def _recv(self, expected_length=-1):
        """
        Receive the response from the slave
        Do not take expected_length into account because the length of the response is
        written in the mbap. Used for RTU only
        """
        to_be_recv_length = expected_length-2+6 #MB/TCP data length must add 6 byte header, sub 2 byte crc
        expected_pdu_length = expected_length-2
        response = to_data('')
        length = 255
        while len(response) < to_be_recv_length: #for H5A: length error issue
            rcv_byte = self._sock.recv(1)
            if rcv_byte:
                response += rcv_byte
                if len(response) == 6:
                    #to_be_recv_length = struct.unpack(">HHH", response)[2]
                    #length = to_be_recv_length + 6
                    if response[5] != expected_pdu_length:
                        print('length field of header error, fix: %s-->%s' %(response[5], expected_pdu_length))
                        response = response[:5] + struct.pack("B", expected_pdu_length)

            else:
                break
        retval = call_hooks("modbus_tcp.TcpMaster.after_recv", (self, response))
        if retval is not None:
            return retval
        return response