What is OPC XML-DA?

OPC XML-DA is a data access standard defined by the OPC Foundation that enables information from the OPC Data Access data model to be transferred between different applications using XML-based technologies.

While the classic OPC DA technology uses the Microsoft COM/DCOM infrastructure, OPC XML-DA uses XML, HTTP, and SOAP-based web services for data exchange.

This approach made it possible to transfer OPC data not only within the same Windows computer or a Windows network configured with DCOM, but also between different applications and systems through web services.

OPC XML-DA was specifically designed to facilitate data sharing between SCADA, HMI, manufacturing systems, enterprise applications, and industrial data collection applications.

Why Was OPC XML-DA Developed?

Transferring data from PLC, DCS, RTU, and other control systems in industrial facilities to higher-level software has been an important requirement for many years.

Although classic OPC DA largely addressed this need with a standardized data access model, its reliance on the COM/DCOM infrastructure could make integration between different operating systems and network environments difficult.

OPC XML-DA aimed to address this issue by using XML-based web technologies to enable OPC data to be shared across a broader system environment.

Web-Based Data Access

Enables industrial data to be transferred to different applications through HTTP and web services.

XML-Based Data

Data can be transported in structured XML messages and processed by different software platforms.

System-to-System Integration

Designed to facilitate data exchange between industrial systems and higher-level applications.

Client / Server Architecture

OPC XML-DA Client applications can access data provided by an OPC XML-DA Server.

OPC XML-DA Data Addressing for EOS Software (X#)

In the EOS SCADA OPC XML-DA client, the data address is created using the text specifying the Tag or data path provided by the OPC XML-DA Server.

X#OPC_XML_DA_TAG_NAME

For example:

X#IOP.dynamic.UI2

The OPC XML-DA data address may consist of multiple sections, with the sections separated by the . or / characters.

How Does OPC XML-DA Work?

OPC XML-DA communication basically consists of an OPC XML-DA Server and an OPC XML-DA Client that connects to this server.

The OPC XML-DA Server receives data from a PLC, DCS, RTU, SCADA, or another industrial system and exposes this data through the OPC data access model. The Client application accesses this data through web services.

XML messages are used for communication. SOAP is one of the primary mechanisms used to transport XML-based service calls, while HTTP provides the transport medium for this communication.

Simplified Communication Structure

PLC / DCS / RTU   →   OPC Server   →   OPC XML-DA Server   →   HTTP / SOAP / XML   →   OPC XML-DA Client

This allows process data from the field to be used by higher-level software through a standardized data access model.

What Data Can Be Accessed with OPC XML-DA?

OPC XML-DA is designed to share real-time process data using the OPC Data Access approach. A data point consists of more than just its numerical value.

Data Values

  • Analog values
  • Numeric values
  • Boolean values
  • Textual data
  • Other OPC data types

Data Information

  • Value
  • Quality information
  • Timestamp
  • Data point / Item information
  • Server status

Basic OPC XML-DA Operations

OPC XML-DA clients can use various service operations to retrieve data from the server and obtain information about data points.

Browse

Allows the client to discover the data area and data points exposed by the OPC server. The client application can browse the available Items.

Read

Used to read the current values, quality information, and timestamps of the specified data points.

Write

Allows values to be sent to data points for which write access is available. This enables control commands or setpoints to be written in supported systems.

Subscription

Provides a subscription mechanism that allows the client to monitor changes in selected data points and receive updates.

OPC XML-DA Features

XML-Based Communication

Enables industrial data to be transmitted within structured XML messages.

HTTP Support

Enables data access through HTTP-based communication infrastructures.

SOAP Web Services

Enables OPC data access operations between different applications through SOAP-based web services.

Real-Time Data

Supports the delivery of process data based on the OPC Data Access model to client applications.

Data Subscription

Enables data changes to be delivered to clients through a subscription mechanism on supported servers.

Data Browsing

Supports Browse operations for discovering and examining the data points exposed by the server.

Cross-Platform Integration

Facilitates integration with different applications through an XML and web services approach that does not depend on COM/DCOM.

OPC DA Data Model Compatibility

OPC XML-DA was designed to expose information based on the OPC Data Access 2.x and 3.0 data access model through XML-based services.

Difference Between OPC XML-DA and OPC DA

OPC DA and OPC XML-DA address the same fundamental need: accessing industrial process data through a standardized approach. However, they use different communication technologies.

