PROFIBUS Manual

The bus is the common medium of transmission.  Medium access control (MAC) serves to ensure that no more than one station transmits on it at any one time. Two types of bus user are present on the PROFIBUS: active masters and passive slaves. Masters receive authorization to access the bus for a specific time. This access authorization is explicitly passed on from one master to another with a token telegram. This process is called token passing. Slaves can send a telegram on the bus once only - in response to a request from a master. This represents master-slave communication. The PROFIBUS FDL access procedure is therefore a hybrid of master-slave and token passing.