Rockwell Automation Publication 1789-UM002K-EN-P - January 2015 147
Develop External Routines Chapter 7
Other Considerations
Consider these suggestions for your external routines.
Pass Tags by Reference
You can pass tags by reference in a synchronous, single-threaded routine. You
should not pass these memory addresses to another thread or process because it is
possible for the originating tag to be deleted. Then, the reference to the
originating tag in the thread or process becomes invalid and causes an
access violation.
External Routine DLL that Uses Other DLLs
If you create an external routine DLL that uses other DLLs, make sure that the
additional DLL files are accessible to the SoftLogix engine at runtime. For
example, assume external routine MyER.DLL uses another file called
MyAdditionalFile.DLL. The Logix Designer application copies MyER.DLL into
the project area and downloads MyER.DLL to the appropriate controller slot
during download. However, the Logix Designer application does not copy or
download MyAdditionalFile.DLL.
To make MyAdditionalFile.DLL available on the target machine, put the
MyAdditionalFile.DLL file into the Windows system32 directory on the target
machine. This makes sure that the file is available when needed. Otherwise, when
the controller attempts to perform the Windows LoadLibrary call for
MyER.DLL it will fail because the MyAdditionalFile.DLL cannot be found.
As a better solution, statically link any additional DLL files that are needed right
into the external routine DLL. This leaves only one file and the Logix Designer
application takes care of copying the DLL to the correct places.
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