Field data bus system

The bus system has a transmission medium with a serial data bus with active bus terminals (14,16) at either end and further bus terminals (18,20) through which the transfer medium passes physically. At least the first and/or second active bus terminal has an arrangement for the regular transmission and/or reception of status messages. At least one terminal checks the status messages from another terminal for absence or error. If a status message is not received within a defined period, or if status messages are received which differ from those for the error-free state of the bus system, the bus system is set into a state which fulfils a predefined safety criterion.