从并口输入数据： 外设检测STB是否忙，忙则等待。发数据，置ACK,检测AFD是否响应，是则清ACK否则等待。 PC检测ACK是否选通，是则置STB否则等待。读数据，处理后发AFD清忙。-input data from the parallel port : Peripherals detection STB whether busy, busy then wait. Fat, home ACK, testing whether AFD response, it is awaiting clearance ACK otherwise. PC detection ACK whether strobe, it is home to wait for STB otherwise. Reading, after the AFD-busy.