你好,
BytesToWrite 是 获取发送缓冲区中数据的字节数, 请检查一下你的串口有没有缓冲区, 或者有没有设置缓冲区， 下面的帖子有具体的内容介绍，你可以参考一下。
https://stackoverflow.com/questions/5991598/usage-of-serialport-bytestowrite
Best regards,
Zhanglong
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