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IBM-PC机与外部的串行通讯是采用标准的Rs232C接口方式,由异步适配器完成。采用串行通讯线路的主要原因在于:降低了通讯线路的价格和简化通讯设备。 串行通讯中的控制规程很多,但IBM-PC机所采用的异步适配器核心是INTEL8251,而8251仅支持同步(BISYNC)和异步(ASYNC)两种方式。IBM-PC机与外界的串行通讯一般都采用异步方式,因而本文也仅限于讨论异步通讯。 异步的含义是传送器和接收器不共享共同的同步信号,也不在数据中传送同步信号。它的定时方法是在一个字符的首尾放置起始符号和停止符号,供接收端用起始符号和停止符号判断一个字符。如图1所示。
IBM-PC and external serial communication using a standard Rs232C interface, completed by the asynchronous adapter. The main reason for using a serial communication line is that it reduces the price of communication lines and simplifies communication equipment. There are many control procedures in serial communication, but the core of asynchronous adapter used in IBM-PC is INTEL8251, while the 8251 only supports BISYNC and ASYNC. The serial communication between IBM-PC and the outside world generally adopts asynchronous mode, so this article is also limited to discuss asynchronous communication. Asynchronous means that the transmitter and receiver do not share a common synchronization signal and do not transmit synchronization signals in the data. Its timing method is to place the start symbol and stop symbol at the beginning and end of a character, for the receiver to use a start symbol and stop symbol to determine a character. As shown in Figure 1.