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 OPC Clients and Servers 
OLE for Process Control (OPC) is the leading technology for software integration on Microsoft platforms. It is based on a proven COM/DCOM architecture, and can be used for integrating a number of different types of data:

• OPC-DA
  Real-time Data Access
• OPC-HDA
  Historical Data Access
• OPC-AE
  Alarms and Events

OPC has its strengths and its weaknesses, which are a result of the underlying COM/DCOM technology.

With OPC clients and servers on the same machine, OPC is an excellent solution. Thanks to COM it is very fast. It can also be efficient, due to the callback mechanism which triggers client updates on data change.

OPC relies on DCOM for communication between machines over a network, and that is where things can get tricky. DCOM is usually fine when all the machines are on the same domain, but when they're not, or if the network is slow, then DCOM can present some challenges.

 OPC Servers 
Our in-house OPC server development is based on a toolkit from Northern Dynamics. By using a toolkit from an industry leader, we can build an OPC server on time and within budget. There are two main aspects to our work:

• Building a custom OPC server to meet a client's requirements, such as an OPC server for a custom communications protocol. Recently, we built an OPC server for the interface to the metering system for Taliman's Tartan A platform in the North Sea.
• Enhancing existing products with OPC server functionality. For example, we have developed an OPC server for a compressor monitoring software package. This OPC server has an in-built OPC client for data aquisition, and creates PNG files showing the compressor characteristics.

 OPC Clients 
We are currently developing a number of OPC client applications:

• Gas Compressibility Client
  We are developing our Gas Compressibility OPC package to work as either an OPC server or an OPC client. More information is available in the Applications area of the site, under Gas Compressibility.

• InterServer Bridge
  Many older Distributed Control Systems (DCS's) only act as an OPC server - they do not have an inbuilt OPC client. Most interfaces to 3rd party systems are only available as OPC servers, and therein lies a problem - how to connect one OPC server to another.

  The InterServer Bridge is basically is a Server-to-Server Bridge which allows two OPC servers to be interconnected. One server is designated as the main data source, while the other is the main data sink. The Interserver Bridge then transfers data from the data source to the data sink. It is also possible of course to transfer data such as setpoints or outputs from the sink back to the source.

  The designation of data sink and data source simply makes it easier to visualize the normal data flow, and also defines which OPC server should be used as the source of the data at startup. For example, we may be interfacing a tank gauging system with a DCS. The tank gauging system makes its data available through an OPC server, and this would be the data source. The DCS would be the data sink.