PLC mainly by CPU module, input module, output module and programmer (see Figure 1), and some PLCs can also be equipped with special function modules, used to accomplish some special tasks.
1. CPU module
CPU module mainly by the microprocessor (CPU chip) and memory components. In the PLC control system plant CPU module is equivalent to the human brain, it constantly collected input signal, the implementation of user programs, refresh the system output; memory used to store programs and data.
2.I / O module
Input (1nput) module and output (Output) module referred to as I / O modules, which are the eyes, ears, hands, feet, is the bridge connecting external field devices and CPU modules.
The input module is used for receiving and collecting the input signal, and the digital input module is used for receiving the digital input signal from the button, the selector switch, the digital dial switch, the limit switch, the proximity switch, the photoelectric switch, the pressure relay, The input module is used to receive continuously changing analogue current and voltage signals from potentiometers, tachogenerators and various transmitters.
The switch output module is used to control the output devices such as contactor, solenoid valve, electromagnet, indicator light, digital display device and alarm device. The analog output module can control the actuator such as regulating valve and inverter.

Figure 1 PLC control system diagram
The operating voltage of the CPU module is generally 5V, while the input / output signal voltage of the PLC is generally higher, such as DC 24V and AC 220V. Spikes and interference noise introduced from the outside may damage the components in the CPU module or make the PLC not working properly. In the I / O module, with optocouplers, photothymbic, small relays and other devices to isolate the PLC’s internal circuits and external I / O circuits, I / O modules in addition to passing the signal, there are level conversion and isolation effect.
3. Compiler
The programmer is used to generate the user program and use it to edit, check, modify and monitor the execution of the user program. Hand-held programmer can not directly enter and edit ladder, can only enter and edit the list program, it is also called the programmer. Its small size, low price, is generally used to program small PLC, or for on-site commissioning and maintenance.
The use of programming software can be directly generated and edited on the computer screen ladder diagram, instruction list, function block diagram and sequential function chart program, and can achieve different programming languages ​​conversion. After the program is compiled and downloaded to the PLC, the program in the PLC can also be uploaded to the computer. Program can save or print, through the network, but also can achieve remote programming and transmission.
4. power supply
PLC generally use 220V AC power or 24V DC power supply. Internal switching power supply for each module DC 5V, ± 12V, 24V and other DC power supply. Small PLCs typically provide 24V DC for input circuits and external electronic sensors (such as proximity switches), and DC power for PLC loads is typically provided by the user.

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