:return: bytes

def __checksum_data(self):
        """
        :return: bytes
        """
        data_bytes = self.data_to_binary()
        if self.rtr:
            rtr_and_size = velbus.RTR | len(data_bytes)
        else:
            rtr_and_size = len(data_bytes)
        prefix = bytes([velbus.START_BYTE, self.priority, self.address,
                        rtr_and_size])
        return prefix + data_bytes