Typical error codes (from my old Modbus spec) returned by the modbus slave are:

  01 Illegal function

  02 Illegal data address

  03 IIlegal data value

  04 Failure in associated device

  05 Acknowledge (should not happen for reads and writes)

  06 Busy, rejected message (device is busy with another message)

  07 NAK (request was rejected)

  08 Memory parity error