What is OPC and Its Role in PLC-Based Automation Systems?
Published on Jun25, 2025 | Category: IntrodunctionShare this Page:
What is OPC? OPC, originally known as OLE for Process Control, is a communication standard designed to enable seamless and secure data exchange between industrial automation systems such as PLCs, SCADA, HMI, and various software applications. Introduced in 1996, OPC was first developed to provide a standardized way for Windows-based systems to communicate with industrial devices using different proprietary protocols like Modbus, Profibus, and others. The original version, now referred to as OPC Classic, relied on Microsoft technologies such as OLE, COM, and DCOM, and was limited to Windows environments. As industrial networks became more complex and platform diversity increased, the OPC Foundation released OPC Unified Architecture (OPC UA), a platform-independent, service-oriented framework. OPC UA integrates all the features of OPC Classic—including real-time data access, alarms and events, and historical data—into one extensible standard. Today, OPC UA plays a crucial role in Industrial IoT (IIoT) and Industry 4.0 by ensuring interoperability between devices and software from different vendors, allowing secure and reliable communication across the entire automation ecosystem.
What Does OPC Stand For?
OPC originally stood for OLE for Process Control, where OLE means Object Linking and Embedding—Microsoft’s technology for software integration. This initial form of OPC, now known as OPC Classic, was introduced in 1996 to enable Windows-based SCADA and HMI software to communicate with industrial controllers like PLCs, using a common interface. However, this version was dependent on Microsoft's COM/DCOM and could only run on Windows platforms.
As industry requirements evolved, the OPC Foundation expanded the specification into a modern, cross-platform, and secure standard called OPC UA (Unified Architecture). With this transition, the meaning of OPC shifted to Open Platform Communications, better representing its broader role in secure, reliable, and platform-independent data exchange across industrial systems.
OPC History
The history of OPC (originally OLE for Process Control) began in 1996 when it was developed by a group of automation industry vendors in collaboration with Microsoft. Its initial goal was to create a standardized interface for industrial devices—like PLCs—to communicate with software applications such as SCADA and HMI systems. The first OPC standards, now known as OPC Classic, were built on Microsoft’s COM/DCOM technology, which limited them to Windows environments.
OPC Classic was divided into different specifications based on data type: OPC DA (real-time data), OPC AE (alarms and events), and OPC HDA (historical data access). These standards enabled integration across devices and systems from different vendors, greatly enhancing industrial interoperability.
As automation technology evolved, the need for a more secure, scalable, and platform-independent solution became clear. In response, the OPC Unified Architecture (OPC UA) was introduced in the mid-2000s. OPC UA combined all the capabilities of OPC Classic into a single framework and added support for cross-platform communication, encryption, complex data modeling, and cloud/IoT integration. Today, OPC UA is the foundation for modern industrial communication in Industry 4.0 and IIoT environments.
OPC History Timeline
1996 – Introduction of OPC Classic:
OPC (OLE for Process Control) was introduced by a collaboration of automation vendors and Microsoft. It allowed Windows-based systems like SCADA and HMI to communicate with PLCs using standardized interfaces based on COM/DCOM technology.
Late 1990s – Growth of OPC DA, AE, and HDA:
OPC Classic evolved into three key specifications: OPC DA (Data Access) for real-time data, OPC AE (Alarms & Events), and OPC HDA (Historical Data Access). These enabled broad integration across industrial systems.
2006 – OPC Unified Architecture (UA) Announced:
The OPC Foundation launched OPC UA to address the limitations of Classic OPC. OPC UA was designed to be platform-independent, secure, and capable of handling complex data structures and modern use cases.
2010s – Industry Adoption of OPC UA:
OPC UA gained traction across multiple industries, including energy, manufacturing, oil & gas, and building automation. It became the preferred choice for modern IIoT and Industry 4.0 architectures.
Present – OPC UA as the Modern Standard:
OPC UA is now the leading industrial communication protocol, enabling interoperability between devices, cloud platforms, and enterprise systems. It supports edge computing, web services, encryption, and scalability for modern smart factories.
Key Differences Between OLE for Process Control and Open Platform Communications
| Feature | OPC Classic (OLE for Process Control) | OPC UA (Open Platform Communications) |
|---|---|---|
| Platform Dependency | Windows-only (uses COM/DCOM) | Cross-platform (Windows, Linux, embedded, cloud) |
| Architecture | Component-based | Service-oriented |
| Security | Basic (limited to Windows security model) | Advanced (encryption, authentication, certificates) |
| Communication Model | Client-Server | Client-Server & Publisher-Subscriber (Pub/Sub) |
| Data Types Supported | Limited (simple types only) | Rich data structures (custom objects, complex types) |
| Use Case | Legacy SCADA/HMI systems | Modern IIoT, Industry 4.0, cloud, edge computing |
Why Are There Different OPC Names Like OPC UA, DA, AE, and HDA?
