串行端口是系统资源的一部分，其本质是作为CPU和串行设备间的编码转换器。当数据从 CPU经过串行端口发送出去时，字节数据转换为串行的位（Bit） 接收数据时，串行的位被转换为字节数据。应用程序要使用串口进行通信，必须在使用之前向操作系统提出资源申请要求(即打开串口)，通信完成后再释放资源(即关闭串口)。串行通信一般可以分为同步和异步两种操作方式。所谓同步方式是指在串口的接收缓冲区中读取规定数目的数据，直到规定数目的数据全部被读出或设定的超时时间已到才返回。如果规定的待读取数据量大且设定的超时时间也较长，而接收缓冲区较小，则可能引起线程阻塞。而异步方式是利用Windows的多线程结构，让串口的读写操作在后台进行，而应用程序的其他部分在前台执行。-The serial port is a part of system resources, and its essence is used as the CPU and the serial code converter between devices. When the data from the CPU through the serial port to send out, the byte data is converted into a serial bit (Bit)　receiving data, the serial data bits are converted into bytes. Applications using serial communication, you must use the resources made prior to application to the operating system requirements (ie open the serial port), after the completion of the release of communication resources (ie close the serial port). Serial communication can generally be divided into synchronous and asynchronous mode of operation. The so-called synchronous mode means that the serial receive buffer to read the required number of data until all the required number of data to be read out or set timeout period has expired before returning. If the specified amount of data to be read and set the timeout time is longer, and the receive buffer is small, it may cause the thread