系统采用单主站方式，在自由口模式下控制串口。
The system adopts single main-station to control the serial ports under free port mode.
通讯方式主要有两种：PPI主从通讯、自由口模式通讯。
Two modes of communication, one is PPI master - slave protocol, one is free port protocol.
详细介绍了在自由口模式下S7 - 200 PLC与计算机实现串行通信的方法，重点介绍了通信协议及其实现方法，并给出了编程实例。
How to implement serial communication between S7-200 PLC and computer based on free port mode was described in detail in the paper.
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