A distributed automation communication system, in particular a real-time critical, cyclic, real-time Ethernet system, which has a plurality of users. Each user has at least one diagnostic unit (7, 8, 15) for the diagnosis and recording of data concerning the data exchange between users. In addition, the communication system (1) has a further user embodied as a central diagnostic terminal (13). The central diagnostic terminal (13) carries out a central diagnosis of the whole communication system (1) by configuration of the diagnostic units (7, 8, 15) of the users and calling up and analyzing the diagnostic data recorded by the diagnostic units (7, 8, 15) of the users.