Schneider programmable controller

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Schneider Programmable Controller 27 / 10

Programmable Controller was originally called PC for short. In order to distinguish it from PC, it is called PLC for short, but it does not mean that PLC can only control logic signals. PLC is specially designed for industrial environment applications, with intuitive, simple and easy to master the programming language environment of the industrial field control device.

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 equipment interface, expansion interface, etc.), external equipment 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 objects to form the PLC control system.

1. Central processor

The central processing unit (CPU) is composed of a controller, an arithmetic unit and registers, and is integrated into a chip. CPU is connected with memory, input/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, M68000, etc. The price of such chips is very low; The CPU of medium-sized PLC adopts 16 bit or 32 bit microprocessor or single chip microcomputer, such as 8086, 96 series single chip microcomputer, etc. The main features of such chips are high integration, fast operation speed and high reliability; Large PLC needs to use high-speed bit chip microprocessor.

The CPU commands the PLC control system to complete various tasks according to the functions assigned 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

The PLC system program determines the basic functions of the 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, so that PLC can work in correct order; The user instruction interpreter converts PLC user instructions into machine language instructions and transmits them to the CPU for execution; Function program and system program call are responsible for calling different function subprograms and their management programs.

The system program belongs to important data that needs to be saved for a long time, so its memory adopts ROM or EPROM. ROM is a read-only memory, which can only read content and cannot write content. ROM is non-volatile, that is, it can still save the stored content after the power is disconnected.

EPEROM is an electrically erasable read only memory. The written content can only be erased by illuminating the lens window on the chip with ultraviolet light. There are also electrically erasable programmable read only memories such as E2PROM and FLASH.

2) User program memory

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

The program after modification and debugging is called user execution program. Since no modification and debugging is required, the user execution program is solidified into EPROM for long-term use.

3) Data storage

During PLC operation, it is necessary to generate or call intermediate result data (such as status data of input/output components, preset and current values of timers and counters) and configuration data (such as input/output configuration, setting input filtering, pulse capture, output table 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 semiconductor memory with high density and low power consumption. Lithium battery can be used as a backup power supply. Once the power is cut off, it can be powered by lithium battery to keep the contents of RAM.

3. Interface

The input and output interface is the interface circuit connecting PLC with industrial field control or detection elements and actuating elements. The input interfaces of PLC include DC input, AC input, AC/DC input, etc; Output interfaces include transistor output, thyristor output and relay output. The transistor and thyristor output are contactless output type circuits, the transistor output type is used for high-frequency small power loads, and the thyristor output type is used for high-frequency large power loads; The relay output is a contact output circuit for low-frequency load.

The field control or detection elements input various control signals to the PLC, such as limit switches, operation buttons, selection switches and switching or analog quantities output by other sensors. These signals are converted into signals that can be received and processed by the CPU through the 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 controlled equipment such as solenoid valve and contactor.