组成PLC的模块是PLC的硬件基础，只有弄清所选用的PLC都具有那些模块及其特点，才能正确选用模块，去组成一台完整的PLC，以满足控制系统对PLC的要求。常见的PLC模块有：CPU模块，它是PLC的硬件核心。PLC的主要性能，如速度、规模都由它的性能来体现。电源模块，它为PLC运行提供内部工作电源，且，有的还可为输入信号提供电源。I/O模块，它集成了I/O电路，并依点数及电路类型划分为不同规格的模块。内存模块，它主要存储用户程序，有的还为系统提供辅加的工作内存。在结构上内存模块都是附加于CPU模块之中。底板、机架模块，它为PLC各模块的安装提供基板，并为模块间的联系提供总线。若干底板间的联系有的用接口模块，有的用总线接口。不同厂家或同一厂家但不同类型的PLC都不大相同。箱体式的小型PLC的主箱体就是把上述几种模块集成在一个箱体内的，并依可能提供I/O点数的多少，划分为不同的规格。
The modules that make up the PLC are the hardware basis of the PLC. Only by understanding which modules the selected PLC has and their characteristics, can the modules be correctly selected to form a complete PLC to meet the requirements of the control system for PLC.
Common PLC modules are:
CPU module, which is the hardware core of PLC. The main performance of PLC, such as speed and scale, is reflected by its performance.
Power module, which provides internal working power for PLC operation, and some can also provide power for input signals.
I/o module, which integrates i/o circuits and is divided into modules of different specifications according to the number of points and circuit type.
Memory module, which mainly stores user programs, and some also provide auxiliary working memory for the system. In structure, memory modules are attached to CPU modules.
Base plate and rack module, which provide base plate for the installation of PLC modules and provide bus for the connection between modules. Some backplane connections use interface modules, and some use bus interfaces. Different manufacturers or the same manufacturer but different types of PLC are not the same.
The main box of box type small PLC integrates the above modules into a box, and is divided into different specifications according to the number of i/o points that may be provided.
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