def _create_request_adu(slave_id, req_pdu):
first_part_adu = struct.pack('<STR_LIT>', slave_id) + req_pdu<EOL>return first_part_adu + get_crc(first_part_adu)<EOL>
Return request ADU for Modbus RTU.

    :param slave_id: Slave id.
    :param req_pdu: Byte array with PDU.
    :return: Byte array with ADU.