The .NET platform required for XML DA is now widely used and OPC server and client applications are being developed on the .NET platform. However, still most applications are built as DCOM based OPC DA server/clients, using .NET wrappers. Despite the many difficulties with DCOM configuration the transition to web services based designs is slow.
With most OPC talk being about the announced OPC Unified Architecture you may find it difficult to decide on how to approach new OPC projects.
This paper tries to shed some light on the related factors. It focuses on the development of OPC clients and explains different approaches and products.
OPC Situation OPC DA (Data Access) is broadly accepted for process data access applications and OPC-DA V2 servers are available for almost any equipment. OPC-DA is based on DCOM and this foundation is becoming a legacy technology. Microsoft will keep supporting DCOM but focuses new developments on web services. .NET3 combines the different Microsoft communication technologies (except DCOM) into one component, the Windows Communication Foundation (WCF).
The OPC standard currently most widely used is OPC-DA V2.05. OPC DA V3 offers some additional features but most client applications don't use these in order to be able to work with the many installed OPC DA V2 servers. The OPC AE (Alarms & Events) and OPC HDA (Historical Data) specifications are used to a much less extent than OPC DA. Other OPC specifications such as OPC DX, Commands and Batch are hardly used at all.
The XML DA specification defines a data access interface in the form of web service SOAP messages. XML DA is designed to provide the Data Access functionality on a web service base and is the only web services based OPC specification released so far.
OPC UA is created as a very general specification that covers all previous OPC specifications and can be implemented with web services communication but also on other communication stacks.
The designer of .Net based OPC application has the choice of using OPC DA, XML DA, or wait for OPC UA.
Download the whitepaper for all the details.