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Datasheets and User Guides

App Notes

Software & Driver


3.1 Modbus Map

Modbus Map Overview

Modbus - Modbus Map [referenceable]

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 and UE9, see the deprecated Modbus system called UD Modbus.

Data Type Constants

TypeInteger Value


Many registers are sequentially addressed.  The Modbus Map gives you the starting address for the first register, and then depending on whether the data type is 16-bits or 32-bits you increment the address by 1 or 2 to get the next value.

Address = StartingAddress + 1*Channel#    (UINT16)

Address = StartingAddress + 2*Channel#    (UINT32, INT32, FLOAT32)

Note that the term "register" is used 2 different ways throughout documentation.  A "register" is a location that has a value you might want to read or write (e.g. AIN0 or DAC0).  The term "modbus register" is generally referring to the modbus use of the term, which is a 16-bit value pointed to by an address of 0-65535.  So most "registers" consist of 1 or 2 "modbus registers".

For example, the first entry in the Modbus Map has the name AIN#(0:254), which is shorthand notation for 255 registers named AIN0, AIN1, AIN2, ..., AIN254.  The data type is specified as FLOAT32, so each value needs 2 modbus registers, an thus the address for a given analog input is channel*2.


Other Versions

The ljm_constants Bitbucket repository contains some up-to-date text versions of the Modbus register map: