def request_pdu(self):
        """ Build request PDU to write single register.

        :return: Byte array of 5 bytes with PDU.
        """
        if None in [self.address, self.value]:
            # TODO Raise proper exception.
            raise Exception

        return struct.pack('>BH' + conf.TYPE_CHAR, self.function_code,
                           self.address, self.value)