2.6 Parameter interchange via XML

The purpose of the parameter interchange via XML is to provide a way to exchange information between Frame Application and DTMs. Typically, in process control systems, multiple client applications like observing, channel assignment, or master configuration, need information about the configuration of a device.

XML is not meant to replace proprietary formats; it is meant to provide access to data that is stored in a proprietary format. Data should be stored locally in the fashion that makes the most sense. XML provides an extendable standard to connect FDT components. The data exchange is done via XML documents. Within these documents XML tags are used to delimit pieces of data. XML leaves the interpretation of the data to the application that reads it. To get a common understanding of the exchanged data FDT uses XML schemas for validation. For the data access are standardized tools like the DOM (W3C's Document Object Model) available.

Figure 2-7 FDT Client/Server Relationship via XML

XML schemas are valid XML syntax themselves and are used to validate XML data. They allow the validation of the document structure and the data types of the elements

W3C's Document Object Model (DOM) is a standard internal representation of the document structure. It aims to make it easy for programmers to access components and delete, add, or edit their content, attributes and style. In essence, the DOM makes it possible for programmers to write applications that work properly on all browsers and servers, and on all platforms. While programmers may need to use different programming languages, they do not need to change their programming model.

An XML parser usually generates the DOM. Microsoft provides such an XML parser.. So the DOM API is free accessible in VC++, Visual Basic and VBScript.

The parsing of XML documents with XML schemas guarantees a valid DOM with well-defined elements.

The FDT developer should always work with the DOM because

• The XML Parser generates the DOM from the transferred XML data
• The schemas guarantee a valid DOM with well defined elements
• The DOM supplies standard tree- and collection-methods for data access
• The DOM can generate the XML data for the data transfer

Note:
In case information is shared across multiple clients it is required to ensure that the configuration information remains consistent across multiple clients by informing all DTMs which have a reference to the same data set about changed data. Example: more than one DTM for the same device on different working stations.