0x80040202 error appears in the OPC Client application when it fails to receive a callback from the OPC Server.
When an OPC Client application is unable to receive callbacks from an OPC Server, users will notice at least two symptoms: The OPC Client application will fail to create an OPC Group altogether. The OPC Client application will not be able to show data updates. Instead, data values will remain unchanged.
OPC supports a report-by-exception (RBX) mechanism whereby the OPC Server sends data updates to the OPC Client (such as an HMI, Historian, APC, ERP, etc.) whenever the data changes (also known as "on data change"). OPC terminology refers to this mechanism as "subscription". OPC Servers are able to achieve subscription updates through the use of asynchronous callbacks. In other words, when the OPC Server detects a change in the data, it immediately "calls" the client back with the data update. This is an asynchronous mechanism because the OPC Client does not know when the OPC Server will send the data. However, if you don't set the security settings properly, these data updates will fail. OPC Client applications typically indicate this failure by setting the Quality value of an item to "Bad".
Callbacks force an OPC Server to actively establish a connection with an OPC Client. In a sense, the OPC Server becomes a Client and the Client becomes a Server.
One simple test to determine whether or not a callback is failing is to force the OPC Client to issue a "Synchronous Cache Read" or a "Synchronous Device Read". If either one of these return values with "Good" quality, then the lack of data updates is likely due to the OPC Server being unable to send callbacks to the OPC Client application.
For the complete step-by-step repair procedure to fix this DCOM error, download the complete whitepaper.
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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