Dynamic Data Exchange

In Windows, Dynamic Data Exchange (or DDE) is currently the primary method for communicating between an operator interface and the communications drivers that talk with real-world devices. DDE was designed by Microsoft to enable Windows programs to communicate with each other, and was extended to work over networks with NetDDE. Some HMI companies have even developed their own variants of DDE (for instance, Wonderware created FastDDE).

A typical computer running an operator interface (Human Machine Interface, or HMI) actually has many tasks running, in addition to the Windows operating system. Most visible are the operator interface screens which only perform the task of presenting data and control screens to the operators. Separate Windows tasks are running which actually communicate with the devices located in the plant. These tasks use DDE to communicate data, so that as a low level communications program receives new values from a sensor, these values show up on the control screen. DDE is also used to relay commands from the operator interface to the correct communication driver, which then sends this data to the particular piece of equipment. NetDDE enables the drivers to actually run on a machine apart from the user interface.

This separation of the HMI and the communications utilities results in a system which is much easier to maintain, as the drivers can be updated as equipment changes without having to modify any of the operator interface screens.

According to Microsoft, DDE is on the way out, to be replaced by Object Linking and Embedding (OLE, pronounced "oh-lay") based on the Component Object Model (COM). The current of OLE is OLE 2.0, and this is one product that Microsoft has no plans to obsolete because of the reusable design idea. The whole concept of OLE was to allow for new technologies without having to modify the base designs.

From the HMI point of view, the difference in the OLE and DDE drivers is not important. Both achieve the desired separation between the operator interface screens and the communications routines.

ASI has written many, many DDE servers. We have installed systems which have had as many as 15 of our DDE servers running on a single NT machine. We have written DDE or OLE servers to communicate with all manner of serial devices, different types of databases, other processes, and bus level products, under all versions of Windows since Windows 3.1. Without a doubt, if you need a specialized DDE or OLE server for a product in your plant, or a product that you design, we can do it for you, and do it right.

Nothing can mess up your application (and your day) like a server written by an amateur. Send us an E-mail, and we will get back to you with information about how one of our servers can work for your product.