Issue Details

• I am using LabWindows/CVI™ as a DDE client and communicating with a 3rd party DDE server. I keep receiving error -25. How can I fix this?
• I receive a "Transaction Failed" error in LabWindows/CVI when I call ClientDDEExecute for some of my DDE Topics. Other times it works. Why does this happen?
• My DDE communication in LabWindows/CVI used to work. Now that I've upgraded my software, I keep receiving the error below:

Solution

This error can be caused by a number of factors. Follow the steps below to narrow down the possible causes and resolve the error:
 

1. Install LabWindows/CVI 2020 f2 or newer if an older version is currently being used.
   • Several bugs that effect DDE communication were reported in LabWindows/CVI 2020. These were fixed in version 2020 f2.
   • Refer to LabWindows/CVI 2020 Known Issues for more information.
2. Verify that a connection to the DDE server can be successfully made.
   • Use the ConnectToDDEServer function and verify that no errors are reported.
3. If the error occurs at ClientDDEExecute, check that the syntax for the command string is correct.
   • Refer to the DDE Server's User Manual to determine that the LabWindows/CVI command matches the exact syntax required for the DDE Topic.
4. Ensure that the DDE connection is properly closed before the code completes execution.
   • Failed transactions may occur if DDE links or connections are not cleanly terminated/closed.
   • If SetUpDDEHotLink or SetUpDDEWarmLink are used, ensure that TerminateDDELink is also called.
   • Verify that DisconnectFromDDEServer is called at the end of communication.
5. If LabWindows/CVI is communicating with a 3rd party DDE server, reinstall the 3rd party software.
   • This will repair any damaged or corrupt 3rd party software files.

Additional Information

If the error remains after following the steps above, consider using a software tool that enables DDE tracing. For example, those with a license for Microsoft Visual Studio can use Microsoft's Spy++ tool. This will highlight the messages that are exchanged between the client and server, which can be useful for troubleshooting the exact point of error.