| Code | Name | Reason |
|---|---|---|
| 01 | ILLEGAL FUNCTION | Unsupported function code |
| 02 | ILLEGAL DATA ADDRESS | Register address or Object ID out of valid ranges |
| 03 | ILLEGAL DATA VALUE | Otherwise invalid request |
Accessing unavailable (temporarily missing) measurement data does not generate a Modbus exception. “Unavailable” value (a quiet NaN for floating point data or 8000hex for integer data) is returned instead. An exception is generated only for any access outside the applicable register ranges.