说明：  通信系统大型实验板上有4个按键，分别为K1、K2、K3、K4，相对应的管脚为PIN46、PIN47、PIN48、PIN49，按下按键K1、K2、K3、K4（定义为key）用示波器或万用表观测被控制的输出端口（定义为tpin）变化，应有电平跳变
(Large-scale experiment on-board communication system with 4 keys, respectively, K1, K2, K3, K4, corresponding to the pin for the PIN46, PIN47, PIN48, PIN49, press the keys K1, K2, K3, K4 (defined as the key) observed with an oscilloscope or a multimeter is controlled by the output port (defined as tpin) changes should be level transition)