Many OPC systems use Distributed Component Object Model (DCOM) for communications over a network. Unfortunately, OPC systems can be affected by several problems when DCOM is used, such as:
- Timeouts
- Security
- Reliability issues.
Although ClearSCADA uses OPC-based technology, it is unaffected by the problems commonly associated with DCOM. This is because the ClearSCADA server is an in-process COM server—it uses data access components to act as the transport mechanism between the server and clients.
Data access components (dbclient.dll) are installed on the clients and use standard Windows TCP/IP sockets. This avoids the problems caused by DCOM as it means that OPC messages are transported between the ClearSCADA server and clients via the TCP/IP socket that is controlled by ClearSCADA.
By installing Data Access Components on client PCs, you set the client PCs so that:
- There is only one way of accessing the ClearSCADA server (via the Data Access Components).
- Access to the server is logged in the Events List and can be tracked
- ClearSCADA has control over the TCP/IP sockets, so it is easy to adjust timeouts and security settings to meet the requirements for your system.
To set up your system so that it is not affected by DCOM related issues, we strongly recommend that you:
- Install any third-party OPC server applications on the same PC as the ClearSCADA server. If the OPC server and ClearSCADA server are installed on the same computer, ClearSCADA can communicate with the OPC server locally and so will not be subject to any DCOM issues.
If you cannot install a third-party OPC application on the same PC as the ClearSCADA server, you will need to configure DCOM. If the OPC server in ClearSCADA is set to run in Synchronous mode, DCOM on the third-party OPC server needs to be configured to allow the incoming communications from the ClearSCADA server. If the OPC server in ClearSCADA is set to run in either of the Asynchronous modes, DCOM on the third-party OPC server needs to be configured to allow incoming communications from the ClearSCADA server and also outgoing communications to the ClearSCADA server.
Again, this can be difficult to implement and for further information on configuring DCOM you should refer to your Microsoft Windows documentation.
For more information on Synchronous and Asynchronous modes, please refer to the ClearSCADA Guide to the Advanced OPC Driver, or the ClearSCADA Guide to the Simple OPC Driver (as applicable).