某些额外模块来增加，每个模块可以包含扩展的输入线和输出线。当然，在PLC控制器的扩展模块上也可以有不同性质的输入和输出例如继电器输出控制器和晶体管输出可以在同一个扩展模块中。21PLC运行过程
PLC的工作就是不断的扫描程序，我们可以把这个扫描周期视为三个重要的步骤，如图84所示。（PLC的工作过程）通常不止这三个步骤，但我们可以专注这三个重要的部分而不必考虑其它的。一般其他的部分是用来检查系统以及更新当前内部计数器和定时器的值。关键词：可编程控制器，PLC，控制器摘自：王伟，张艳丽主编《电气工程与自动化专业英语》，机械工业出版社，第八章（P84P88）Chapter8ProgrammableLogicCo
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f外文翻译
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