# OPC Data Access (OPC DA)

Protocols · `OPC DA` · verification: source checked

OPC Data Access (OPC DA) is one of the OPC Classic specifications, which are based on Microsoft Windows COM/DCOM technology for exchanging data between software components. OPC DA defines the exchange of process data including values, time information and quality information. It is defined separately from OPC Alarms & Events (OPC A&E), which covers alarm and event messages and variable states, and OPC Historical Data Access (OPC HDA), which covers query methods and analytics on historical, time-stamped data.

## Specification

| Property | Value |
|---|---|
| Transports | other |
| Architecture | client server |
| Encoding | binary |

### Data types

| Name | Description |
|---|---|
| value | Data value exchanged as defined by OPC DA. |
| time | Time information exchanged together with the value. |
| quality | Quality information exchanged together with the value. |

## Connected

- Related: [OPC UA](https://machineschema.com/protocol/opc-ua.md)

## Study notes

- OPC Classic specifications rely on Microsoft COM/DCOM (Distributed Component Object Model) on Windows for data exchange between software components.
- OPC Classic is split into three separate specification groups: Data Access (DA), Alarms & Events (A&E) and Historical Data Access (HDA).
- The document states that Microsoft de-emphasized COM and DCOM in favour of cross-platform service-oriented architecture (SOA).
- The OPC Foundation released OPC Unified Architecture (OPC UA) in 2008; it integrates all functionality of the existing OPC Classic specifications.
- Reasons given for OPC UA include a single set of services for exposing data models, support on non-Microsoft and embedded systems, transport of high-level structured data, and secure access through firewalls.
- The document gives no port numbers, packet formats, throughput figures or version numbers for OPC DA.

## Sources

1. [Classic - OPC Foundation](https://opcfoundation.org/about/opc-technologies/opc-classic/) (accessed 2026-10-25)

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