Specifications

42 RSLinx – Training Guide
DDE Continued - Application, Topic, Item
Each Dynamic Data Exchange question consists of three parts:
• To whom am I talking (i.e., which APPLICATION gets the question)?
• What subject are we talking about (i.e., what is the general TOPIC)?
• Which data do I require (i.e., what specific ITEM are we talking about)?
In other words, who is talking? what's the general topic?
what specific data?
Windows DDE uses the words: Application, Topic, Item to describe the DDE
conversation that takes place between programs running under Windows
(e.g., RSLinx and Excel).
Definitions of Application, Topic, and Item:
Application: When you use a Microsoft Windows application to obtain data from
another Windows application, you must provide the name of the Application you wish to
respond to your question. The Application refers to the program you are running, such
as RSLinx would be the application that some client (such as Excel) would talk to (it
usually refers to the executable program file, e.g., RSLinx.exe).
To use the previous example, when you pose a question to your group of people, you
must tell them whom you want a response from. When you ask RSLinx for data from a
PLC on a highway, the Application name to use is RSLinx. (RSLinx.exe is the name of
the RSLinx executable file and thus the name to use for DDE communications no matter
what you have named the RSLinx icon).
Topic: Once the Application is known, we must determine which Topic to discuss. To
use the previous example, when you pose a question to an individual, you must select a
general Topic to talk about. With DDE, available Topics are determined by the
Application. The person asking the question must choose an available topic, or data
exchange cannot take place.
In RSLinx, a Topic will consist of the information necessary to get us communicating
with a particular PLC.
A DDE Project is a storage container for one or more DDE Topics, as a folder holds files
in Windows. By grouping topics together in a project, you can make multiple topics
available at the same time.