User`s guide

2 • Introduction to PCIS-DDE
Client applications can use DDE for one-time data transfers or for
continuous data exchanges in which updates are sent as soon as new
information is available. For one-time data transfers, the client application
only requests the “snapshot” data from the server application. For
example, as a macro for report generation is executed in Excel, a link to
another DDE program will be set up to request the specified data. The link
will be terminated after the requested data is received. Then the received
data are used to generate reports.
The continuous data exchanges mode is also named “hot link”. While a
client application sets up a link to another DDE program, it requests the
server application to advise the client whenever a specific item's value
changes. These data links will remain active until either the client or server
program terminates the link or the conversation. It is a very efficient means
of exchanging data because once the link has been established no
communication occurs until the specified data value changes. InTouch
uses DDE to communicate with I/O device drivers and other DDE
application programs.
For InTouch, if the tagname are defined as I/O type, they can read or write
their values to or from another DDE compliant Windows program. For
example, InTouch can read or write their values to Excel, and Excel can
also read or write data to InTouch Database. Whenever the data from
source are updated, the remote data are updated automatically as soon as
new information is available. DDE can be used to dispatch control
instructions to process-connected instruments. With this ability, two or
more related applications can be combined together to make up a large
size of super application. For example, Excel spreadsheet can perform the
optimal calculation for production. Thus, Excel may read data from InTouch
database, which are accessed from I/O controllers or sensors. Reference
to the data, the Excel spreadsheet performs some complicated calculation.
InTouch reads the calculated result back from Excel and then uses this
optimal value to control various production parameters.