说明：  本函数的功能是从计算机接收一串字符串，在单片机处理筛选出需要的字符串， 校检接收字符是否正确，无误后，将字符串发送回计算机
(This function is received from the computer a string, in the single-chip processing out the need for screening of string, Checking the received symbol is correct, correct, send the string back to the computer)