Layer Model of the Foundation Fieldbus

DCS Communication
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This fieldbus follows a model which is composed of several layers. Here, the main elements are provided by

  • the physical layer
  • the communication stack, and
  • the application

Within the layer model of the Foundation FB, the application is the lowest level, and it is composed of the function block model and the device description. The next-highest layer is the communication stack, on which the application sets itself directly. At this point, the user can access a different range of services, depending on which blocks are implemented in a device.

Furthermore, the services and functions brought by the application and the application layer can be used by the system management for processing of its tasks. This communication behavior makes it possible to guarantee correct cooperation between the individual components of the bus. This also includes time synchronization of all of the measurement and control tasks of the respective field devices. The effect of the communication stack is based on distributed communication. This guarantees several points:

  • Each controlling field device can exchange data with other devices. This includes the reading of measuring values and the derivation of set values.
  • All field devices are serviced in time. The definition as a timely process provides that there are no negative effects during the processing of the control circuits.
  • Two or more devices can never access one and the same bus simultaneously.

All of this is made possible by the use of a centrally located communication control.

In this representation, the physical layer forms the highest level. This assumes a relevance within the Foundation FB. In addition to these classical layers, the Foundation FB is also characterised by its dependence on the layer model of the ISO/OSI standard. With this, the layers 3 to 6 are not realized, even if they exist in most fieldbus systems.

However, the tasks still occurring within these unused layers are covered by the communication stack. The seventh layer, i.e. the highest level within such a layer model, is composed of two components, the fieldbus access sublayer and the fieldbus message specification.