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Addressing Scheme

The syntax of the addressing scheme is not specified in OPC XML-DA, instead OPC leaves this issue open to the implementation of the server. OPC XML-DA only specifies the two XML elements ``ItemPath'' and ``ItemName'' that together uniquely identify the address of the OPC Item. The specification allows to use only ``ItemName'' for addressing and omit the element ``ItemPath''.

The most natural way to implement the IGUANA addressing scheme in OPC is to place the address into the ``ItemName'' element. 103 The hierarchical structure of the IGUANA addressing scheme allows hierarchical browsing in the object hierarchy. However, the format of the IGUANA addressing scheme may prove inappropriate as hierarchical addresses are commonly formatted in a URL like style, which consist of character strings, delimited by the slash (``/''). Another way is to format the address similar to variables in object-oriented programming languages by delimiting the addressing elements with the dot (``.'') character, such as server1/node2/datapoint3 or server1.node2.datapoint3. Therefore IGUANA addresses may be translated, as shown in figure 51.

Figure 51: Translating the IGUANA Addressing Format to a URL Like or a ``Programming style'' format
\begin{figure}\centering
\includegraphics[scale=0.7]{graphics/iguana-opc-addrformat.eps}\end{figure}

An IGUANA address may contain any printable character, with the exception of the three special characters ``!'', ``.'' and ``@'', which are used as delimiters. This means that the slash or dot character may theoretically be part of the IGUANA address. Therefore it has to be escaped104, so that possible occurrences of the slash character are not confused with the address delimiter.

For retrieving fieldbus data, the encoding style has to be concatenated to the translated IGUANA address. This way a fieldbus value may be retrieved in a particular IGUANA encoding style. An example address is shown in figure 52.

Figure 52: Translating the IGUANA addressing format to a URL like format
\begin{figure}\centering
\includegraphics[scale=0.7]{graphics/xmlda-ex-address.eps}\end{figure}


next up previous contents
Next: OPC XML-DA Faults and Up: Representing the IGUANA Interface Previous: Data Encoding, Data Types   Contents
Hermann Himmelbauer 2006-09-27