Thanks for posting on PI Square. I assume that you are asking with regards to the PI OPC DA and HDA interfaces.
DA and HDA are two different kinds of OPC Servers with different connection protocols. DA which stands for Data Access is the most common OPC Server. HDA stands for Historical Data Access. As you can guess from the naming, the OPC HDA Server allows the user to retrieve historical data from the OPC Server unlike DA which can only retrieve the most recent value for a tag.
For more information about these servers please refer to these links:
What is DA and HDA? Ref KB: 976
The interfaces you are asking about are built to connect to their specific OPC servers since the connection protocols are different. With PI OPC DA interfaces, you can connect only to the OPC DA Servers. The PI OPC HDA Interface can connect to the OPC HDA Servers as well as the OPC DA Servers. We would recommend using the PI OPC HDA interface to connect OPC HDA Servers only.
Please let us know if you have further questions or need further clarification.
Thanks Jesse Ru...
Retrieving data ...