有些PLC中的电源，是与模块合二为一的，有些是分开的，其主要用途是为PLC各模块的集成电路提供工作电源。同时，有的还为输入电路提供24V的工作电源。电源以其输入类型有：交流电源，加的为交流220VAC或110VAC，直流电源，加的为直流电压，常用的为24V。
四、底板或机架：
大多数模块式PLC使用底板或机架，其作用是：电气上，实现各模块间的联系，使能访问底板上的所有模块，机械上，实现各模块间的连接，使各模块构成一个整体。
五、PLC的外部设备
外部设备是PLC系统不可分割的一部分，它有四大类
1.编程设备：有简易编程器和智能图形编程器，用于编程、对系统作一些设定、监控PLC及PLC所控制的系统的工作状况。编程器是PLC开发应用、监测运行、检查维护不可缺少的器件，但它不直接参与现场控制运行。
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