Introduction to OPC HDA Data Objects
All data returned from OPC HDA servers can be stored in MATLAB® as an OPC HDA data object. The HDA data object allows for convenient data storage, manipulation, and visualization. The data elements themselves are represented by one or more value, quality, and timestamp values, all associated with an item ID.
When you perform read operations on OPC HDA servers, you request data for one or more
item IDs on that server over a specified time range. For each item requested, the OPC
server returns zero or more data object elements stored as triplets of Value (the sensor
reading or item value), Quality (the quality of the value stored), and TimeStamp (the
time the data was logged by the server). The Value, Quality, and TimeStamp properties
M-by-1 vectors. The data type of the Value property
depends on what the server returns to MATLAB. See Conversion Between MATLAB Data Types and COM Variant Data Types.
Each read operation thus returns an array of OPC HDA data objects, one for each item requested. Elements of a data object array are not guaranteed to have the same number of Value, Quality, and TimeStamp triples, because the server might not have logged data at the same time for all items requested.