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Windows is a "message based", multitasking operating
system. Multitasking means that you can have more than one application
running at a time and "message based" (also referred to
as "event driven") means that while an application is
running, it is simply sitting in memory dormant, waiting for event
messages to be sent to it by the operating system or from another
running application. When you press a key on your keyboard or click
your mouse, you are generating events. At the moment an event occurs,
the operating system sends a message describing the event to the
application that is the target of the event (i.e. the top level
window for keyboard and mouse events). Messages are sent by writing
them to a message "queue" for the application. When the
system is idle (i.e. no programs are busy doing anything), Windows
passes control to each running application in turn to give it a
chance to process any event messages that it finds in its message
queue. When the application is finished processing all of its messages
it passes control back to the operating system so that the next
running application can process its event messages. The important
point here is that while one application is busy doing something,
all other applications are unable to process any of their own event
messages until the first application finishes.
DDE is simply a way for one application to send messages directly
to another. When one program passes data or sends a DDE command
to another it is simply sending a message that will get placed in
the other application's message queue. The receiving application
will not be able to act on that message until the sending program
goes idle thus allowing it to process its messages.
If you understand what is going on behind the scenes, you can
avoid some common mistakes. For example, suppose you have a device
that responds to a prompt sent via the serial port. When the device
receives the prompt it sends back a string of data. You also want
to write an Excel macro that will send a command to the Wedge causing
it to send the prompt out the serial port and then you want Excel
to read the response back from the device by performing a DDERequest
to the Wedge. You might come up with the following Excel macro that,
although it appears to be quite logical, will never work.
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Sub PromptMyGizmo()
Chan =DDEInitiate("WinWedge", "COM1") 'open
a DDE link with WinWedge
DDEExecute Chan, "[SENDOUT(’?’,13,10)]" '
send a prompt out the serial port
MyData =DDERequest(Chan, "Field(1)") '
read back the response from Field(1)
Sheets("Sheet1").Cells(1, 1).Formula = MyData '
write the data to cell A1 in Sheet1
DDETerminate Chan 'terminate the DDE link
End Sub
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