code: def _create_request_adu(slave_id, req_pdu):
      
      first_part_adu = struct.pack('>B', slave_id) + req_pdu
  
      return first_part_adu + get_crc(first_part_adu)
docs: Return request ADU for Modbus RTU.
  
      :param slave_id: Slave id.
      :param req_pdu: Byte array with PDU.
      :return: Byte array with ADU.