The fluctuation of AC voltage in 3708E is within the range of+10% (+15%), and the PLC can be directly connected to the AC network without taking any other measures. The central processing unit (CPU) is the control center of the PLC. It receives and stores user programs and data input from the programmer according to the functions assigned by the PLC system program; Check the status of power supply, memory,/O, and alert timer, and be able to diagnose syntax errors in user programs. When the 3708E PLC is put into operation, it first receives the status and data of various input devices on site through scanning, and stores them in the I/O image area respectively. Then, it reads user programs one by one from the user program memory, interprets commands, and executes logical or arithmetic operations according to the instructions. The results are sent to the/O image area or data register. After all user programs such as 3708E are executed, the output states or data in the output registers of the I/O image area are finally transmitted to the corresponding output devices, and the operation is repeated until it stops. In order to further improve the reliability of PLCs, 3708E has also adopted redundant systems with dual CPUs or voting systems with three CPUs for large PLCs in recent years