If an error occurs in a request from the master, the slave will respond with the function code with 0x80 added to it, and one of the following exceptions:

Code	Meaning
0x01	Illegal Function
0x02	Illegal Address. The address(es) requested do not exist
0x03	Illegal Value.
0x04	Slave Device Failure.
0x05	Acknowledge. The slave can use this to indicate that it has received the request, and is processing it, but it will take longer than the response timeout. The master can use a Poll Program Complete message to determine if the processing is complete.
0x06	Slave Device Busy. The master should retry the request later.
0x07	Negative Acknowledge. The slave cannot perform the programming requested.
0x08	Memory Parity Error
0x0A	Gateway Path Unavailable
0x0B	Gateway Target Device Failed to Respond

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References
Simply Modbus - Exception Codes