PLC(英文ProgrammableLogicController的缩写)的类型繁多，功能和指令也不尽相同。但其结构和工作方式则大同小异，一般由主机、输入/输出接口、电源、编程器、扩展接口和外部设备接口等几个主要部分构成。如图1所示。如果把PLC看作一个控制系统的核心，外部的各种开关信号或模拟信号均为输入变量，它们经输入接口寄存到PLC内部的状态寄存器和数据储存器中，而后按用户程序要求进行逻辑运算或数据处理，最后以输出变量的形式送到输出接口，从而控制输出设备。
(1)主机
主机部分包括中央处理器(CPU)、系统程序储存器和用户程序及数据存储器。
CPU是PLC的核心，起着总指挥的作用，它主要用来运行用户程序，监控输入/输出接口状态，作出逻辑判断和进行数据处理。即取进输入变量，完成用户指令规定的各种操作，将结果送到输出端，并响应外部设备(如编程器、打印机、条码扫描仪等)的请求以及进行各种内部诊断等。
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