one. System composition. two. The role of each part. 1. CPU computing and control center "heart" role. Long: When the program input from the programmer is stored in the user program memory, then the CPU According to the system functions (system program memory interpreter compiler), the user program translated into PLC internal recognized user compiled program. Horizontal: input status and input information input from the input interface, CPU will be stored in the working data memory or input image register. Then by the CPU data and procedures organically together. The results stored in the output image register or working data memory, and then output to the output interface, control of external drives. Composition: CPU by the controller , operator and registers. These circuits are integrated on a single chip. CPU through the address bus, data bus and I / O interface circuit connected. 2. Memory Semiconductor circuit with memory function. Divided into system program memory and user memory. System program memory is used to store system programs, including management programs, monitoring programs, and interpreter compilers that compile user programs. By the read-only memory, ROM composition. Manufacturers use, the contents can not be changed, power does not disappear. User memory: divided into user program storage area and working data storage area. Comprised of random access memory (RAM). User's use Power outages disappear. Commonly used high-efficiency lithium battery as a backup power supply, life expectancy is generally 3 to 5 years. 3. Input / output interface (1) Input Interface: The optocoupler consists of two LEDs and a phototransistor. LED: In optocoupler input with the change of electrical signals, The light-emitting diode produces the same optical signal as the input signal. Photoelectric three tubes: the light signal conduction, the degree of conduction depends on the strength of the optical signal. In the optocoupler linear work area, the output signal and the input signal has a linear relationship. Input interface circuit work process: When the switch is closed, the diode light, and then the transistor under the light irradiation guide Pass, input signal to internal circuit. When the switch is off, the diode is not emitting light and the transistor is not conducting. Input signal to internal circuit. That is, through the input interface circuit to external switching signals into PLC internal acceptable digital signal. (2) output interface PLC relay output interface circuit Working process: When the internal circuit outputs a digital signal 1, a current flows, the relay coil has a current, and then the normally open contact closes to provide the current and the voltage for the load to turn on. When the internal circuit outputs the digital signal 0, current flows, Relay coil no current, Then the normally open contact is disconnected, Disconnect the load current or voltage. That is, through the output interface circuit to the internal digital circuit into a signal so that the load action or not action. Three types: Relay output: a contact, short life, low frequency, AC and DC load Transistor output: non-contact, long life, DC load