This error appears in the OPC Client application when it fails to find the OPC Server. The two most common causes are:
When an OPC Client application is unable to find the OPC Server, users receive an error informing them that the OPC Client application could not obtain the CLSID of the OPC Server. Depending on the software product that you are using, this error may appear in various different formats as follows:
The inability to establish the first connection with an OPC Server is both the first and most common error that Integrators encounter when using OPC. When an OPC Client application tries to connect to an OPC Server, the OPC Client application must be able to identify the exact application (OPC Server) to which it will connect. Each OPC Server is identified by a Class Identification (CLSID). The CLSID is a type of Globally Unique Identifier (GUID), which is a 128-bit number that identifies an application. A CLSID is similar to your own government-issued identification, such as a Social Security Number (SSN) or Social Insurance Number (SIN), which identifies a specific person.
Most OPC Client applications attempt to display a list of available OPC Servers on the target PC. This process is called Browsing, and is the process whereby the OPC Client application is able to view the OPC Servers that are installed on the remote PC. When the OPC Client performs a Browse, it actually connects to a copy of OpcEnum, which resides on the remote PC, and retrieves the list of available OPC Servers (refer to Figure 1). This list includes the ProgID (human friendly name) and the CLSID (the numerical identification) of each OPC Server. At this point, the OPC Client does not actually connect to the OPC Servers directly. Consequently, the retrieval of the list is independent of the state of each OPC Server and whether it is operational or not.
Figure 1: OpcEnum is used to browse for OPC Servers that are located on the same PC as OpcEnum.
If the OPC Client application is unable to find the CLSID, for any reason, it will not be able to connect to the OPC Server and use any of the messages in Section "2 Symptoms" (above). The OPC Client application typically retrieves the CLSID from one of two sources:
4. OPC Marketplace Observations
A failure to interact either with the Registry or with OpcEnum will result in an error. Section "6 Repair Procedure" (below) explains how to fix the problems that either of the above causes. Nevertheless, keep the following in mind when working with software products that do not seem to behave as you expect.
4.1 Preferred OPC browsing
OPCTI recommends that all programmers enable their OPC Client applications to behave as follows:
4.2 Some applications are unable to connect to OpcEnum
OPCTI recommends that all programmers enable their OPC Client applications to connect to either a local or remote copy of OpcEnum, as per section "4.1 Preferred OPC browsing" (above). However, there are many OPC Client applications that cannot connect to OpcEnum. This is because the programmers who wrote these OPC Client applications decided not to use OpcEnum for browsing. If this is the case integrators will have to do one of the following:
4.3 Some applications can’t connect to the Windows Registry
Some OPC Client applications are unable to connect to the Windows Registry regardless of the user’s identity. This is because the vendor did not add this functionality. These applications can typically only connect to a list of OPC Servers that the vendor preselected. There is typically nothing that the user can do in this case because the limitation lies with the application and not with OPC or DCOM.
5. Repair Procedure
Download the whitepaper for the complete solution by clicking on the "Download" button at the top of the page.
For automated troubleshooting, see www.OpcRescue.com
For automated troubleshooting, refer to OPC Expert, a software application to troubleshoot OPC connections. If OPC Expert fails to connect, it automatically informs you of the problem, diagnoses the cause, and suggests a solution. OPC Expert is free to download, does not require installation, and does not make any changes to Windows registry, so it is safe to use on your computer. Download at www.OpcExpert.com.
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