Exploiting data link layer features to realize communication models in FieldBus system

The high complexity of actual process control systems in factory automation has led to the need of suitable communication systems to interconnect field and control devices, in order to overcome most of the limits of the actual point-to-point connections. Serial communication systems known as FieldBuses represent a valid solution to these problems. At present the IEC/ISA FieldBus protocol, which is currently being standardized, represents an international standard for process control networks. Although the standard has been almost completely defined, it is still considerably difficult to understand, above all as regards implementation of the communication models offered by the Application layer through the access medium mechanisms available at the Data Link level. This mapping is a key point in the definition of a standard, as only correct mapping between application layer communication models and data link layer mechanisms can guarantee that all the time-critical requirements of application processes will be fully met. The aim of this paper is mainly to analyze the communication models currently offered by the Application layer and their implementation through the mechanisms present in the Data Link layers. The results of the analysis will show that the models currently offered by the standard are insufficient for the implementation of certain realistic process control system scenarios. In order to bridge the gaps pointed out, the paper proposes a further communication model, specifying its implementation through the Data Link mechanisms.