中央处理单元(CPU)是可编程逻辑控制器的控制中枢。它按照可编程逻辑控制器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。
Central processing unit (CPU) is the control center of PLC. It receives and stores the user program and data input from the programmer according to the function given by the PLC system program, checks the status of power supply, memory, I / O and alert timer, and can diagnose the syntax error in the user program.
当可编程逻辑控制器投入运行时，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。
When the PLC is put into operation, first it receives the status and data of each input device on the site by scanning, and stores them in the I / O image area respectively, then reads the user program one by one from the user program memory, and after command interpretation, the results of executing logic or arithmetic operations according to the instructions are sent into the I / O image area or data register. After all the user programs are executed, the output status of the I / O image area or the data in the output register are finally transmitted to the corresponding output device, which circulates until it stops running.
为了进一步提高可编程逻辑控制器的可靠性，近年来对大型可编程逻辑控制器还采用双CPU构成冗余系统，或采用三CPU的表决式系统。这样，即使某个CPU出现故障，整个系统仍能正常运行。
In order to further improve the reliability of PLC, in recent years, double CPUs or voting system with three CPUs are used in large PLC. In this way, even if a CPU fails, the whole system can still operate normally.