OPC brings XML-DA to the workshop layer
Control Solutions International July, 2002author (s): Jim Luth i
CYS
Note. Recently I studied an article about OPC XML in MES, translated an article, take it out
Why use XML?
In recent years, XML has become a popular data coding and exchange format due to its open and systematic independence. OPC Foundation Selects XML as an export of existing OPC-COM interfaces as an accurate data transmission, mainly hopes to:
Using the current OPC-COM applications can be conveniently integrated in the workshop LAN.
However, if you want to use DCOM through the Internet, you will might encounter great troubles, DCOM is close to the security registration mechanism with Windows, and it adopts dynamic allocation TCP / IP port (generally unable to cross the company's firewall)
Corresponding, XML is a definition design based on the W3C based on the Internet protocol. It usually transmits the HTML web page via HTTP, but other ftp / smtp can also be used.
Problems performed in non-Microsoft environments
Microsoft Designed COM / DCOM is a modern-based remote procedure call mechanism that can make components, process, and communication mechanisms in Microsoft Environments. Although Microsoft has been open to other implementations, COM / DCOM never really runs outside of Microsoft, and it is also not available on the Microsoft platform.
As a standard XML of W3C is an unpredictable platform for all modern platforms. Even the latest SOAP and Web service standards are built on XML (OPC-XML-DA), which supports many platforms and computing environments.
Better connectivity
The real-time data of manufacturing applications like MRP / ERP is usually obtained by executing server users in the OPC-COM interface. The problem is that most of these high-level applications do not need to perform an OPC-COM interface to interact with an OPC-COM server. And many times these applications are running in non-Microsoft environments, making him impossible to communicate via COM.
Figure OPC-XML provides integration of workshop layers to manufacturing enterprises
Connectivity is the key to these major enterprise systems, and many of these applications are now using XML as a method of its preferred data exchange. The OPC also includes XML, real-time OPC data will be very easy to pass through the indoor layer to the manufacturing equipment.
SOAP, Web Services & Microsoft .NET
The OPC-XM Association has already decided to limit the definition of OPC-XML to convert existing OPC-DA definitions. Therefore, this will be a definition including only XML-DA, while other OPC-COM interfaces such as alarms, events, and historical data are also converted accordingly. By defining a well-known DA interface, it should not cost too much to define the definition of XML Schemas's XML Schemas. However, why still take two years to define? Two reasons: Tools and versatility limits.
Figure 2 A simple XML-DA package can connect multiple existing OPC-COM servers
Defining XML Schemas is easy, but OPC is primarily inter-use, and the definition XML Schemas does not guarantee the versatility of many vendors.
The Committee has to define communication protocols, discovery mechanisms, error handling, and all other as the OPC DCOM interface is defined.
Two years ago, the Committee also found that XML is more cumbersome than the high-level COM programming that has been accepted.
Fortunately, it is not just an inconvenience that the OPC Committee is facing XML. The main software investors Microsoft has always worked hard to build its new standards and new tools on XML.
First SOAP (Simple Object Protocol) IS is built on XML, then WSDL (Web Service Description Language) is built on the SOAP layer. Place together, these definitions have established real interoperability.
Figure 3: OPC provides example source code for member customers and servers
With the definition of web services, the OPC-XML committee chooses to use Microsoft .NET to complete its reference definition (example code). As the author of the XML-DA customer example, I can confirm Use high-level programming tools .NET to create and use web services like XMLDA. Help use as soon as possible
With new technical definitions, you can better use new tools and platforms, which look like XMLDA is suitable for future, but not now. However, there will be such factors to accelerate using this interface:
1 Software Suppliers in Manufacturing Soon Soon use new programming tools such as .NET, not because of OPC, but due to higher productivity of programmers. Once the tools that support Web services are used, the industrial standards are supported. Web services, such as XML-Da, of course, will also be adopted.
2 If all existing OPC vendors must upgrade existing OPCCCM servers to XML-DA, it may take a long period of time. XML DA standard allows packaging of existing OPC COM servers. One XML DA wrapper can It is a general, independent approach from any vendor's OPC COM server. So a supplier, as long as you provide such a guarantor, you can quickly enable the OPCCM server XML in the world. I hope all OPC vendors This technology can be used immediately.
3 OPC is a truly provider code to help OPC members develop. The first example customer and server example, will be released while defining, which is written on the .NET platform.
In the future OPC-XML definition
OPC will first release web-based XMLDA. Other function blocks corresponding to OPC COM (ALARM & Events, Historical Data Access, Batch ETC.) will also be published. With XML DA, new Definition will allow packaging to upgrade the existing COM-based server as soon as possible.
An important new mode
As a new programming mode, XML Web Services has been supported by many new tools, mainly from companies such as Microsoft, IBM, Sun, Microstems. The XML-DA standard from the OPC is a web service interface that is designed to cross the latency data through the soft component through the LAN or Internet. By using the wrapper, hundreds of existing OPC-COM servers will be available in a service manner through XML-DA to accelerate it as a standard acceptability.
Come from:
http://blog.9cbs.net/chaiysh/archive/2003/11/27/21794.aspx
