PROFIBUS Manual

The bus cycle time TDP in a multi-master system is dependent on various factors:

•how many class 2 masters are active in the system alongside the class 1 master?

•how much acyclic data is transferred by the class 1 master or the class 2 master?

 

To keep the bus cycle time constant - equidistant - the following procedure is used:

 

The class 1 DP master commences cyclic data exchange. It can signal this by sending a global control (GC) telegram. First it sends - if necessary - a GC to signal its operating state. Cyclic data for all DP slaves will then follow. It is possible to calculate the time required for cyclic data as for a single master. Subsequently, the class 1 master processes one acyclic service (MS1) and passes the token (TC) on to any class 2 master. This master in turn processes another acyclic service (MS2) and passes the token back to the class 1 master.

 

In a non-equidistant cycle, the class 1 master will immediately continue here with cyclic communication. The duration of a bus cycle therefore depends to a great extent on the presence of a second master and the need for acyclic services.

 

With an equidistant bus cycle the class 1 master waits until the cycle time has elapsed before it starts another bus cycle.  It does this by executing an "active pause" (ASP). It keeps sending an FDL status request telegram to itself. This ensures that the bus remains active and controlled for all other bus users. Only when there is too little time for such a telegram - e.g. remaining time is less than TSL - does the cyclic master execute a "passive pause" (PSP) until the bus cycle time has elapsed. At this precise microsecond, it is now possible - if desired - to retransmit a clock telegram or start up cyclic data exchange.

 

For an equidistant cycle to be maintained, this time must be selected to be long enough and the number of permitted acyclic services with a class 2 master must be restricted.