(the client). The OPC client uses Dionex Integral analyzer data for custom applications. The Chromeleon PA OPC interface specifications are based on the Axeda FactorySoft OPC Toolkit. The above programs, as well as the datasource and instrument server can be installed on a single computer or on separate computers, as in the following example. This OPC Server is accessed by an OPC Client application on a WIN2K(AnonymousLogon). This client cannot be changed in any way (cannot create user, or change DCOM configuration). Following instructions to configure OPCEnum, DCOM and Computer, I am able to view the OPC server from my client, but not to connect. Integrate secure, reliable communications in your LabVIEW applications with the LabVIEW OPC UA Toolkit. This toolkit allows communications and interoperability with third-party devices and supervisory systems supporting the OPC UA specification. It incorporates a series of features for the creation of OPC UA clients, servers, and security management.
Introduction
The OPC XML-DA standard and the currently widely used OPC DA V2 standard address the same tasks but the server interface is quite different, due to their different base, web services respectively DCOM. Developers of OPC client applications have to be aware of these differences. The sample client applications show how the Read and Subscribe/Poll functions are used from different kinds of applications. The VB.NET and C# samples use the .NET web services support, the HTML/ASP scripts use the MSSOAP toolkit.
Background
Read the XMLDA.NET White Paper for a description of how web services differ from DCOM and how this influences the design of client applications. Evaluation versions of .NET based OPC XML DA servers a clients can be downloaded.
Using the Code
VB and C# applications typically use the .NET features to access web services. Desktop and ASP.NET applications can access OPC XML-DA servers in the same way. XML-DA has a stateless design that fits well into ASP.NET applications. The code snippet shown below uses the Read function to periodically read the value of two items from the server. The XML-DA Read function is self contained and not dependent on any previous calls.
Optional parameters are a source for common mistakes. The .NET web services interface has an 'enable' parameter with each optional value type parameter. If this parameter is not set to 'true', then setting the optional parameter has no effect.
The usage of the XML-DA functions is quite simple but initializing the request lists with the required definitions for hundreds of items need quite a bit of code. In applications with many items, it is better to define the request lists in an XML file and in the application only call a helper method that initializes the request list object from the XML file. Advosol's XML-DA Client Component offers such support. The code snippet below shows how a read call is done using the SDK helper classes. Download the XML-DA Client SDK evaluation version to explore this further.
HTML and ASP applications need to make the OPC XML-DA server access from client-side respectively server-side scripts. The source code download contains samples with JavaScripts and VB scripts. They show how to use the Read and the more complex but also more efficient Subscribe/Poll functions. The XML-DA web service call is made by building the SOAP request message using the MSSOAP toolkit. The code snippet shows how a Read request message is created.
Free Opc Client
License
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