| SID | Description |
| 1 | The length in the Modbus header does not match the length needed |
| by the Modbus function code. | |
| Each Modbus function has an expected format for requests and responses. | |
| If the length of the message does not match the expected format, this | |
| alert is generated. | |
| 2 | Modbus protocol ID is non-zero. |
| The protocol ID field is used for multiplexing other protocols with | |
| Modbus. Since the preprocessor cannot handle these other protocols, | |
| this alert is generated instead. | |
| 3 | Reserved Modbus function code in use. |