Modbus Exception: Illegal Data Value

What does 03 hex mean?

This Modbus exception is returned when the master device sends a data value that is outside the permissible range or in an unacceptable format for the specified Modbus address. This means the value attempting to be written is invalid according to the parameter's constraints or data type, preventing a successful write operation.

• The Modbus master is writing a value to a register that is numerically out of its defined limits (e.g., writing 5000 to a register with a max value of 1000).
• Data type mismatch between master and slave (e.g., master sends a signed integer to an unsigned integer register, or floating point to an integer register).
• The value sent is not a valid enumeration for a specific parameter (e.g., writing '5' to a parameter that only accepts '0', '1', or '2').
• Parameter scaling factor misunderstanding (e.g., writing '1000' when the parameter expects '10.0' and master doesn't account for implicit scaling by 100).
• Modbus master attempts to write to a register that requires specific conditions (e.g., inverter in STOP state) for value acceptance.

Click steps to track your progress.

1. 1

   1. Consult the inverter's Modbus communication manual to determine the valid data range, data type, and scaling factor for the specific Modbus register being written.

2. 2

   2. Verify the Modbus master's logic (e.g., PLC programming, HMI data entry limits) to ensure the sent value adheres to the register's specifications.

3. 3

   3. Use a Modbus sniffer tool to capture the Modbus write command and inspect the exact data value being transmitted.

4. 4

   4. Confirm that the data formatting (e.g., integer, float, endianness) is correctly handled by the Modbus master for the target register.

5. 5

   5. Check if the inverter's operating state (e.g., RUN or STOP) affects the acceptance of the parameter value and adjust accordingly.

6. 6

   6. Test writing simpler, known-good values (e.g., 0 or 1) to the problematic register to isolate the issue to the value itself.