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Fieldbus Data and Addressing Scheme

Fieldbus data resides on fieldbus nodes which provide one or multiple values and properties that may be read and/or written. This data has to be aggregated and abstracted in some way, so that it can be provided as a consistent data structure on the Internet/fieldbus gateway.

The easiest way would be to create a physical representation of the fieldbus nodes and to provide this structure on the gateway. This approach is impressively simple and easy to implement but has the following shortcomings:

Fieldbus-specific:
Different fieldbus systems implement different data structures62. Therefore data aggregation based on physical structures may not be consistent over different fieldbus systems which makes data abstraction fieldbus-specific and hence limits interoperability between systems.

Physical/Logical considerations:
In many cases, the physical structure does not comply with the logical structure. For example, a single fieldbus node may implement lighting and heating controls which have no logical connection. Data structures based on the physical implementation will therefore lead to confusing addressing schemes.

A common fieldbus abstraction is built around the value, which is the very basic element of fieldbus nodes. Echelon for example introduces a concept of so-called ``data points'' that basically represent pieces of information. These data points reside in a fieldbus node and contain the value that can be read and written with some accompanying properties, such as a timestamp, the data type or the status of the value as depicted in figure 28.

Figure 28: Data Points
\begin{figure}\centering
\includegraphics[scale=0.6]{graphics/datapoints.eps}\end{figure}

Common interface specifications for Internet/fieldbus gateways, as described in chapter 3, use very similar abstraction techniques.

Internet/fieldbus gateways may also aggregate data points to a structure that is presented to client applications. This way, data point elements in the gateway, such as the value, are only place holders63, for their counterparts in the fieldbus node. Such a data structure in an Internet/fieldbus gateway has to be configured, which means that fieldbus data is assigned to certain data points that are aggregated to some data structure in the gateway. The following basic data structures may be used:

Flat:
This data structure is depicted on the left in figure 29. The advantage of this arrangement of data points is its simplicity - it is easy to set up and to maintain. The drawback is that the structure gets very complex when the gateway contains a high number of data points. Browsing the structure may become very inefficient or complicated64.

The addressing scheme of data points in this structure is simple. However, every data point must have a unique address, which demands an address space that is big enough when high numbers of data points are assigned to an Internet/fieldbus gateway.

Hierarchical:
Hierarchical data structures, as shown on the right in figure 29, are used to arrange data in a tree-like structure with parent elements that contain zero or multiple child elements. The top level data point is called ``root data point''. Every child may have only one parent.

Hierarchical trees may be used to represent physical, geographical or logical structures. The addressing scheme itself describes the location of a data point in the hierarchical structure. The address itself consists of the child's address and all parent addresses, which are separated by delimiters65.

Figure 29: Flat and Hierarchical Data Structures
\begin{figure}\centering
\includegraphics[scale=0.7]{graphics/flat_hierarchical.eps}\end{figure}

Fieldbus systems may have multiple viewpoints66, therefore Internet/fieldbus gateways may assign one data point to multiple data structures. This way, multiple data point trees may coexist in one gateway and offer clients different perspectives of fieldbus systems.


next up previous contents
Next: Localization and Units Up: Generalized Fieldbus Interfaces Previous: Operations for Fieldbus Access   Contents
Hermann Himmelbauer 2006-09-27