Extending the Schema
MTConnect was designed with various extension points that are created as substitution groups. These extension points allow for new elements to be created that have certain properties that must be inherited from the parent to provide consistency for an application. There are three places where the MTConnect schema can be extended using an additional namespace: 1. New components can be added, 2. new data item types can be added, and 3. new asset types or measurements can be added. Each requires extending one of the three main XSD schema file. The XSD can be extended using a separate namespace in a separate XSD file and importing the base MTConnect XSD or you can use the schema generator to create the file for you.
Adding a new Component
Adding a new Data Item Type
The header must look something like the following:
<xs:schema xmlns:xs='http://www.w3.org/2001/XMLSchema' xmlns='urn:example.com:ExampleStreams:1.2'
<xs:import namespace='urn:mtconnect.org:MTConnectStreams:1.2' schemaLocation='/schema/MTConnectStreams_1.2.xsd'/>
The header needs to be modified slightly to include the new namespace
e and MTConnect must remain the default namespace. The statements on lines 2 and 3 show how this is done. The MTConnect namespace
m should always be assigned to MTConnect, this will make sure the agent operates correctly when generating XML.
The schema locations should also point to the local storage for these XSD files. It is best to have the agent serve the files up directly so there are no version issues when you deploy the files. I will cover agent configuration later in the document.
Line #5 contains the import statement for the MTConnect schema. This will include the base schema into the extended schema and resolve all the references. Specifying it as in the header will work with all validation tools with the C++ Reference MTConnect agent.
Next we are going to add a new data item called CommonVariable. This can be specified in the Devices.xml as follows:
<DataItem type="e:COMMON_VARIABLE" category="EVENT" id="cv1" name="reg_1" />