stm32f407本实验串口2将不停的打印信息到电脑上，同时接收从串口发过来的数据，把发送过来的数据直接送回给电脑。(Stm32f407 this experiment serial port 2 will print the information to a computer, coming from a serial port to send and receive data, the data was sent directly back to the computer.)