What is OPC .NET 3.0 (WCF)?
OPC .NET 3.0 (WCF) (formerly known as OPC Express Interface (Xi)) is the continued evolution of OPC Foundation native support for Microsoft platforms that previously included: the OPC Foundation Automation Wrapper for Visual Basic 6 and the OPC .NET 2.0 API for VB.NET and C#.It bridges the gap between Microsoft.NET and the world of OPC Classic.
OPC .NET 3.0 is the result of collaboration among several OPC vendor companies to provide a simple .NET interface for OPC Classic servers. It was developed to allow client applications toeasily use the latest .NET features to access existing OPC Classic servers.
OPC .NET 3.0 is an OPC standard C# Application Programming Interface (API) designed to simplify client access to OPC Classic servers.It also includes a formal WCF Interface, however, there is no compliance or certification program for this interface. The OPC Classic interfaces are still the primary means to ensure multi-vendor interoperability.
The OPC .NET 3.0 WCF Interface combines the semantics of OPC DA, OPC A&E, and OPC HDA into a single interface and provides additional support for historical alarms and events. The use of WCF provides an increased level of security through built-in authentication, authorization and data encryption. It also simplifies access through firewalls, requiring only two ports to be opened. Additional security features have been built into theOPC .NET 3.0 WCF Interface to enable secure server access by the client.
The OPC .NET 3.0 WCF Interface also provides for automatic reconnection to the server when transient communication failures cause disconnection. This feature preserves the client context with the server and eliminates the need for the client to rebuild its configuration in the server.
The OPC .NET 3.0 API has been designed to insulate the client application from the network interface. Using this API, the client application will no longer have to deal with the details of maintaining a connection with the server and exchanging requests and responses with it. Code that is distributed with the API and the WCF interface will perform these functions for the client application.
OPC .NET 3.0 is distributed with three generic client applications that have been developed to provide a model for developing client applications. They have also been developed for use as test clients to ensure newly developed OPC .NET servers operate as required.
On the server side, a generic wrapper is available for OPC DA, A&E, or HDA servers. The wrapper code is divided into two components; one that handles the WCF interface and one that performs the mapping between OPC .NET and OPC Classics calls.
The code that handles the WCF interface is referred to as the Server Base. It defines a consistent behavior for all OPC .NET servers, and can be used as the basis for native .NET servers. The wrapper is distributed in source form to allow customization if desired.
The wrapper is capable of wrapping one or more of the OPC Classic servers (OPC DA, HDA, and/or A&E) at the same time. The wrapper may be installed on the same machine as the wrapped servers, or it can be configured to access them remotely machine via DCOM.
The OPC .NET WCF Interface and Client API are documented to support the use of IntelliSense by programmers. Help for classes, methods, and parameters are dynamically provided to the programmer during development. In addition, a Microsoft .CHM file has been generated to provide a standalone specification for these interfaces.
To address the problem of finding servers on the network, the OPC .NET 3.0 code set also includes a Discovery Server. The Discovery Server provides a list of available OPC .NET 3.0 servers to client applications. The Discovery Server can automatically detect OPC .NET 3.0 servers using the Microsoft Peer Name Resolution Protocol (PNRP). PNRP is available on XP and later operating systems. Alternatively, the Discovery Server can be configured with the address of the Discovery server. Further, multiple Discovery Servers can be running at the same time to provide for redundancy.
Similarly, client applications also detect the Discovery Server using PNRP or through manual configuration. If a Discovery server is not used, client applications can be manually configured with the list of available OPC .NET servers.
OPC .NET 3.0 (WCF) Provides:
- Native .NET access to OPC DA (Data Access), OPC HDA (Historical Data Access) and OPC A&E (Alarms and Events)
- Secure, firewall-friendly communications via WCF, the .NET replacement for DCOM.
- Auto-recovery mechanisms that reconnect the client to the server after transient communication failures without loss of the client context with the server.
- Full compatibility with OPC Classic
About WCF
Windows Communication Foundation (WCF) is the .NET replacement for DCOM. It provides a framework for building service-oriented applications, decoupling the service interface from the underlying protocol. As a result, the methods of the server oriented application can be configured to use named pipes, TCP, HTTP, and HTTPs at the same time without any changes to the application code.
Additionally the service-oriented application can be hosted by IIS, by a console application, or by a Windows Service without changes to the service. To help developers, the OPC .NET Wrapper and the OPC .NET Discovery server are distributed as console applications and as Windows Services. The console application versions are intended for use during development and the Windows Service versions are intended for production use.