the invention relates to a computer - implemented diagnostic procedure for a field bus system in which bus users exchange data via a field bus in accordance with a predetermined communication protocol , the data being exchanged in the form of data packets , comprising the following procedural steps : recording states of the bus users ; detecting a data packet which is transmitted via the field bus between the bus users ; analyzing the data packet and deriving an updated state of the bus users in dependence on the preceding state of the bus users and the content of the data packet ; recording the updated states of the bus users ; repeating steps to in order to detect the current states of the bus users in an in - line manner .