PLC structure
主要由CPU（中央处理器）、存储器和输入\输出模块三部分组成
1.CPU
Function : control and monitor all operations within the PLC to complete
composition : a controller, and the register operator
works : by the address data, address and control buses to the memory, input \ output interface circuit is connected
2. Memory
Type : read only type (EPRORM, EEPROM), read \ write type (RAM)
read only type: the PLC operating system stored here, curing process by the manufacturer, usually can not be modified
read \ write type (RAM): Main Used to store intermediate computing structure and data and system management.
Input \ output interface
Function : It is a bridge between the weak current signal in the PLC and the strong current signal in the industrial field.
1. Protective function : Use the internal electrical isolation circuit 1 to isolate the industrial site from the inside of the PLC.
2. Regulate the signal : Regulate different signals (such as strong and weak electrical signals) into signals that can be processed by the CPU (5V, 3.3V or 2.7V)
Input interface circuit
Composition : wiring terminal, signal conditioning and conversion circuit 2 , status display circuit, electrical isolation circuit and multiple switch module.
Types of access equipment : input signal separation signal, analog signal. When the input terminal is a discrete signal, the device type can be a limit switch, a button, etc. When the input terminal is an analog signal, the device type can be a pressure sensor, a temperature sensor, etc.
Output interface circuit
Composition : multi-channel switch module, signal latch, electrical isolation circuit, status display circuit, output replacement circuit and wiring terminals.
Output form : relay output 3 , transistor output 4 , thyristor output 5
Access equipment type : output signal can also be discrete signal and analog signal, when its output terminal is discrete signal, its access equipment can be solenoid valve coil, the LED, the contactor coil,
when its output is an analog signal, which is a flow valve access device, an analog meter, the DC drive, a flow controller.
The working principle of PLC
principle
PLC is a kind of controller that stores programs. After the user compiles the control program according to the specific control requirements of a certain object, the program is input into the PLC (or downloaded to the PLC by the computer) with a programmer to register it in the user program memory. The control function of PLC is realized by running the user program.
Way of working
主要分三个阶段：输入扫描、程序执行、输出刷新。
Input scanning : Before executing the program, the PLC scans the input terminals first, and reads all input signals into the input image register of the register-input state in sequence. This process is scanning. When PLC is running the program, the required input signal is not the information on the input terminal, but the information in the input image register. And the sampling result will not change during this working cycle, and will only change in the next scanning phase.
Program execution : After the PLC completes the scan, the program starting from address 0 will be scanned and executed one by one. The result is stored in the output image register.
Output refresh : After executing all the user programs, the PLC will send the contents of the output image register to the latch for output, drive the user equipment, and the scan time depends on the number of output modules.
Scan time : The process of the above three stages is the program scan time. It is mainly affected by three factors: 1. The clock speed of the CPU. 2. The number of I/O modules. 3. The length of the program.
- Electrical isolation devices are mainly used to isolate the mechanical or electrical input signals of the industrial site from the signals of the PLC's CPU, which can ensure that excessive electrical interference signals and surges do not enter the PLC's microprocessor and play a protective role. ↩
- Convert industrial field signals (such as strong AC220V signals) into electrical signals (weak electrical signals that can be identified by the CPU) ↩
- PLC load power supply can be DC power supply or AC power supply↩
- The PLC load power supply is a DC power supply, and its output frequency is relatively high ↩
- The PLC load power supply is an AC power supply. ↩