USB是主从总线，上位机不读，下位机无法上传。所以当上位机调用ReadData时，下位机才能上传数据，当然，下位机应该准备好数据，如果没有准备好，上位机程序会一直等待下去，像停机一样。-USB is the main from the bus, not to be PC, the next crew unable to upload. So when PC calling ReadData, where crew can upload data, of course, the next crew should be ready, if not ready, the PC process has been to wait and stands as the same.