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OPC is used by automation professionals for secure data transfer and process control. One OPC specification from the OPC Foundation is OPC Xi (OPC Express Interface). OPC Xi defines a .NET interface with the combined functionality of OPC DA, HDA, AE and a rather simple data model. OPC Unified Architecture in comparison allows the same functionality, including a .NET interface and a similar data model to OPC DA, HDA, and AE, but also supports platform independence and optionally complex information modeling capabilities.
What is OPC Xi?
OPC Xi can securely transport data among automation applications on separate computers. It can only run in .NET environments and thus not in all plant network environments. To transport data, Classic OPC relies on DCOM, however OPC Xi works with Windows Communication Foundation. This means that Managed code applications don’t need a .NET wrapper for DCOM interoperation, but a wrapper from COM to .NET on the server, adding complexity to the server environment.
OPC Xi and Networks
The use of WCF allows OPC Xi to be functional with small and large networks. OPC Xi can be configured for communication across firewalls and Network Address Translation (NAT).
Security and OPC Xi
In addition to the security implied in WCF (authentication, authorization and encryption), OPC Xi has additional security measures that restrict write, read, and subscribe access to servers. This is done based on authentication and protocol of the client application that is attempting to access the server using http or tcp.
OPC Xi Specification
The OPC Xi specification is defined in a Windows Helpfile. Different versions of this file without obvious version information are available from OPC Foundation official website.