继电器控制,设8255为A口输入,C口输出,延时5s,是否有键按下,若有则转exit,查询8255的PA0是否为高电平,若不是则继续,定时时间到，子程序返回-Relay control, set up 8255 for the A port input, C output, delay 5s, whether there is any key pressed, if so, then switch to exit, the PA0 query whether the 8255 high, if