OPC DA Overview

The “OPC DA” data connection allows WIN-911 to connect to a wide variety of HMI/SCADA systems by using a generic data exchange medium, OPC DA server. WIN-911 serves as a generic OPC DA client. WIN-911 only supports Data Access (DA) servers not Alarm and Event (A&E) servers.  WIN-911 is capable of browsing tags in an OPC DA server and importing them into the WIN-911 configuration. The browse function can be limited by the OPC DA server.  


It is important to note that some of the terms (ItemID, Machine Name, and Server Class) are components of Microsoft's OPC address nomenclature.  Although these definition fields are part of the WIN-911 configuration, they are governed by constraints set forth by Microsoft, the OPC Foundation, and the particular server WIN-911 is subscribing to.


Typically, the OPC server must be running before WIN-911 can establish a connection.  Unlike previous versions of WIN-911, if at any time it loses connection to the OPC server, WIN-911 will attempt to re-establish its connection and continue to do so until the server is back online and the data is restored.  


The OPC DA data source passes raw values to WIN-911.  This means WIN-911 must be configured to alarm on appropriate set-points, therefore owning the alarms.  The OPC DA connection supports Items or tags without alarm set points configured.  Items can be used in reports to monitor values.  


The OPC DA connection also supports watchdog timers.  WIN-911 can monitor an item's changing value by setting the timeout value greater than the time it takes the tag's value to change.


What is OPC DA?

In 1994 a group of vendors representing a broad spectrum of disciplines in the industrial segment formed what is now known as the OPC Foundation. The OPC Foundation put forth the goal of developing a single client/server specification that would allow any vendor to develop software and applications that could share data in a fast, robust fashion, and do it in a way that would eliminate the proprietary schemes that forced these same vendors to duplicate development efforts. The OPC Foundation developed the specification called Data Access Specification 1.0a which was released in early 1996. Using this specification, vendors were able to quickly develop client server software.


OPC (OLE for Process Control) builds on the strengths of OLE (Object Linking and Embedding) to provide a complete data sharing medium that meets the rugged demands of industry. It allows a client to browse all OPC servers for data both on the local node as well as networks and provides a filtering mechanism to help narrow the scope. Data can then be imported and linked to the source for data sharing and manipulation. The communication process is faster than DDE and provides for a much greater amount of data sharing while maintaining integrity and security.

Based on Microsoft's OLE (now ActiveX), COM (component object model), and DCOM (distributed component object model) technologies, OPC consists of a set of standard interfaces, properties, and methods for use in process-control and manufacturing-automation applications. The ActiveX/COM technologies define how individual software components can interact and share data. Backed by Microsoft's NT technology, OPC provides a common interface for communicating with diverse process-control devices, regardless of the controlling software or devices in the process.


The OPC DA Conversation

There are three parts to an OPC DA Address: the Machine Name or IP Address, Server Class, and the ItemID.  The Machine Name specifies the network node that the server resides on, the Server Class specifies the server that contains the data, and the ItemID is the specific data point within the server.  


Preparing Your Computer for Remote OPC DA

Setup DCOM

Run the DCOM configuration utility by selecting Start and typing dcomcnfg in the Search Programs and Files text entry box.  Highlight Component Services, then Computers, then right-click My Computer and select Properties which will bring up the Component Services dialog.  Select the Default Properties tab and check Enable Distributed COM on this computer.  In addition, make sure that the Default Authorization Level shows Connect and the Default Impersonation Level is set to Identify.  Uncheck the additional security for reference tracking box (match the settings below). Set default security right by selecting the Default Security tab.  


Configure Access Permissions for the computer's DCOM via the Default COM Security tab. On the Access Permission window you can add individual users and groups to grant access to this particular computer on the DCOM level.  Make sure you select the correct domain or workgroup user list from the pull-down menu. Here you select your computer's domain or workgroup. For initial test purposes include Everyone in the Grant Access list.


Tech Support

You can contact WIN-911 Tech Support at [email protected] or call (800) 331-8740 between the hours of 8:00 AM and 5:00 PM Central, Monday through Friday.    

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