如果结東字符不在输入缓冲器中，那么在结束字符已经被接收或暂停时间到之前，操作将不能完成。读字符串操作完成时，端口状态显示读操作完成的原因(结束字符已经被接收或暂停时间到)。如果暂停时间被置为0，读字符串COMM_REQ保持在待处理状态直到被请求的字符串接收完或由指定结束字符终止。如果这个COMM_ _REQ由于某种原因失败，没有数据被送回到输入数据区。从内部输入缓冲器读出的数据保留，可以被后面的读请求检索。

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