Understanding OPC DA Interface Callback Requirements for PI Advised Tags

Understanding OPC DA Interface Callback Requirements for PI Advised Tags

In the landscape of industrial automation, integrating software systems with real-time data has become crucial. The use of OPC (OLE for Process Control) DA (Data Access) interfaces is common for facilitating communication between various industrial devices and software systems like the PI System. A common challenge faced by engineers when using the PI OPC DA interface involves understanding when and why a callback mechanism is required for data flow.

Advised Tags and Callback Requirements

When configuring PI tags in the PI System using an OPC DA interface, you can set these tags to be "advised", meaning the tag is supposed to automatically receive updates from the OPC server. To achieve this, the mechanism often relies on a callback function from the OPC server to inform the PI system of any changes.

In essence, the callback is critical because it serves as the OPC server's way to push data dynamically to the PI interface, ensuring that real-time updates are efficiently communicated. Without a successful callback, the interface may not receive these updates, disrupting the intended data flow.

Challenges with DCOM Errors

Frequently, users encounter DCOM (Distributed Component Object Model) errors while interacting with the OPC Client tool. Such errors can arise if there are issues with permissions between the system running the OPC server and the PI interface node trying to access it. If the OPC Client tool experiences a DCOM error when using a "refresh" operation, it's often because the necessary user account is not linked correctly between the OPC server and the system attempting the connection.

However, switching to an "Asynchronous" option in the tool can help bypass these issues temporarily. This workaround might allow data retrieval despite the underlying permissions problem, but it doesn’t resolve the root cause.

Steps to Resolve DCOM Errors

For the PI OPC DA interface to establish a seamless data flow from OPC to the PI server, the following should be ensured:

1. Correct User Account Setup: The user account running the PI OPC DA interface needs to match or have appropriate permissions as the OPC server's user account. Any logging failures related to the account must be rectified.

2. Review DCOM Configuration: Update the DCOM permissions on the OPC server machine to allow the account running the PI OPC DA interface access.

3. Consult OSIsoft DCOM Guides: Reference materials like the OSIsoft DCOM Configuration Guide are invaluable resources that provide step-by-step instructions for configuring DCOM correctly for OPC interfaces.

Conclusion

Understanding and correctly setting up the callback processes and DCOM permissions is crucial for the successful operation of advised PI tags through an OPC DA interface. Paying close attention to user account configurations and leveraging the guidance available through official documentation can help mitigate issues, ensuring real-time data flows smoothly from OPC servers to the PI System.