中央处理单元(CPU)是PLC 的控制中枢，它按照PLC 系统程序赋予的功能接收并存储从编程器键入的用户程序和数据、检查电源、存储器I/O以及警戒定时器的状态；并能诊断用户程序中的语法错误。当PLC 投入运行时，首先它以扫描的方式接收现场各输入装置的状态和数据，并分别存入I/O 映象区，然后从用户程序存储器中逐条读取用户程序，经过命令解释后，按指令的规定执行逻辑或算数运算的结果送入I/O 映象区或数据寄存器内，等所有的用户程序执行完毕之后，最后将I/O 映象区的各输出状态或输出寄存器内的数据传送到相应的输出装置，如此循环运行直到停止运行。(The central processing unit (CPU) is the control center of the PLC, which is in accordance with the function of PLC system program to receive and store the type in the user program and data from the programmer, check the power supply, memory I/O and warn the timer state;And diagnose the grammatical errors in the user program.When PLC in operation, the first it receives in the form of scanning field state of each input device and data, and deposited in the I/O image area respectively, and then passes from the user program memory read the user program)