What is OPC XML- DA?
OPC XML-DA stands for OPC XML Data Access. It was designed to incorporate SOAP and Web services that server data to clients in XML format. While it was not broadly adopted, there are still a few OPC XML-DA servers out there, and you can use OPC Data Client to communicate to them.
Because of the similarity to the original OPC-DA specification and the versatility of the OPC Data Client, the same code that is used to interact with OPC-DA servers can be used to interact with OPC XML-DA servers by just specifying the URL of the XML-DA server.