Feature OPC DA OPC XML-DA
Core technology COM / DCOM XML / SOAP / HTTP
Data access OPC Data Access OPC Data Access model
Use of XML Not the primary communication mechanism One of the primary communication technologies
Web services Not part of the core architecture Part of the core architecture
Windows dependency High due to COM/DCOM Does not depend on COM/DCOM
Typical use Windows-based SCADA, HMI, and automation applications Web-based and cross-system data integration

OPC XML-DA and OPC UA

Although OPC XML-DA and OPC UA may appear to have similar purposes, they are not the same technology. OPC XML-DA was developed to provide the OPC DA data access model through XML and web services technologies.

OPC UA, on the other hand, was developed later by the OPC Foundation as a platform-independent and more comprehensive architecture. OPC UA was designed to cover not only real-time data access, but also information models, security, alarms and conditions, historical data access, and other services.

Therefore, OPC UA is generally considered when designing a new industrial system, while OPC XML-DA support may still be important for integrating existing OPC XML-DA servers found in legacy systems.

Important Note

OPC XML-DA is not a new-generation OPC technology commonly preferred for new systems today. However, OPC XML-DA servers can still be found in existing industrial facilities and legacy applications.

Therefore, for SCADA, data acquisition, and integration software, OPC XML-DA support can still provide value in terms of compatibility and data access for existing systems.

Where Is OPC XML-DA Used?

OPC XML-DA can be used particularly in applications where industrial data needs to be shared between different software systems.

SCADA Systems

Can be used by SCADA applications to retrieve real-time process data from OPC XML-DA servers.

Manufacturing Systems

Can be used to transfer information obtained from production equipment to different software systems.

Data Acquisition and Reporting

Can transfer real-time process data to data acquisition, analysis, and reporting applications.

Enterprise Integration

Can be used to transfer plant-level data to higher-level applications and enterprise systems.

Legacy System Integration

Can be used to connect existing OPC XML-DA servers to new data acquisition or software systems.

Web-Based Applications

In suitable architectures, can enable industrial data to be transferred to different applications through web services.

Advantages of OPC XML-DA

  • Provides XML-based data exchange.
  • Uses widely adopted web technologies such as HTTP and SOAP.
  • Does not directly depend on COM/DCOM infrastructure.
  • Can facilitate data integration between different applications.
  • Supports the sharing of real-time process data based on the OPC DA data access model.
  • Supports basic data access operations such as Browse, Read, Write, and Subscription.
  • Can be used particularly for integration with existing OPC XML-DA systems.

Limitations of OPC XML-DA

OPC XML-DA provided an important approach in its time for bringing OPC data into web-based environments. However, it has some limitations in terms of today's industrial communication requirements.

  • The structure of XML and SOAP messages can introduce greater overhead in high-speed data communication.
  • It does not provide a comprehensive information model covering all modern industrial communication requirements.
  • OPC UA provides a more comprehensive architecture for modern security and platform-independence requirements.
  • OPC UA may be a more suitable option in many newly developed systems.
  • Nevertheless, the need to integrate with existing OPC XML-DA systems may continue.

OPC XML-DA Client and Server

OPC XML-DA Server

A component that provides data obtained from industrial systems to client applications through OPC XML-DA services.

  • Provides data points
  • Supports Read operations
  • May support Write operations
  • Supports Browse operations
  • May provide Subscription services

OPC XML-DA Client

An application that accesses data provided by an OPC XML-DA Server. SCADA, data acquisition, reporting, or custom software applications can operate as OPC XML-DA Clients.

  • OPC XML-DA Server discovery
  • Browsing data points
  • Reading data
  • Writing data
  • Monitoring data changes

OPC XML-DA Tools

This section will provide free tools, test applications, sample software, and utilities that can be used when working with OPC XML-DA.

Tools developed for OPC XML-DA Client and Server testing, data reading, data writing, Browse operations, connection testing, and troubleshooting communication issues will be published here.

Coming Soon

OPC XML-DA tools and sample applications will be developed and published in this section.

Summary

OPC XML-DA is an OPC technology that enables real-time data from industrial automation systems to be transferred to different software systems through XML-based web services.

By combining the data access approach of OPC DA with XML, HTTP, and SOAP technologies, OPC XML-DA was developed particularly for sharing data between different applications and connecting existing industrial systems to web-based applications.

Although OPC UA is a more modern and comprehensive alternative for new projects today, the need to access OPC XML-DA servers in existing facilities means that this technology can still remain relevant for industrial software and data acquisition applications.