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Modbus Exception Codes 01 to 04: Root Causes & Diagnostic Recovery Checklist

Demystifying Modbus exception responses: Exception 01 (Illegal Function), Exception 02 (Illegal Data Address), Exception 03 (Illegal Data Value), and Exception 04 (Slave Device Failure).

  • In an exception response, the slave sets the most significant bit of the function code (FC + 0x80) followed by a 1-byte error code.
  • Exception 02 is overwhelmingly caused by 0-based protocol addressing vs 1-based documentation notation.
  • Exception 04 indicates the slave received a valid request but encountered an internal microcontroller fault.

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