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Modbus Map

We distribute a constants file called "ljm_constants.json" that defines all information about the Modbus register map.  The dynamic filter & search tool below pulls data from that JSON file.

  • Name:  A string name that can be used with the LJM library to access each register.
  • Address:  The starting address of each register.  Valid through LJM or with direct Modbus.
  • Details:  Click to get text pulled from the description field in the JSON.
  • Type:  Specifies the datatype, which also tells you how many registers each value uses.
  • Access:  Read-only, write-only, or read & write.
  • Tags:  Used to associate registers with particular functionality.  Useful for filtering.

For a U3/U6 with firmware less than 2.0, or for the UE9, see the deprecated Modbus system called UD Modbus.



What is the order of I/O ports when writing to and reading from a multi-port register like DIO_STATE? is it CIO,EIO,FIO, where FIO is the 8 LSBs?


Yes.  To verify, you can jumper a wire between VS and FIO0, then jumper between GND and FIO0.  You should see the LSB change from 1 to 0 when reading DIO_STATE (assuming FIO0 is set to input).


Could somebody please make this list available as Excel sheet of PDF.


If you want something you can browse, search, or even print, I suggest you use "ljm_constants.json" which is on your computer if you have run our installer.  This is a text file so you can open it in about anything.

The other thing we can do is use our internal autoscribe software to generate a printable modbus map.  See if the new Printable Modbus Page is useful, and if so we can fill in the rest of it.