Description of the Modbus RTU status words
Layout, status flags and update timing of the Modbus RTU status words
Layout
The Modbus RTU status words consist of 2 bytes.
Status word | High byte | Low byte |
|---|---|---|
Master status word | Empty | Status flags (aggregated) |
Slave status word | Exception code | Status flags (individual) |
Status flags
In a master status word, a status flag is set if the corresponding error occurred for at least one of the slaves connected to that master. A raised status flag does not indicate that the master itself is not working properly. In a slave status word, a status flag is set if the corresponding error occurred for that slave.
0x01 (SERIAL_COM_ERROR) | A serial communication error occurred. |
0x02 (RESPONSE_TIMEOUT) | A slave did not respond in time. |
0x04 (SHORT_FRAME) | The data frame sent by a slave is too short. |
0x08 (CRC_ERROR) | The CRC value appended to a message by the sender does not match the CRC value calculated by the receiver. |
0x10 (ILLEGAL RESPONSE) | The response of the slave is incompatible with the corresponding request of the master. |
0x80 (SLAVE_EXCEPTION) | A slave exception occurred. The exception code can be found in the high byte of the status word of the corresponding slave. |
Update timing
The master status word is updated after each complete Modbus RTU cycle.
The slave status word is updated as soon as the corresponding slave has been processed.