Microsoft Windows DDE allows the continuous and automatic exchange of data between different Windows applications on the same machine without the need for any user intervention. For example, your company's Production group might use a spreadsheet application to graphically represent plant-floor data (product output). This could be dynamically updated with the latest live data using DDE to read values directly from CitectSCADA.
Windows DDE uses the DDE protocolMessaging format consisting of a set of messages and guidelines used for communication between the server and an I/O device. The communication protocol determines how and the I/O device communicate; the type of data to exchange; rules governing communication initiation and termination; and error detection. to send messages between applications that share data.
Dynamic Data Exchange occurs between a DDE client application (which requests the data or service) and a DDE server application (which provides the data or service). The DDE Client starts the exchange by establishing a conversation with the DDE server, and requesting data or services. The DDE server responds to these requests by providing the data or services to the DDE Client. The DDE Client terminates the conversation when it no longer needs the DDE server's data or services.
Note: As the DDE protocol is not designed for high-speed data transferTransfer of information from one location to another. The speed of data transfer is measured in bits per second (bps)., the use of DDE is only appropriate when data communication speed is not critical.
Note: When reading or writing to CitectSCADA tags using DDE, you might unknowingly add to your CitectSCADA license point count. Once the dynamic point count is greater than the license point count, the software protection mechanism will terminate CitectSCADA runtime. Therefore, when accessing tags via DDE, it's important to remain aware of how many points you have used. For details, see license point count in the Installation and Configuration Guide.