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.
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.
在危机的全球化、法律现实主义运动的盛行和紧急权力法制化的兴起等因素的影响下，自由模式在20世纪初期开始衰微。
Liberal pattern was decline at the beginning of 20 century with the factors of the rise of legal realism and emergency power legalization etc.
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