8250串行通讯实验：键盘输入一个字符，将其ASCII码加1后发送出去，再接收回来在屏幕上加1后的字符显示出来，实现自发自收。-8250 serial communication experiment: a keyboard input character, its ASCII code plus 1, after sending out, and then receive back on the screen plus 1 character after the show, the reali