异步通信与同步通信的主要区别是什么，说明通信双方如何联络。
参考答案：
同步通信和异步通信的主要区别是前者有公共时钟，总线上的所有设备按统一的时序，统一的传输周期进行信息传输，通信双方按约定好的时序联络。后者没有公共时钟，没有固定的传输周期，采用应答方式通信，具体的联络方式有不互锁、半互锁和全互锁三种。不互锁方式通信双方没有相互制约关系；半互锁方式通信双方有简单的制约关系；全互锁方式通信双方有完全的制约关系。其中全互锁通信可靠性最高。
It was a bitter, cold evening in northern Virginia. The old man’s beard was 11 by winter’s frost while he waited for a ride across the river. The wait seemed endless. Then he heard the weak sound of hooves(马蹄) advancing along the frozen path. 12 , he watched as several horsemen rounded the bend. He let the first one pass by, then another. As the last rider drew near where the old man sat, the old man 13 the rider’s eye and said, “Sir, would you mind giving an old man a ride to the other side of the river?” Reining(勒住) his horse, the rider replied, “Sure thing.” Seeing the old man unable to 14 his half-frozen body from the ground, the horseman helped the old man onto the horse and took him to his 15 . On the way, the horseman said: “Sir, I notice that you let several other riders pass by. I'm 16 why, on such a bitter winter night, you would wait and ask the last rider.”The old man looked at the rider and replied, “I know people pretty good. When I 17 the eyes of the other riders, I immediately saw there was no 18 for my situation. But when I looked into yours, kindness and compassion(同情) were obvious. I knew that your gentle spirit would give me help in my time of 19 .”Those heart-warming comments 20 the horseman deeply. “I'm most grateful for what you have said,” he told the old man. “May I never get too busy in my own affairs that I fail to respond to the needs of others with kindness and compassion.”
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