PROFIBUS Manual

To allow an engineering tool access to a slave at all times, it is switched onto the bus as a class 2 master and must explicitly establish a connection with the slave. Several master class 2 can establish a MS2 connection at the same time to the same slave. The connection has not to be planned in advance, but for this purpose, further SAPs are reserved on the master and slave:

 

SAP: service access points of master

 

SAP: service access points of  DP slave

 

A connection is explicitly established with a DPV1_Initiate and closed again with am DPV1_Abort. For a connection to be established, the following services and telegrams must be transmitted:

 

DPV1_Initiate: at SAP = 49 (resource manager)

 

A poll cycle at SAP = 49 with response data:

 

If the resource manager has no more free SAPs, it reports this at the FDL layer with RS (= no resource available). The master begins with a number of poll cycles at the new SAP = Server_SAP until a response is given or until the Min_Send_Timeout has expired.

 

 

A negative response:

 

If the response is positive, payload data can now be exchanged.