Here, the abbreviation FB stands for the term “fieldbus”, so that Foundation FB relates to a fieldbus. This can be used flexibly and is found frequently, especially in the process automation sector. Since the development of this technology, fieldbuses have been needed to reduce the costs of installing a network. In addition to saving costs, they save time as well on account of the simplicity of the planning process as well as through improvements and securing of company operations.
Historical Background to Foundation Fieldbus
The project to development of a uniform fieldbus and create an international standard was initiated in 1992 on the part of Forma ISP. At the same time, the members of the French Flux Information Processus founded an international Organization known as WorldFIP. Initially, this federation represented a counterweight to the ISP, however two years after the foundation, a joint formation, the Fieldbus Foundation was founded for reasons of technology and economic policy. The aim of this foundation is still to find a fieldbus standard in which an IEC uniform fieldbus is created, i.e. one which can be especially used in explosive applications. Because of this, the Foundation FB shall include the FIP specification as well as the ISP specification.
The Layer Model of the Foundation Fieldbus
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.
Communication Control with the Foundation Fieldbus
According to the specification developed by the “Fieldbus Foundation”, control of the communication processes within the fieldbus is performed by two types of transmission:
- Clocked data transmission: This mainly controls tasks which are evaluated as time-critical. A fixed processing schedule is set up to perform execution of the resulting tasks in due time and without access problems. This schedule is created by a system administrator during configuration of the bus system in accordance the the FF specification. During the clocked data transmission, all devices run synchronized in time, whereby the point in time and the sequence are specified in advance. This is also known as a deterministic system.
- Non-clocked data transmission: Here, parametrization and diagnosis especially takes place within the communication control. After all, transmission of this information and data must be possible when it is required. However, the transmission of the device parameters and the diagnosis information is not time-critical, i.e. acyclic, so that with Foundation FB the non-clocked data transmission takes place exclusively in the time gaps of the clocked transmissions.
Tasks that are still to be performed with Foundation Fieldbus
No standard so far has been decided with regard to the topology and the definition of the associated subnets, as in many items the execution of the FF system still cannot be equated with the specifications of the IEC file model. The IEC fieldbus thus solves the communication tasks with the intrinsically safe H1 bus and the H2 bus above it. As such, this is a bus system with two components. In regard to the H1 bus, the fieldbus of the FF system already operates in the same way as the IEC model, but the classification of the H2 bus has not yet been completed. The only item specified is the use of the fast or high-speed Ethernet for solving problems with regard to the Foundation FB.