The OPC-DA standard was very progressive for its time, but in the modern era it has a few significant problems.

This is why the OPC Foundation has replaced the old standard with OPC-UA, a more modern equivalent.

There are several significant advantages of UA over DA, including…

• OPC-DA depends on D-COM, a Windows only component that is very complex to administer, often fails after Windows updates, and is being actively phased out by Microsoft. UA does not.
• OPC-UA is cross-platform, where OPC-DA would only work on Windows systems.
• OPC-UA has it's own security layer, rather than depending on Active Directory
• OPC-UA has finer-grain security controls, less overhead and some modern extensions that improve the system overall.

If you're working with older systems, you can use an OPC-UA Wrapper - a product that effectively upgrades your DA server to a UA one. Since this can run on the same server as your OPC-DA machine, avoid all of the complexity of DCOM and allows much easier access to your data.