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OPC XML Webservices

Sub Category Summary
XML DA Gateways

The OPC XML-DA Gateways offer a way to upgrade OPC solutions step-by-step to XML-DA web services. 

XML DA Developer Kits

Client development components and server toolkits help quickly develop .Net based XML-DA client and server applications.




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Category Description

OPC Web Services

Web Services are currently the only high-level application-to-application communication technology embraced by virtually all platform providers including Microsoft, IBM, Sun, and Linux.
XML (eXtensible Markup Language) is one of the key enabling technologies for achieving global interconnectivity and it has industry-wide support as a standardized way of handling data among systems using a variety of operating systems. On a higher layer the SOAP specification defines remote procedure call messages in a platform independent fashion, using XML. Web services such as XML DA or UA base on SOAP and are defined in WSDL documents.

 

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

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

The XML DA specification defines SOAP messages for requests and responses. With .NET2 the communication is restricted the HTTP transport. With .NET3 (WCF) the communication can be configured in a wide range to include TCP and Pipe transports and many security options. The WCF basicHttp binding is compatible with .NET2.

 

 

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 a UA compliant communication stack.

 

The wide flexibility of UA allows servers to be structured in different ways. The UA client development tends to be complex because clients need to be able handle all UA allowed features.
So far UA is mainly used as a PLC interface. There are not yet many UA .NET applications.

 

 

OPC .NET 3 (Express Interface)

The Express Interface (Xi) specification was created in 2009 by a number of OPC vendor companies and end of 2009 the OPC Foundation adopted Xi as the OPC .NET 3.0 (WCF) specification.
In March 2010 the specification version 1.1 was released and the first products became available.
In October 2010 the specification version 1.20 was released.

 

Xi is a Microsoft .NET based interface designed for secure and reliable access to real-time and historical process automation system data. Xi provides a standard, .Net based interface for “classic” OPC server functionality, OPC Data Access (DA), OPC Alarms & Events (A&E) and OPC Historical Data Access (HDA) and represents a natural progression of Microsoft communication technology from Microsoft Component Object Model (COM) to .Net.

Xi provides the same functionality as the classic OPC servers while addressing some of the known shortcomings of classic OPC. The classic OPC servers are based on COM communications, which was state of the art when the OPC specifications were created, but since the introduction of OPC DA in 1996, Microsoft has moved from COM to .Net communications. COM is efficient for local server access but can be difficult to configure for remote server access and problematic for communication through firewalls.

Xi is designed for WCF and can use all current and future WCF communication and security features. Xi adds additional security features designed to combine high security with high performance.


Thanks to the simplicity there are already a number of Xi products on the market.

XML DA and Express Interface Comparison

OPC XML DA Express Interface (Xi / OPC .NET 3.0)
Specification defines SOAP messages (WSDL). Can be implemented with .NET2 web services or WCF (.NET3) Specification defines WCF (.NET3.5) contracts.
OPC DA (Data Access) functionality only.
The specification is optimized for simplicity.
Combines DA, HDA and AE functionality into one interface.
The specification is designed for secure communication with high performance.
Subscriptions cannot be modified. This interface simplification can complicate the application development. Client access to data is managed in lists. These can be dynamically modified.
Specification is OPC copyright, available to OPC Foundation members only. Sample code is available to OPC Foundation members. The XML DA WSDL file is available to non-members. Specification and server base code is public domain. It can be downloaded from www.expressinterface.com.  The OPC Foundation provides client and server sample code to its members.
XML DA communication is state-less. There is no Connect/disconnect. Clients can e.g. use only the Read method. Clients need to establish a server context (with proper security) before data can be exchanged.
Clients can easily be created from WSDL with web services tools. Clients should be based on a Xi SDK for compliance reasons.
Secure communication is possible but not enforced. Secure communication is enforced and offers many options.
Only HTTP communication when implemented as .NET2 web service. WCF implementation can use all WCF communication options. All WCF communication options available on server and client machine can be used. For communication with non-Windows systems only basicHttp binding can be used.

 

 

Advosol XML DA Products
Advosol was first to deliver software components for XML-DA clients and servers and now offers matured software in version 5. Version 1 was based on the OPC XML-DA pre-release specificationa and allowed users and us to create advanced solutions and build up know-how. Version 2 was released shortly after the XML-DA standard V1.0 was available. It was tested at the Spring 2004 OPC Interoperabiliy workshop with all available XML-DA products. Version 3 offers enhanced performance, functionality and ease of use. Version 4 uses.NET Framework 2.0. Version 5 supports Windows Vista .NET3 (WCF) and allows the communication to be configured for either high performance through TCP or Named Pipe transports or for high security with X.509 certificate based authentication and/or encryption.
The Advosol software products are compliance tested with the OPC Foundation test tools.

Advosol offers software development tools for XML-DA clients, XML-DA Servers and Gateways that provide a sound base for new applications and an easy migration path for the thousands of OPC DA products in use today.

XML-DA server-side Gateway The XDAGW-SS gateway enables OPC-DA servers to be accessed from XML-DA clients. Using the gateway is the quickest way to make an existing OPC DA server accessible from a remote location.
XDAGW-SS can be configured to restrict access to a item subset or allow only read access.
XML-DA client-side Gateway

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.

OPC Exchange Server The Exchange Server aggregates data from multiple OPC DA servers and exchanges data between these servers.
Accessed as an XML DA server the Exchange Server acts as a gateway web service to multiple OPC DA servers.

XML-DA Client Component The XMLDA.NET client component allows XML-DA clients to access local OPC DA V2 server directly, without going through XML serialization, offering highly improved performance.
Clients can be developed as state of the art web services clients even if OPC DA COM server need to be accessed.

XML-DA Server Toolkit Gateways can be used to wrap OPC DA servers to XML-DA, but a native XML-DA server offers higher performance and better configuration options.
The XDASrv Server Toolkit makes the development of native XML-DA servers simple an quick. The generic XML-DA server is provided as a .Net web service. The user only has to develop a .Net assembly with a few item configuration and device access methods. Optional methods can be implemented to optimize performance and realize demanding applications.

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