The reliability of a distributed processing system is enhanced through the use of a highly reliable data communication system. Each node of such a system is connected to the other nodes by a local data network, including duplicated data network media, such as coaxial cables. Each node includes a maintenance processor for controlling which of the duplicate network media is active and which is standby. Regular inter-node data packets are transmitted over the active network medium. Control data packets to the maintenance processor are sent from a control node over the standby network medium. The maintenance processor can also totally stop transmission of data from the associated node to both data network media.