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。箱体式的PLC还有I/O扩展箱体，它不含CPU，仅有电源及I/O单元的功能。扩展箱体也依I/O点数的多少划分有不同的规格。除上述模块，PLC还有特殊的或称智能或称功能模块。如A/D（模入）模块、D/A（模出）模块、高速计数模块、位控模块、温度模块等等。这些模块有自己的CPU，可对信号作预处理或后处理，以简化PLC的CPU对复杂的程控制量的控制。智能模块的种类、特性也大不相同，性能好的PLC，这些模块种类多，性能也好。通讯模块，它接人PLC后，可使PLC与计算机，或PLC与PLC进行通讯，有的还可实现与其它控制部件，如变频器、温控器通讯，或组成局部网络。通讯模块代表PLC的组网能力，代表着当今PLC性能的重要方面。掌握PLC性能，一定要了解它的模块，并通过了解模块的性能，去弄清楚PLC的性能。除了模块，PLC还有外部设备。
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.
The box type PLC also has an i/o expansion box, which does not contain CPU, but only has the function of power supply and i/o unit. The expansion box also has different specifications according to the number of i/o points.
In addition to the above modules, PLC has special or intelligent or functional modules. Such as a/d module, d/a module, high-speed counting module, position control module, temperature module, etc. These modules have their own CPU, which can preprocess or post process the signal, so as to simplify the PLC CPU's control of complex programmed control quantities. The types and characteristics of intelligent modules are also very different. PLC with good performance has many kinds of modules and good performance.
Communication module, which can communicate between PLC and computer, or between PLC and PLC after being connected to PLC. Some can also communicate with other control components, such as frequency converter and temperature controller, or form a local network. The communication module represents the networking ability of PLC and the important aspect of PLC performance today.
To master the performance of PLC, we must understand its module, and understand the performance of PLC by understanding the performance of the module.
In addition to modules, PLC has external devices.
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