Modbus Exception: Write to Read-Only Register/Coil

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This Modbus exception is returned when the master device attempts to write data to a register or coil that is configured as read-only by the inverter. The inverter rejects the command because it cannot modify the data at that address. This typically applies to monitor values or fixed system status indicators that are not user-editable.

• The Modbus master is attempting to write to a status register (e.g., actual motor current, output frequency) that is inherently read-only.
• Incorrect register mapping in the Modbus master's configuration, treating a read-only parameter as writable.
• Parameter (holding register) is configured as read-only by a specific inverter setting (e.g., P1-08 (Motor Rated Current) might be read-only if auto-tuning is enabled).
• A Modbus master's programming error where a write command is mistakenly issued to a read-only address.
• Corrupted Modbus frame due to communication line noise, leading to the inverter misinterpreting a read request as a write.

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1. 1

   1. Refer to the inverter's Modbus communication manual to identify which registers are designated as read-only and which are writable.

2. 2

   2. Review the Modbus master's programming (e.g., PLC, HMI) to correct any instances where a write command is directed to a read-only register or coil.

3. 3

   3. Use a Modbus sniffer tool to capture the Modbus write command and confirm the target register address and that it's indeed a write operation.

4. 4

   4. Check if any inverter parameters (e.g., P7-05 (Parameter Lock), or internal control modes) might temporarily render a normally writable register as read-only.

5. 5

   5. Distinguish between actual process data (read-only) and configurable parameters (writable) in the Modbus master's mapping.

6. 6

   6. If the register is intended to be writable, confirm that its corresponding address for writing is correctly referenced, as some inverters sometimes use different addresses for read vs. write operations.