In computing, Dynamic Data Exchange (DDE) is a technology for interprocess communication used in early versions of Microsoft Windows and OS/2.
DDE was partially superseded by Object Linking and Embedding (OLE), and is currently maintained in Windows systems only for the sake of backward compatibility.
[1] At the time, the only method for communication between the operating system and client applications was the "Windows Messaging Layer."
DDE extended this protocol to allow peer-to-peer communication among client applications, via message broadcasts.
Internal changes in the cell due to Excel actions would then be signaled (in reverse) to the calling application via additional message broadcasts.
In addition, Wonderware also sold an enhanced version of NetDDE to their own customers that included support for TCP/IP.