Fieldbus network multiplexing system

A fieldbus network multiplexing system is provided to perform the role of a main master board when the main master board has an event like an operation fault and power-off, by comprising a subsidiary master board. A fieldbus network downloads and stores a control engineering program and a field bus constitution program from a host computer(21), and includes a first master board(22) having a main processing unit(MPU) operating a field bus control system by transmitting constitution information of slaves to a fieldbus main controller(FMC). The first master board comprises a dual link unit(DLU), and performs a role of an operation master board of the fieldbus network by operating the field bus control program and the field bus constitution program downloaded from the host computer. At least one multiple master board is connected to the host computer by an Ethernet, and receives the data through the dual link unit, and performs a role of a slave board while monitoring the state of the first master board, and performs a role of the operation master board when the first master board has a system switching event.