OPC Servers support a registration mechanism to put the appropriate entries into the ClearSCADA registry. Registration occurs automatically whenever an application that supports the OPC protocol is installed. This section of the guide provides information for administrator level users should registration not appear to have occurred as expected, or should ClearSCADA not be able to access an OPC server.
ClearSCADA can communicate with three types of OPC server—In Process, Local, and Remote (see Configuring the OPC Server Properties). In Process servers are registered using RegSvr32; Local (executable) servers are registered through the /RegServer command line option; Remote servers need registering on both the server and client machines. Registration on ClearSCADA systems is done through the ClearSCADA Control Panel applet.
The OPCEnum service is installed automatically when you install ClearSCADA as a server. The OPCEnum service offers an effective method of locating remote OPC servers, provided that the service is also available on the remote devices.
When searching for remote OPC servers, ClearSCADA will initially use the OPCEnum service. The ClearSCADA Simple OPC Driver will attempt to connect to the OPCEnum service on the remote machine. If the OPCEnum service is not available on the remote machine(s), ClearSCADA will revert to using the Category Manager to browse for OPC servers.
This feature was introduced by the OPC foundation during the OPC DA 2 specification. Previously, a server included an extra OPC key under its Class Identifier (CLSID) to indicate that it was an OPC server. New servers should register themselves using both mechanisms—that is, using the Category Manager and using the OPC key under the CLSID.
Some very old servers only use the OPC key technique and as such, are not visible to ClearSCADA (run OLE View in ‘Expert’ mode to see the OPC categories that are listed in the ‘Grouped By Component Category’ section). Old OPC servers that are not visible to ClearSCADA need upgrading if ClearSCADA is to communicate with them using the OPC protocol.