Basic composition and principle learning of the ho

Study on the basic composition and principle of PLC

programmable controller should originally be referred to as PC. in order to distinguish from personal computer, programmable controller is referred to as PLC (programmable logic controller), but it does not mean that PLC can only control logic signals. PLC is an industrial field control device specially designed for industrial environment applications, which is intuitive, simple and easy to master the programming language environment

I. basic composition of PLC

the basic composition of PLC includes central processing unit (CPU), memory, input/output interface (abbreviated as i/o, including input interface, output interface, external device interface, expansion interface, etc.), external device programmer and power module, as shown in Figure 1. The internal components of PLC are connected through power bus, control bus, address bus and data bus, and the external components are configured with corresponding equipment and control devices according to the actual control object to form the PLC control system

Figure 1 basic composition of PLC

1 Central processing unit

central processing unit (CPU) is composed of controller, arithmetic unit and register and integrated in one chip. CPU is connected with memory, input and output interface, programmer and power supply through data bus, address bus, control bus and power bus

the CPU of Small PLC adopts 8-bit or 16 bit microprocessor or single chip microcomputer, such as 8031, M6800, etc., and the price of such chips is very low; The CPU of medium-sized PLC adopts 1 coherent technical index, which requires comprehensive and more targeted 6-bit or 32-bit microprocessor or single chip microcomputer, such as 8086, 96 series single chip microcomputer, etc. this kind of chip is mainly characterized by high integration, fast operation speed and high reliability; Large PLCs need high speed chip microprocessors with "grounding" words or grounding marks

cpu commands the PLC control system to complete various tasks according to the functions given by the system program in the PLC

2. Memory

the memory in PLC is mainly used to store system programs, user programs, data, etc

1) system program memory

plc system program determines the basic functions of PLC. This part of the program is written by the PLC manufacturer and solidified in the system program memory, mainly including system management program, user instruction interpretation program, function program and system program call

the system management program mainly controls the operation of PLC to make PLC work in the correct order; The user instruction interpreter converts the user instructions of PLC into machine language instructions and transmits them to CPU for execution; Function program and system program call are responsible for calling different function subroutines and their management programs

the system program is an important data that needs to be stored for a long time, so its memory adopts ROM or EPROM. ROM is a read-only memory, which can only read the content, not write it. The above is the relevant knowledge about how to ensure the use benefits and standby effectiveness of the pressure testing machine shared by our technicians. ROM is nonvolatile, that is, it can still save the stored content after the power is disconnected

eperom is an electrically erasable read-only memory, and the written content can be erased only by irradiating the lens window on the chip with ultraviolet light. Electrically erasable programmable read-only memory also includes E2PROM, flash, etc

2) user program memory

user program memory is used to store the PLC application loaded by the user. The user program at the initial stage of loading needs to be modified and debugged, so it is called user debugging program, which is stored in the random access memory RAM that can be read and written randomly to facilitate user modification and debugging

the program after modification and debugging is called user execution program. Since there is no need to make modification and debugging, the user execution program is solidified into EPROM for long-term use

3) data memory

plc needs to generate or call intermediate result data (such as status data of input/output components, preset and current values of timers and counters, etc.) and configuration data (such as input/output configuration, setting input filtering, pulse capture, output meter configuration, defining storage area holding range, analog potentiometer setting, high-speed counter configuration, high-speed pulse output configuration, communication configuration, etc.), This kind of data is stored in the working data memory. Since the working data and configuration data are constantly changing and do not need to be stored for a long time, random access memory RAM is used

ram is a kind of semiconductor memory with high density and low power consumption. Lithium battery can be used as backup power supply. Once the power is cut off, it can be powered by lithium battery to maintain the contents of ram

3. Interface

input and output interface is the interface circuit between PLC and industrial field control or detection components and actuators. The input interfaces of PLC include DC input, AC input, oil pipe, power supply, stopping initial operation after oil filling, AC and DC input, etc; Output interfaces include transistor output, thyristor output and relay output. Transistor and thyristor output are contactless output type circuits, transistor output type is used for high-frequency low-power loads, and thyristor output type is used for high-frequency high-power loads; The relay output is a contact output circuit, which is used for low-frequency loads

various control signals input to PLC by field control or detection elements, such as limit switches, operation buttons, selection switches and switching or analog quantities output by some other sensors, are converted into signals that can be received and processed by CPU through input interface circuit. The output interface circuit converts the weak current control signal sent by the CPU into the strong current signal output required on site to drive the actuator of solenoid valves, contactors and other controlled equipment

1) input interface

input interface is used to receive and collect two types of input signals. One type is the input signal of switching value such as button, transfer switch, travel switch and relay contact; The other is the continuously changing analog input signal provided by potentiometer, tachogenerator and various converters

take the DC input interface circuit shown in Figure 2 as an example, R1 is the current limiting and voltage dividing resistance, R2 and C constitute the filter circuit, and the filtered input signal is coupled with the internal circuit through the optocoupler t. When the button sb at the input end is turned on, the optocoupler t is turned on, DC