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DDE(Dynamic Data Exchange) had been widely treated as one of the interfaces before OPC appeared though the OPC technology had infiltrated as the interface between the applications now. A lot of our company also must developed the DDE server before developing the OPC server, and use it now.
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Commentor / M.Ike |
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OPC and DDE |
DDE is supported with Windows, and does the data transfer by using the shared memory. DDE communicates by using three parameters of the application name, the topic name, and the item name. And, the interface named complete and interchangeable DDEML(Dynamic Data Exchange Management Library) to original DDE was prepared since Win32, and API group to be able to mount the DDE transaction easily came to be offered though DDE was originally a protocol of the message base. A present DDE server uses this DDEML.
Well, data can be easily read and written by using the DDE server when the custom applications such as Visual Basic and EXCEL are used. Especially, it is possible to achieve it by the automatic link function of DDE only by describing the expression in the cell in the property setting and EXCEL of the control in Visual Basic for reading. DDE Server is suitable for easily constructing the client for OPC Server to need the implementation of COM on the client side.
However, there are some problems in DDE.
- Eecrease of correspondence software(For example, DDE is unsupported in Visual Studio.NET. )
- The data transfer is slow.
- The security management is not done
- It is necessary to start the server beforehand
On the other hand, OPC can cancel all these problems. OPC is an interface of an international standard, and typical SCADA software supports OPC (Of course, it is possible to use it also with Visual Studio.NET). Moreover, the data transfer performance can be high, and security by DCOM manage. And, when it starts automatically by the access from the client because it is COM application, and the access from all clients disappears, it ends automatically.
A suitable OPC server for a standard system that doesn't depend on the device and equipment now is often used, and there are a lot of customers for whom the DDE server is used so that Visual Basic and EXCEL may easily access it on the other hand, too.
Then, to make the best use of each merit, the server made of our company OPC schedules the version to have both the DDE interface and to shortly improve. Please expect it of a new OPC server that supports the best application construction according to the usage and the environment by all means.
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Inside MELSEC
Advanced OPC