The OPC Historical Data Access (HDA) standard provides an interoperable platform to store and exchange historical process data. This standard differs from the OPC Data Access (DA) specification that deals only with real-time data. OPC Toolbox™ software provides a client interface to historical data access servers via the MATLAB environment. This client interface lets you:
Retrieve data from HDA servers into MATLAB®
Preprocess that data for common analysis tasks
Visualize the data for easy interpretation
There are several types of OPC HDA historians:
Simple trend data servers function only as basic raw data storage. The data itself would be of the type commonly made available by an OPC data access server and would take the form of value, quality, and timestamp triplets.
Complex data compression and analysis servers provide data compression in addition to raw data storage. These servers are used where large volumes of process data are expected and storage space would be a limiting factor.
Analysis servers are capable of providing analysis and summary information. They can support the updating of data and store the history of those updates. Storing data annotations may also be supported.
OPC Toolbox provides capabilities for reading raw and processed data from servers. Updating data on an HDA server and retrieving annotations is not supported.
Measurements from process end points (sensors, PLCs, etc.) are represented in the OPC HDA infrastructure as "items". Each item has a unique item ID on the server, and therefore can be accessed uniquely. To best arrange the items, the server orders the items into a logical listing called a "name space." These name spaces often take the form of a hierarchical tree in which groups of similar items are arranged into logical categories:
An item is usually represented by its fully qualified item ID (FQID) within the name space. An FQID is usually comprised of each level of the item's hierarchy separated by periods. For example:
In some cases, as in very small or simple historians, a hierarchical structure is not used. Instead all items are presented as a flat list of items.