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XML-DA Client to OPC-DA Server Gateway
The server-side XML-DA gateway (XDAGW-SS) allows XML-DA clients to access OPC-DA servers.
The XML-DA Gateway offers a way to access remote OPC DA servers and to upgrade applications step-by-step to web services technology.
On Windows Vista the gateway runs as a .NET3 (WCF) service. It can be accessed from any XML DA client and .NET3 based clients can configure the communication to use either a high performance TCP or Named Pipe transport or a secure protocol that authenticates and encrypts the web service access.
XDAGW-SS is compliance tested for XML-DA V1.0.1 and OPC-DA V2.05 and 3.0. If connected to an OPC-DA V3 server the OPC DA V3 features such as WriteVQT() are used, otherwise the server is accessed OPC-DA V2.05 compliant.
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The XDAGW-SS Server-side Gateway is an XML-DA Server with an integrated OPC DA client.
The requests from XML-DA clients are forwarded to the OPC DA server and the results are passed back to the clients. The XML-DA server handles the data buffering according the OPC XML-DA specifications.
The OPC DA server is still being accessible by local OPC DA clients.
Test Tools
Security permissions need to set correctly for the gateway to work. Problems can be located with the provided test tools:
- Configuration and Diagnostic Utility
- XML-DA Test Client
- XML-DA Test Client for secure access
- OPC-DA Test Client
- OPC-DA Simulation Test Server ( OPC DA V2.05 and V3.0 compliant)
The XDAGW-SS gateway can be combined with the XDAGW-CS Client-side gateway to form an XML-DA Bridge that allows OPC-DA clients access to remote OPC-DA servers.
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Gateway Configuration
The XDAGW-SS gateway is simple to use. In many cases only the ProgID of the OPC server has to be defined.
Optional configurations settings can restrict the OPC server access to read-only or to a subset of items. It is also possible to make Impersonation settings and define credentials for the OPC server access.
The gateway is provide in version for ASP.NET1, ASP.NET2 (optional with WSE3) and .NET3 (WCF). The .NET3 version can be configured for compatibility with any XML DA web service or different bindings for access from .NET3 based XML DA clients. The XDADW-CS client side gateway is for example such a client. .NET3 supports many bindings such as TCP or Named Pipes for high performance or X.509 certificate secured communication for authenticated and/or encrypted web service access.
Configuration and Diagnostic Utility
The XDAGW-SS configuration settings are defined in the AppSettings section of the web.config file.
The configuration utility displays the settings and changes can be made interactively instead of modifying the web.config file with a text editor.
The diagnostic features test the access to the OPC DA server and the XML DA gateway web service.
The .NET3 configuration is also in the web.config file and is modified with the WCF configuration utility.
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Up to 9 OPC servers can be pre-configured to allow a quick and simple configuration changes by just selecting one of the pre-configured servers. There can be multiple definitions for the same OPC server, one e.g. with full access and another with restricted access. |
When to use the OPC XML-DA Gateways?
The XML-DA Gateways enables XML-DA clients to access devices with OPC-DA servers, respectively OPC-DA clients to access local or remote XML-DA servers. The gateways offer a simple way to upgrade step-by-step to the new XML-DA standard. The gateway can be bypassed with minimal changes when XML-DA servers/clients get available.
The gateway is a permanent solution when devices are never be upgraded because:
- the vendor offers XML-DA only in a newer model
- the device lacks the resources for OPC XML-DA
- local OPC DA clients need access to the device
Why use OPC XML-DA?
OPC XML-DA provides the foundation for multi-platform interoperability because it is based on XML instead of the Windows specific DCOM. It simplifies sharing and exchange of data amongst the various levels of the plant hierarchy (low level devices and up to enterprise systems), and to a wider range of platforms.
Unlike previous OPC data access specifications, the OPC XML-DA specification describes data exchange in an environment which assumes no persistent connection between the client and the server. This makes it suitable for a much wider range of communication systems and, since XML is text based it passes firewalls.
The XML-DA Bridge combines the Server Side gateway with the XDAGW-CS Client Side Gateways to allow OPC-DA clients access to remote OPC-DA servers. Running on .NET 3 (WCF) the communication can be configured for:
- ASP.NET2 web service compatibility
- high performance TCP or named Pipe transport
- secure data transfer using X.509 certificates or other mechanisms
The XML-DA Bridge connects remote OP DA servers/clients using established standards. DCOM issues can be minimized without proprietary tunneling solutions.
Read the gateway overview for more information.
Security
Due to the Web Services architecture access to OPC XML-DA servers is not as restricted to a local area network as it is for COM based OPC DA servers. Security issues are therefore more important. OPC XML-DA may also be used for data exchange with higher level systems. Such systems typically are only concerned with a small data subset.
The gateway implements effective security measures. It can be configured to restrict the server access to an item subset. E.g. make some items read-only or allow only a subset of the items to be accessed.
The XDAGW-SS gateway also supports security on SOAP level for the authentication and encryption of the messages. This security handling is based either on .NET3 (WCF) or the Microsoft Web Enhancements WSE 3.0. Both implement the industry standards WS-Security 1.0 and 1.1, WS-Trust, WS-Secure Conversation, WS-Addressing and Message Transmission Optimization Mechanism (MTOM).
The user can define the security policies to optimize the handling according the application requirements.
| System Requirements |
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Windows Vista, Windows XP, Windows 2000 or Windows 2003 Server |
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IIS Web Server
either with ASP.NET1
or ASP.NET2 and optionally WSE 3.0 for secured access.
or .NET3 (WCF) on Windows Vista |
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