Introduction

The Open Linking and Embedding for Process Control (OPC) consortium released several open standards, which address interfaces for vertical integration in industrial automation. These standards can be used to build Internet/fieldbus interfaces which are placed on gateway devices as shown in figure 1

Figure 1: Internet/fieldbus interface on a gateway

Historically, OPC used the Distributed Component Object Model (DCOM) for the underlying communication technology. DCOM has the disadvantage of being platform specific: it is only available for Microsoft Windows based systems. Other platforms, such as Linux, can therefore not retrieve fieldbus data from DCOM based servers. Another disadvantage of DCOM is that it can not easily bypass firewalls, hence access will often be limited to certain segments of a corporate network.

In the last years, a new technology, called SOAP Web services, emerged. [LIV02] defines a Web Service as: ``a method or function that is available for other applications to access over the Internet.''. Web services enable Remote Procedure Calls (RPC) and have the following key features:

High level of interoperability:

Web services technologies are all based on strictly defined open standards¹.

High networking abilities:

As an underlying communication protocol, Web services utilize Internet protocols such as the Hyper Text Transfer Protocol (HTTP) or the Simple Mail Transfer Protocol (SMTP). These protocols have high networking abilities and may moreover penetrate firewalls.

Protocol legible by humans:

The Simple Object Access Protocol (SOAP)² is based on the Extended Markup Language (XML), which is legible to humans. This way, testing and debugging of Web services is far easier than with binary protocols.

Documentation:

Another underlying technology of SOAP is the ``Web Service Definition Language'' (WSDL) which may be used to define the service, especially by constraining the format of the SOAP protocol. WSDL utilizes the XML Schema language for defining these SOAP messages³. These WSDL documents can be utilized by frameworks to generate stubs that provide a base for accessing a Web service.

Validation:

WSDL in combination with a validating XML parser enable the validation of SOAP messages. This way, custom code will never receive syntactically or semantically erroneous data, which should improve the stability of the service.

SOAP Web services are seen as a successor to several alternative technologies such as DCOM and are already broadly accepted by the industry. More information about the SOAP protocol can be found in [SEE02] and [LIV02].

The OPC consortium reacted on this technological evolution by adopting SOAP Web services for their standards. One recent addition of OPC is the "XML Data Access Version 1.0" (XML-DA 1.0) standard. This standard deals with access of underlying fieldbus technologies and covers the following aspects:

Information model:

The specification provides a simple information model, based on ``OPC Items'' which represent a piece of information, similar to fieldbus data points. These items can be arranged hierarchically.

Data types:

OPC XML-DA adopts several XML-Schema based data types, such as integer, float, date/time specific types. Moreover it defines arrays which are based on these basic types.

Operations:

The standard specifies 8 operations such as reading/writing and browsing which can be used to access the underlying fieldbus.

Subscription:

The specification further introduces a mechanism to retrieve only changed items, called ``Subscription''. Clients may thus subscribe to items and use a dedicated polling operation to retrieve changed data.

The standard does not address security, instead it relies on underlying Web service technologies⁴. More information about OPC XML-DA can be found in [HIM06] and [OPCXMLDA].

Although OPC XML-DA is based on open and standardized technologies, it can nevertheless be tedious to build services based on this standard. Therefore several OPC frameworks are available that introduce simple building of client and server applications. However, most of these frameworks are not freely available, moreover most of them are based on Microsoft's .Net framework and are therefore platform dependent.

Due to these limitations, the PyOPC framework was developed, which fully implements the OPC XML-DA standard, enabling developers to build OPC XML-DA based applications in an easy way.