Illegal data value (Modbus Exception)

What does 3 mean?

A value contained in the Modbus query data field is not an allowable value for the server (drive). This indicates a structural fault in the remainder of a complex request, such as an incorrect implied length. It does NOT mean a submitted data item is outside an application's expectation, as the Modbus protocol doesn't interpret specific register values.

• Attempting to write an out-of-range value to a specific Modbus register (e.g., motor speed limit above max RPM).
• Writing an invalid data type to a Modbus register (e.g., attempting to write a string to a numeric parameter).
• Modbus master sending a command to a read-only register with a write function code.
• Drive operating mode preventing a specific parameter change (e.g., writing to P1-01 while drive is running).
• Incorrect scaling factor applied by the Modbus master for a specific parameter (e.g., decimal values expected as integers).

Click steps to track your progress.

1. 1

   Review the Modbus communication log for the exact register address and data value being written at the time of the fault.

2. 2

   Consult the drive's Modbus register map to confirm the valid data range, data type, and read/write permissions for the specified register.

3. 3

   Ensure the Modbus master (e.g., PLC, HMI) is sending values within the specified limits and in the correct format (e.g., 0-100% for P1-02).

4. 4

   Verify the drive's current operating status (e.g., idle, running) and if the target parameter (e.g., P0-10) can be modified in that state.

5. 5

   Test writing a known valid value to the problematic Modbus register to isolate if the issue is with the value itself or the register access.