What is OPC Classic?
OPC Classic (OLE for Process Control) is an industrial communication standard that uses Microsoft technology (COM/DCOM) to exchange process data between automation systems. It was developed in the late 1990s for integrating SCADA, PLC, HMI and other systems within Windows environments.
OPC Classic underpins many industrial installations, but is increasingly being replaced by OPC UA because of limitations in platform independence and security.
🧠 How does OPC Classic work?
- Client-server architecture
- An OPC Server fetches data from a PLC or other device
- An OPC Client (such as a SCADA system) reads or writes data via the server
- Based on Microsoft DCOM
- Runs only on Windows and uses COM/DCOM for communication
- Data exchange within or between Windows machines requires complex configuration
- OPC specifications
- OPC DA (Data Access) → real-time data
- OPC HDA (Historical Data Access) → historical data
- OPC A&E (Alarms & Events) → alarm messages and events
OPC Classic is fast and widely supported, but vulnerable when network settings are insecure.
🏭 Use of OPC Classic in OT networks
- Many legacy SCADA systems use OPC DA for real-time data communication
- HMIs retrieve data via OPC Servers on Windows
- Historian systems use OPC HDA for data logging
- MES and ERP systems integrate via OPC A&E or middleware
Typical locations:
- Levels 2–3 in the Purdue Model
- Within process networks (control domain)
🔍 OPC Classic vs. OPC UA
| Aspect | OPC Classic | OPC UA |
|---|---|---|
| Technology | COM/DCOM (Windows only) | Platform-independent binary/HTTP/SOAP |
| Security | Limited (depends on DCOM) | Built-in encryption, authentication, authorisation |
| Network flexibility | Difficult outside the LAN/DMZ | Works across LAN, WAN and cloud |
| Configuration | Complex (DCOM settings) | Flexible and scalable |
| Use today | Legacy systems | More modern OT/IT integration |
🔐 Security considerations
- DCOM is difficult to secure and requires many open ports
- Not suitable for Remote Access over the internet or DMZ
- Use only within segmented zones with Firewall and Access Control
- OPC Classic is not end-to-end encrypted
- Microsoft is ending DCOM support in future Windows versions → migration to OPC UA is recommended
OPC Classic requires additional measures for safe deployment within modern OT architectures.
📌 In summary
OPC Classic is a widely used legacy standard for data communication in OT networks, based on COM/DCOM. Although powerful within local Windows environments, it has clear limitations in security, scalability and future-proofing.