A PLC consists of several main parts, including a CPU, power supply, input circuits, output circuits, memory, and communication interface circuits, as shown in the figure.
PLC internal structure
I. CPU
The CPU of a PLC is actually a central processing unit, which can perform various data operations and processing, convert various input signals into input registers, and then perform logical operations, timing, counting, arithmetic operations, data processing and transmission, communication networking, and various other operations. It compiles the programmed code, executes commands, and sends the results to the output terminal to respond to various external devices.
II. Memory
The memory in a PLC system is mainly used to store system programs, user programs, and operating status data. PLC memory includes system program memory and user memory.
1. System program memory (ROM)
The system program memory (ROM) is used to store system management programs, monitoring programs, and internal system data. The PLC is pre-installed in the read-only memory (ROM) or PROM before leaving the factory and cannot be changed by the user.
2. User Memory (RAM)
RAM consists of two parts: user program memory (program area) and data memory (data area).
RAM stores various temporary data, intermediate results, and user programs. This type of memory is typically composed of low-power CMOS-RAM, and its stored contents can be read and modified. The stored contents are lost upon power failure and are generally retained by a lithium battery. In other words, user memory is used to store various programs written by the user using a PLC programming language for specific control tasks.
Depending on the type of memory selected (RAM, EPROM, or EEPROM), the user can modify and add to the program. The user data memory stores the ON/OFF status and data of the devices used by the user. The size of the user memory is related to the capacity of the user program and is an important indicator of PLC performance.
To facilitate reading, writing, and modification, PLC user programs are typically stored in CMOS static RAM, with a lithium battery maintaining a continuous power supply to ensure that the program is not lost after power failure.
The working data stored in RAM consists of data that changes frequently and is frequently accessed during PLC operation, in order to meet the needs of random access. In the PLC's working data memory, there are logic data storage areas for input/output relays, auxiliary relays, counters, timers, etc. The states of these devices are determined by the initial value settings of the user program and the running status.
Depending on the requirements, some data is maintained in its current state using a backup battery when power is lost. This storage area that can retain data when power is lost is called the data retention area.
III. Digital Input/Output Interfaces (I/O)
The input/output interface of a switch quantity is an interface for connecting to the control electrical appliances in the industrial production site.
The input/output interfaces of the digital signals employ opto-isolation and RC filtering, achieving electrical isolation between the internal and external circuits of the PLC and reducing electromagnetic interference, while also meeting the matching requirements of various signals in industrial settings.
For example, the digital input interface circuit uses an optocoupler circuit to convert the control signals from field input devices such as limit switches, manual switches, and encoders into digital signals that the CPU can accept and process.
1. Input Interface
The input interface is used to receive and collect external input signals and convert these signals into internal information that the CPU can accept.
The input interface circuit can acquire three main types of signals: active switches, passive switches, and analog signals. Buttons, contactor contacts, and limit switches are all passive switches, while proximity switches and transistor switching circuits are active switches. Analog signals are generated by potentiometers, tachogenerators, and various transmitters.
2. Output Interface
The output interface circuit is a bridge between the PLC and the external load. It can convert the PLC's output signals into control signals that can drive external circuits to control the power on and off of electrical appliances such as contactor coils.
The switching output circuit includes three output forms: relay output, thyristor output, and transistor output.
The relay output has a slow response but a large load capacity. The maximum current output of each port is 2A, and it can be connected to DC or AC loads.
Transistor outputs have a fast response but low load capacity, with each port outputting a current of tens of milliamps. They can be connected to DC loads. The equivalent circuit is shown in the figure.
IV. Power Module
The power supply module of a PLC can process external input power and convert it into DC power required for the operation of the PLC's internal circuits such as CPU, memory, and input/output interfaces.
Many PLCs use DC regulated switching power supplies, which can provide multiple independent voltages for internal use and also power external inputs (such as sensors). Depending on the model, some PLCs use single-phase AC power, usually 220V, while others use DC power, usually 24V.
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