XML DA

In October 1999 the OPC Foundation announced that it will publish XML schema based on OPC specifications that define application interoperability and communications between disparate industrial automation devices, systems and applications running across the manufacturing enterprise.
In July 2003 the first version of the XML DA specification was released.
XML DA specifies a rather simple web service with eight methods that handle all of the features typically used in the OPC Data Access applications.

The XML DA specification didn't get the attention it deserves, mainly because the OPC Foundation announced the United Architecture specification shortly after the release of XML DA and allocated its resources on this new specification. Another reason is performance concerns. Due to the XML based communication XML DA is much slower than the COM based OPC DA. This caused users to decide for proprietary tunnel solutions instead of XML DA. However, it needs to be noted that XML DA is actually fast enough for many OPC data access applications.

XML DA Characteristics

• Simple to implement
• Limited functionality, Data Access only
• Covers the requirements of most OPC Data Access applications
• Can be used with web service extensions for secure communication
• Much slower than COM based OPC
• Products available

Windows Vista with .NET3 (WCF) eliminates many of the web services short comings. The communication can now be configured for either high performance or security.

XML DA is a good starting point to bring remote accessibilty to OPC applications.

OPC UA

In January 2004 the OPC Foundation tasked a work group to create the Unified Architecture (OPC UA) specification.

The OPC UA specification was to address the shortcomings of XML DA:

• Integration of all OPC specification allowing alarms and historical data to be accessed in much the same way as current values.
• A communication suitable on device and enterprise level.

These requirements made the OPC UA specification a daunting task and not all of the eleven parts of the OPC UA specification were released at end of 2006.
OPC UA is structured to have the required features for all OPC specifications (Data Access, Alarms&Events, Historical Data, Batch, Commands, Security) in it's base. Therefore the OPC UA base is complex and designed with a network database. A network database by itself is complicated. Due to the difficulty implementing and maintaining the network database model, the simpler relational databases are widely used instead.

To achieve the goal of OPC UA to be suitable on device and enterprise level OPC UA had to be defined for multiple communication implementations. OPC UA is therefore an 'inner' specification that can be accessed through different communications stacks (communication layer implementations) such as web services or a binary communication for high performance device level communication.

OPC UA server and clients not only need to be OPC UA compliant but also need to use the identical communication stack.

It remains to be seen when stable OPC UA implementations will be available and if the complexity of the OPC UA base can be encapsulated to make OPC UA suitable for simple data access applications.

The XDAGW-CS gateway allows OPC DA clients to access XML-DA servers. Only minimal configuration is required. XDAGW-CS is OPC DA V2.05 and V3 compliance tested.

The XML-DA Bridge consists of a pair of gateways, the XDAGW-SS gateway and the XDAGW-CS gateway.
Existing OPC DA clients can access OPC DA servers over the Internet.