Communication control unit switched between active and stand-by functions automatically in response to monitored status of a different communication control unit connected to a common network

摘要

A system comprised of multiplexed communication control units and a communication control unit for such multiplexing, where the switching between the stand-by unit and the active unit at the time of an abnormality can be performed in a short time and the cause of a breakdown obstructing operation can be grasped from the condition of the transmission line and the switching made conditional on that condition, thereby enabling highly reliable communication. The communication control units are connected in a multidrop configuration and both of the active unit and the stand-by unit are kept activated (communicable in state). The stand-by communication control unit transmits a frame through the transmission line at a timing not obstructing the communications, so it is possible to detect an abnormality in the active unit and rapidly switch to the stand-by unit at the time of an abnormality using the unit switching processing unit.