说明：  S7-200 PLC之PPI协议 通过硬件和软件侦听的方法，分析PLC内部固有的PPI通讯协议，然后上位机采用VB编程，遵循PPI通讯协议，读写PLC数据，实现人机操作任务。这种通讯方法，与一般的自由通讯协议相比，省略了PLC的通讯程序编写，只需编写上位机的通讯程序资源
(S7-200 PLC via PPI protocol of the hardware and software methods of listening, analyzing the internal PLC inherent PPI protocol, then the host computer using VB programming, follow the PPI protocol, read and write PLC data to achieve man-machine task. This communication method, and the general freedom of communication protocols, communication is omitted PLC programming, just the preparation of PC communication program resources)