History
OLE for Process Control (OPC), which stands for Object Linking and Embedding (OLE) for Process Control, is the original name for a standards specification developed in 1996 by an industrial automation industry task force. The standard specifies the communication of real-time plant data between control devices from different manufacturers.
As of November 2011, the OPC Foundation has officially renamed the acronym to mean "Open Platform Communications" although they also use the tagline "Open Productivity & Connectivity" on their website.[1] The change in name reflects the applications of OPC technology for applications in Process Control, discrete manufacturing, building automation, and many others. OPC has also grown beyond its original OLE (Object Linking and Embedding) implementation to include other data transportation technologies including XML, Microsoft's .NET Framework, and even the OPC Foundation's binary-encoded TCP format.
The OPC Foundation has defined interfaces to Data Access Servers, Event Servers, Batch Servers, and History Data Access Servers. These servers have information that is valuable to the enterprise, and is currently being provided to enterprise applications via OLE/COM based interfaces.
XML, the eXtensible Markup Language, and XML-based schema languages
provide another means to describe and exchange structured information between
collaborating applications. XML is a technology that is more readily
available across a wide range of platforms. OPC XML DA (Data Access)
is the OPC Foundation's adoption of the XML set of
technologies to facilitate the exchange of plant data across the internet, and
upwards into the enterprise domain.
Overview
Fig. 1 Illustrates a simple diagram of the Smart OPC XML Server. At the front stands a HTTP listener who is responsible to accept client XML SOAP requests and transfer them to the OPC XML DA and OPC XML HDA Web services. Both Web services are respectively connected to the OPC XML DA and OPC XML HDA modules. Below the OPC XML DA and OPC XML HDA modules are situated loaded plugins for each one of them.
The HTTP listener supports both HTTP and HTTPS schemes. For HTTPS scheme you must provide a server certificate. Also HTTP listener supports different authentication schemes like Digest, Negotiate, NTLM, Integrated Windows Authentication, Basic and Anonymous. HTTP listener can check client certificate thumbprint for authorization. User identity from HTTP listener is transferred to the ASP .NET runtime where the authorization is made.