Glossary · Industrial communication
OPC Classic DA, HDA and A&E
Also known as: OPC DA, OPC HDA, OPC A&E
German: OPC Classic DA, HDA und A&E
In industrial communication, OPC Classic DA, HDA and A&E are the three main OPC Classic specifications: Data Access (DA) for current values with quality and timestamp, Historical Data Access (HDA) for archived data, and Alarms & Events (A&E) for alarm and event notifications.
- Industrial communication
- Standards
In one sentence
OPC DA (current values), OPC HDA (history) and OPC A&E (alarms and events) are the three main OPC Classic specifications.
Example
A SCADA system reads live tags via OPC DA, fetches last week's trend from the historian via OPC HDA and receives alarms via OPC A&E.
How it applies
- Engineering: Each specification needs its own server and client implementation. A product that supports OPC DA doesn't automatically support HDA or A&E.
- Data quality: OPC DA delivers each value with a quality code and a timestamp. Applications should evaluate the Quality status instead of using values blindly, especially after communication loss.
- Migration: In OPC UA the three functions map to data access, historical access and alarms and conditions within one server. Mapping is not always one-to-one, so migration needs testing.
- Documentation: The documentation team lists each supported specification with its version and states which items, history ranges and event types a server provides.
OPC DA vs. OPC HDA
OPC DA answers "what is the value now?"; OPC HDA answers "what were the values between two points in time?" Mixing them up in requirements leads to missing trend data.