To make a Profibus Mater function correctly on an FX3U PLC, code is required. This involves reading the manual, understanding the example programs, understanding how the Profibus slaves work, and then writing PLC code to suit the slaves.

If the user is planning to simply block transfer data to and from the FX3U Profibus DP master, and wants to use GX Developer then there is an easy solution (so long as the user has access to both GX IEC Developer and GX Developer).

Design Considerations

The trick with this solution is to create the Profibus configuration in the normal way using GX Configurator DP but then use the GX IEC Developer as an intermediary to get to the final solution of GX Developer code.

  • Create the Profibus configuration in GX Configuration DP.
  • Set the data registers for block data transfer using the Setup menu then PLC and GX IEC Developer settings.
  • From the File menu Export a POU for GX IEC Developer.
  • Create a bank FX3U project in GX IEC Developer and from the Project menu and then the Other feature, import the *.asc file that was created by the export from GX Configurator DP.
  • The IEC project now contains the necessary code for the PLC so it can be compiled (using the “rebuild all” feature), and then downloaded to the PLC.
  • Finally the PLC code can be imported to GX Developer from the PLC and the second to last line containing the [FEND] can be deleted.
  • This PLC code will now work as desired.

NOTE: if features such as byte swapping are implemented the source data registers in the PLC used for outgoing data to the slaves, will not be located in the data registers expected. Example: D200 was selected in the Profibus configuration as the start address for outgoing data but the code generated used D500 as the source data that was then byte swapped into the chosen D200.

NOTE: the obvious problem with this solution is that GX IEC Developer is needed but if it is available it does make setting up a Profibus network quite simple.

Further Information

For further information on how the FX3U-64DP-M Profibus Master works, its features, functions and example PLC code, please refer to the FX3U-64-DP-M manual or contact our support desk (

You can download this Tip in PDF format here

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