docs: 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