The OPC standard is divided into several specifications, each targeting a specific type of data exchange in industrial automation. These versions were developed over time to handle real-time data, historical data, alarms, and more. Here's a breakdown of the most common OPC types:
- OPC DA (Data Access) – The most widely used OPC specification, OPC DA is designed for real-time data communication between devices such as PLCs and HMI/SCADA systems. It enables reading and writing of live values like temperature, pressure, or motor status.
- OPC AE (Alarms & Events) – This specification handles alarm and event messaging. OPC AE is used to monitor process conditions, trigger alarms, log system events, and manage operator notifications in real time.
- OPC HDA (Historical Data Access) – OPC HDA allows applications to retrieve and analyze historical process data stored in databases or data historians. It is used for reporting, trend analysis, and long-term storage.
- OPC UA (Unified Architecture) – Introduced later, OPC UA is a modern, platform-independent, and secure protocol that unifies the functionality of OPC DA, AE, and HDA into one extensible framework. It supports real-time data, historical access, alarms/events, complex data structures, and more—suitable for IIoT and Industry 4.0.
These different OPC specifications were developed to meet specific industry needs. While OPC DA, AE, and HDA are part of the older OPC Classic standard and mostly limited to Windows environments, OPC UA overcomes those limitations and is now the recommended standard for modern automation systems.
What is OPC DA (Data Access)?
OPC DA stands for OPC Data Access. It is one of the earliest and most widely used OPC specifications, developed to enable real-time data communication between devices like PLCs, sensors, and industrial software such as SCADA and HMI systems. OPC DA is part of the OPC Classic standard and is based on Microsoft COM/DCOM technology, which means it works primarily on Windows operating systems.
OPC DA allows reading and writing of live process data—for example, the current temperature from a sensor, the pressure in a pipeline, or the status of a motor. It is ideal for applications where real-time monitoring and control are needed.
Example: A SCADA system uses OPC DA to read live temperature values from a PLC and display them to the operator. If the operator adjusts a setpoint, the SCADA system writes the new value back to the PLC using OPC DA.
What is OPC AE (Alarms and Events)?
OPC AE stands for OPC Alarms and Events. It is a specification designed to handle non-continuous data like alarms, system events, and operator actions. While OPC DA focuses on continuous real-time data, OPC AE deals with discrete messages that occur based on specific conditions or system states.
OPC AE is useful in environments where monitoring alarms, tracking system events (like startup/shutdown), and logging operator activities are essential for safety and control. It also allows for setting priority levels, severity, timestamps, and acknowledgment of alarms.
Example: If a motor overheats, the PLC sends an over-temperature alarm to the OPC AE server, which then logs the event and notifies the SCADA system to alert the operator with a popup message and sound.
What is OPC HDA (Historical Data Access)?
OPC HDA stands for OPC Historical Data Access. This specification allows access to historical (archived) data collected from process control systems. OPC HDA is used for analysis, reporting, auditing, and trending purposes, rather than real-time control.
It is particularly useful in industries where long-term data is required for performance analysis, troubleshooting, quality assurance, or compliance reporting. OPC HDA connects SCADA or analytics software to data historians or databases storing past process values.
Example: An engineer wants to analyze the past week's pressure readings in a pipeline to identify patterns or faults. The software queries the OPC HDA server for historical data stored in the database and displays it in a trend chart.
What is OPC UA (Unified Architecture)?
OPC UA stands for OPC Unified Architecture. It is the modern evolution of the OPC standard and was designed to overcome the limitations of OPC Classic (DA, AE, HDA). Unlike previous versions, OPC UA is platform-independent, secure, and extensible. It works not just on Windows but also on Linux, embedded systems, cloud, and IoT devices.
OPC UA unifies all the core functions of OPC DA, AE, and HDA into a single protocol and supports complex data types, encryption, authentication, and firewall-friendly communication. It is ideal for modern Industry 4.0 and IIoT environments where security, scalability, and cross-platform support are essential.
Example: A cloud-based monitoring system connects to multiple PLCs from different vendors using OPC UA. It securely reads live data, accesses historical logs, and receives alarms—all through a single unified interface.
