# 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
--- | --- | ---
Client status word | Empty | Status flags (aggregated)
Server status word | Exception code | Status flags (individual)

## Status flags

In a client status word, a status flag is set if the corresponding error occurred for at least one of the servers connected to that client. A raised status flag does not indicate that the client itself is not working properly. In a server status word, a status flag is set if the corresponding error occurred for that server.

0x01 (SERIAL_COM_ERROR) | A serial communication error occurred.
0x02 (RESPONSE_TIMEOUT) | A server did not respond in time.
0x04 (SHORT_FRAME) | The data frame sent by a server 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 server is incompatible with the corresponding request of the client.
0x80 (SERVER_EXCEPTION) | A server exception occurred. The exception code can be found in the high byte of the status word of the corresponding server.

## Update timing

The client status word is updated after each complete Modbus RTU cycle.

The server status word is updated as soon as the corresponding server has been processed.