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OPC Unified Architecture

 

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 OPC UA Overview
The OPC Foundation announced the UA architecture vision in 2003 and formed the UA working group in January 2004. The first parts of the specification were released in June 2006.
The current UA specification consists of 11 parts. In Mai 2015 it was made public so that not only OPC Foundation members can download the specification from the OPC Foundation web site.

OPC UA is much more complex than previous OPC specifications and is designed to:

  • Use cross-platform capable communication instead of Windows DCOM
  • Combine the OPC DA, A&E, HDA functionality into a single set of services
  • Model complex data structures for collaboration with other standards organizations
  • Be implementable on different platforms, from embedded systems to enterprise systems

The OPC Foundation web site lists books that describe the OPC specifications.

 

So far most UA products are for embedded systems. This is understandable because there are no established standards for these systems while for Windows/.NET environments the DCOM based classic OPC solutions are widely used.

OPC Specifications Overview

 

Classic OPC
The initial OPC specifications, based on DCOM communication, are now called Classic OPC. The OPC DA (Data Access) specification was released in 1996. It is for the communication of current data and still is the most widely used OPC specification. Most applications use the specification version 2.05 despite the newer version 3.0.
Similar specifications were released for Historical Data (OPC HDA) and Alarm & Events (OPC AE).

XML DA
After the introduction of XML Web Services and SOAP the OPC Foundation released the XML DA specification in 2003. The release was overshadowed by the announcement of OPC UA.
The XML DA specification is based on SOAP and specifies communication records for a functionality similar to OPC DA.
The XML DA specification didnít find wide acceptance but is established in areas such as e.g. Wind Farms.

OPC UA
The OPC UA architecture concept was announced in 2003 and in 2006 draft specifications were released. In 2009 the initial software kit became available.
In 2015 the UA specification was made public and in 2016 the UA communication stack source code became Open Source.

OPC .NET (Xi)
OPC .NET (Xi) specifies a .NET WCF based interface with the functionality of Classic OPC DA/HDA/AE. The specification and sample code were released in 2010.
OPC .NET (Xi) has not been widely accepted but itís the only OPC .NET API specification that can be used for Classic OPC DA/HDA/AE and OPC UA.

OPC UA Specification
The UA specification is structured to be implementation independent and adaptable to new technologies. While this has important advantages, it makes reading harder. Several specification documents have to be consulted for complete information.

  • The Services part defines the UA core services in an implementation independent way.
  • The Mappings part describes how the UA core services are mapped to the communications interface.
    Currently two mappings are defined, Native (TCP/IP based) and Web Services ( .NET3/4 based )
  • The Access Types parts detail the data structures passed to/from the services for
    Data Access, Historical Access, Alarms and Conditions, Programs.
  • The Address Space Mode and Information Model part describe the UA core.
  • The Profiles part defines possible subsets of the UA functionality

The current status of the UA specification is V1.3 for all parts.

The UA specification documents can be downloaded from the OPC Foundation web site. Since Mai 2015 the documents are available to the public. OPC Foundation members and non-members are prohibited from copying and redistributing this specification. All copies must be obtained on an individual basis, directly from the OPC Foundation.

The Accelerated Adoption work group was created in May 2009 with the task to create additional profiles, samples and documentation to make the development of UA server and client applications simpler and quicker. It appears that the work was suspended without concrete results.

 

OPC Foundation UA Developer Kits
The OPC Foundation OPC UA Software Development Kit is available for OPC Foundation members.
The UA SDK V1.0 has been released 2009-07-31.
This release of the SDK supports the released versions of Parts 1 through 9 of the UA Specification (basically DA functionality only).

The SDK V 1.1.320 Beta was released Sep-2-2010
The Compliance Test Tool (CTT) V1.0 Release Candidate was released Sep-16-2010
The SDK V 1.1.324 Beta was released Nov-2-2010
The SDK V 1.1.331 Beta was released Aug-24-2011
The SDK V 1.2.334.3 was released Sep-2013

The SDK V 1.2.334.6 was released Nov-2013

The SDK V 1.2.335.0 was released Dec-2014

The SDK V 1.2.336.0 was released Jun-2015

 

The OPC Foundation now only maintains the UA communication stack software. Other parts of the SDK, such as the COM interop converters, are provided as sample code that needs to be tested and maintained by the member companies. The complex SDK software is provided with little documentation, making the use and adaptation difficult.

In April 2016 the OPC Foundation made the UA SDK source code available as an Open Source repository at http://github.com/opcfoundation, with very restrictive license conditions.

 

OPC UA Basics
The UA core implements a network database with nodes that describe items and item attributes. The nodes can be linked in a complex manner, allowing all kinds if information structures to be modeled. Network databases are difficult to implement and therefore the more restricted relational databases are far more widely used.
The OPC Foundation implemented a reference UA server and a SDK. Probably all UA server products will base on this implementation. On this base the UA server development will probably not be much more complex than the development of a DA, AE or HDA server based on a server toolkit. The server developer can model the server according his preferences and can ignore other UA features.
The UA client development/configuration is significantly more demanding than it is for OPC DA clients. This is mainly because UA server can be modeled in different ways. The server developer can choose his favored structure and build the server accordingly. For client applications to be universal, they must be able to handle all UA features that different servers may use.

 

OPC UA Licensing
In 2015 the UA specification was made public and can be downloaded from the OPC Foundation web site by members and non-members. Still OPC Foundation members are not allowed to distribute with their UA products. Each user has to individually download the specification from the OPC Foundation web site.
In April 2016 the OPC Foundation made the UA SDK source code available as an Open Source repository at http://github.com/opcfoundation.

The UA stack software is provided under a dual license policy. For OPC Foundation members the OPC Foundation RCL License and for non-members the GNU GENERAL PUBLIC LICENSE V2, which requires all applications containing the UA stack software to be distributed under the same open source license.
UA SDK sample code is provided under the OPC Foundation MIT license.

 

OPC UA Products
So far most UA products are for embedded systems. This is understandable because there are no established standards for these systems while for Windows/.NET environments the DCOM based classic OPC solutions are being widely used for many years.
Advosol currently offers an OPC UA Server Toolkitthat bases on the OPC Foundation UA Communication Stack and the Advosol OPC DA .NET Server Toolkit. The same application plug-in .NET assemblies can be used with the Advosol OPC DA/AE, OPC HDA, UA and XML DA server toolkits. Users of Advosol toolkits for classic OPC can simply upgrade to UA.
For client developers Advosol offers UA option OPCDA.NET-UA, OPCHDA.NET-UA, OPCAE.NET-UA. Applications developed for classic OPC DA can access UA servers through the same API.
The PaXi client component implements the OPC .NET (Xi) interface and supports access to Classic OPC and UA servers.

 

XML DA Alternative
For applications that only need Data Access (OPC DA) functionality XML DA is a viable alternative. Based on .NET3 XML DA has the same communication security as UA and DCOM is not used, eliminating the frustrating DCOM issues. Advosol offers XML DA components that make the XML DA server/client development simple. Gateways to/from OPC DA are also available.
All Advosol XML DA products are provided in versions for .NET2 and .NET3 (WCF). The secure and efficient WCF communication configurations can be used when client and server are .NET3 based.

 

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