Connect to ClearSCADA Using OPC - OPC Protocol Details

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OPC is based on Microsoft DCOM, and the terms In Process Server, Local Server, and Remote Server are all options when a client connects to a server. These options are available in the OPC Server database object. More specifically they are the flags arguments passed to the CoCreateInstanceEx call under WIN32.

• In Process means that the server is a Dynamic Link Library located on the local machine. This is the only option that can be used to connect to an ClearSCADA server because ClearSCADA OPC Servers are provided as an In-Process DLL.

• Local Server means that the server is an executable located on the local machine. This is usually the option used with 3rd party OPC Servers, because most are provided as an EXE.

• Remote Server means that the server is an executable located on another machine. To successfully launch or connect to a remote server, user account details have to be entered to specify which user account should be used in accessing the remote machine.

You can only specify DCOM security for Remote Server activations, for In Process and Local Server activations security credentials passed to the CoCreateInstanceEx call are ignored.

To provide additional security, the OPC foundation defines a private security interface. If you select the option "Use Private Security", the OPC client driver will attempt to log on to the OPC Server using the private security interface. The ClearSCADA OPC server supports this interface and maps private security log on requests directly onto ClearSCADA user accounts. So for a system using ClearSCADA to monitor server statuses, ensure that either the everyone user has read access to the database, or private security is enabled on the OPC server definition that uses an ClearSCADA user who has read privilege.

OPC Servers should support a registration mechanism to put the appropriate entries in the registry. ClearSCADA registration is done through the ClearSCADA Client Connections control panel applet (also available in the Start Menu). In process servers are registered using RegSvr32. Executable servers are registered through the /RegServer command line option. Registration of remote servers is more difficult, in that the server has to be registered on the server and client machines.

In terms of OPC server browsing, ClearSCADA uses the category manager to browse for OPC servers. This technique was introduced by the OPC foundation some time during the OPC DA 2 specification. The previous technique was for an OPC server to include an extra OPC key under the CLSID to indicate that this is an OPC server. New servers are supposed to register themselves using both mechanisms, i.e. using the category manager and using the OPC key under the CLSID. Some very old servers only use the OPC key technique and will not be visible to ClearSCADA. If you run OLEView (an OPC Tool) in "Expert" mode you should see OPC categories in the "Grouped By Component Category" section.

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