The application of programmable controller PLC characteristics and structure

Programmable logic controller is originated in the 60 s, hereinafter referred to as PLC ( 可编程逻辑控制器) 。 At present, has been widely used in metallurgy, mining, machinery, etc. Its characteristic is: 1, programming is convenient to learn 2, 3, 4 the use of convenience operation is stable and reliable, low requirements for environment with programmable controller to implement control, pictured above, the essence of which is according to certain algorithm, input/output transformation and the transformation to physical implementation. In the transformation, physical implementation can be said to be two basic points of PLC controller control. And the transformation is, in fact, information processing, information processing is the most commonly used microprocessor technology, physical implementation requires the PLC input should eliminate interference signal is adapted to the industrial field. Output should be amplified to the level of industrial control, for the actual control system easy to use. This requires that the I/O circuit is designed. The analysis according to the essential points of the PLC controller control, PLC adopts the typical computer structure, mainly by the CPU, RAM, ROM and specially designed input and output interface circuit, etc. CPU is the brains of the PLC, it as shown by the central processor ( CPU) And storage, etc. 1, the central processor ( CPU) The central processor ( CPU) Generally consists of control circuit, logic unit and register, these circuits are integrated on a chip. The CPU through the address bus, data bus and control bus and storage unit. Input/output ( I / O) Interface circuit is connected. It mainly completed the following functions: ( 1) The input signal is sent to PLC stored. ( 2) Deposit in order to take out the user instruction, to compile. ( 3) Complete the user instructions provided various operations. ( 4) Send the results to the output side. ( 5) Response to a variety of peripheral devices ( If change, Cheng Qi, printer, etc. ) The request. 2, PLC internal storage memory has two kinds: one kind is the RAM ( The random access memory) , can at any time by the CPU to read, write for it; Another kind is a ROM ( The read only memory) , the CPU only read but not written. RAM is mainly used to store data, the intermediate result of staging and users are debugging program, ROM main storage monitoring program and the user has debugging good, these procedures are prior to burn in the ROM chip, after startup to run the program. 3, input and output interface circuit it plays the role of the communication between PLC and peripheral equipment. User equipment needs input of PLC controller control signals, such as limit switches, buttons and other operation or analog sensor output switch ( Through modulus transform into the machine) And so on, through the input interface circuit can convert these signals into the CPU signal receiving and processing. Output interface circuit for the weak current signal processor output high voltage signal into the scene need to drive the solenoid valve, contactor is accused of equipment such as actuators. The structure of the commonly used interface circuit as shown in the figure below. Input/output interface circuit in order to guarantee the normal work of the circuit, the interface circuit has the following characteristics; 1) Input adopts photoelectric coupling circuit, 2) can greatly reduce the electromagnetic interference (emi) Output also USES photoelectric isolation and there are three ways, namely, relay, transistors and brake pipe. This makes the PLC can be suitable for all kinds of different requirements of users. Such as low speed, high power load generally adopts relay output; Will be used on high speed large power thyristor output; High speed low power transistor output available. And power amplification output circuit, etc. , and made some output circuit module type, pluggable, change rise very convenient.
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