to the conununication line of the first network and sent by the send/receive means connected to the communication line of the second network, the first network being any one of the networks, the second network being another one of the networks, the identifier of the data frame being reset by the first node before the data frame is sent to the second node; an identifier setting means for setting the identifier of the data frame received by the first send/receive means to a value which represents that the identifier is set before the data frame is sent by the second send/receive means; and a relay inhibiting means for checking whether the identifier of the data frame received by the first send/receive means is set and inhibiting the data frame to be sent by the second send/receive means if the identifier is set, wherein a third node receives a data frame from a forth node, the third node being any one of the nodes included in the networks, the fourth node being another one of the nodes included in the networks, the identifier of the data frame being reset by the fourth node before the data frame is sent to the third node, the third node comprising: a first check means for checking whether the network which includes the forth node is the same as the network which includes the third node; a second check means for checking whether the identifier of the data frame received from the fourth node is set; and a verification means for receiving results of the checks from the first check means and the second check means and invalidating the received data frame if both the results are ‘YES’ or both the results are ‘NO’.