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Foundation Fieldbus OSI Model Versus OSI Model
How Fieldbus Works ?
refers to the passing of data from one device to another, may it be a field device, operator console or a configurator. This is the communication protocol part of Fieldbus. The application is the automation function the system performs. By standardizing part of the application, Fieldbus has gone further than any other communication standard, ensuring interoperability between conforming products.
OSI Model :
layers immediately above and below in the stack. The stack interfaces upwards to the application, and downwards to the transmission media.
process (AP). The AP consists of two parts, one user portion, which is the functionality, and one communication portion. In Fieldbus the user portion is the actual device function, such as measurement or control (function blocks), or the user
OSI Reference Model
•Layer 6: Presentation –
System and Network Management
control application can be built upon, and a part that provides optimization of operation and diagnostics of problems. It coordinates functions in all layers, controls overall device operation and startup.The purpose with system management kernel is to provide functions for:
of them), the others act as agents. In case a manager fails, one of the agents will assume the manager role. For the system management to perform its task, it must cooperate with the system management in other stations on the network. A
simple device may implement only a part of the system management functions.
Physical Device Tag assignment
Station address assignment
Function block binding
The purpose of scheduling is to minimize delays due to communication. Such delays are pure dead time which make control difficult. Scheduling also insures that variables are sampled and function blocks are executed on a precisely period basis, so that the delay is constant. A constant delay is a must, since a change in delay would require re-tuning. This way, tight closed loop control is achieved with time left over for background traffic. Scheduling also insures accurate trending and predictable alarm detection.There are three scheduled functions:
Foundation Fieldbus Network Management
monitors and controls operation of network resources.
one station to another. It is modeled as a managing system (playing the manager role), a managed system ) that plays the agent role), which has a Management Information Base (MIB). MIB is the logical store for information and resources
used to support network management.The end-user is primarily concerned with the physical connection and the application. The physical layer, as previously mentioned, is already standardized and will not change. It will surely expand to include other media such as radio, but it will not change. Since all suggested Fieldbus protocols propose almost identical solutions for the user application, the Fieldbus is ‘ready’ as far as the user is concerned. Users may now go ahead and learn the installation, application and operation aspects of Fieldbus without being afraid of having wasted their time. The uncertainty lies in the application and
data link layers, and in the management functions. However, those are transparent to users, and only of concern to product developers.
Foundation Fieldbus OSI Model
Fieldbus, (and most other LANs) has no interconnection between networks, which is the purpose of these layers. This simplification makes Fieldbus faster and easier to implement in devices with limited processor power, such as field
instruments.The three remaining layers, and provided functional and procedural characteristics are:
1. Physical Layer (PhL); media independent activation, maintenance, and deactivation of physical links that transparently pass the bit stream for communication; it only recognizes individual bits, not characters or multicharacter frames. The standard defines types of media and signals, transmission speed and topology including number of nodes, and device power (only in Fieldbus).
2. Data Link Layer (DLL); transfers data between network entities; activation, maintenance and deactivation of data link connections, grouping of bits into characters and message frames, character and frame synchronization, error control, media access control and flow control (allowing several devices to share the network). The standard defines type of media access control, frame formats, error checking and station addressing*.
7. Application Layer (APP); gives access to a set of local and communication services for serving the distributed system — interconnection between the APs and the user. Standard defines message formats and services available to the AP.