Programmable Controller (PLC) should originally be abbreviated as PC, but to distinguish it from the personal computer's abbreviation PC, it is abbreviated as PLC (Programmable Logic Controller). However, this does not mean that a PLC can only control logic signals. PLCs are industrial field control devices specifically designed for industrial applications, featuring an intuitive, simple, and easy-to-learn programming language environment.
Basic components of a PLC
The basic components of a PLC include a central processing unit (CPU), memory, input/output interfaces (I/O, including input interfaces, output interfaces, external device interfaces, expansion interfaces, etc.), an external device programmer, and a power supply module, as shown in Figure 1. The various internal units of the PLC are connected via power buses, control buses, address buses, and data buses. Externally, appropriate devices and control units are configured according to the actual controlled object to form the PLC control system.
1. Central Processing Unit
The central processing unit (CPU) consists of a controller, an arithmetic logic unit (ALU), and registers, all integrated on a single chip. The CPU connects to memory, input/output interfaces, a programmer, and a power supply via data bus, address bus, control bus, and power bus.
Small PLCs use 8-bit or 16-bit microprocessors or single-chip microcomputers for their CPUs, such as the 8031 and M68000. These chips are very inexpensive. Medium-sized PLCs use 16-bit or 32-bit microprocessors or single-chip microcomputers, such as the 8086 and 96 series single-chip microcomputers. These chips are characterized by high integration, fast processing speed, and high reliability. Large PLCs, on the other hand, require high-speed bit-slice microprocessors.
The CPU directs the PLC control system to complete various tasks according to the functions assigned by the system program within the PLC.
2. Memory
The memory inside a PLC is mainly used to store system programs, user programs, and data.
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 stored in the system program memory. It mainly consists of system management program, user instruction interpretation program, function program and system program call.
The system management program mainly controls the operation of the PLC, ensuring that the PLC works in the correct sequence; the user instruction interpreter converts the PLC's user instructions into machine language instructions, which are then transmitted to the CPU for execution; and the function program and system program call are responsible for calling different function subroutines and their management programs.
The system program is important data that needs to be stored for a long time, so its memory uses ROM or EPROM. ROM is a read-only memory, which can only read content and cannot write content. ROM is non-volatile, meaning that it can still retain the stored content after the power is turned off.
EPEROM is an electrically erasable read-only memory. It requires ultraviolet light to illuminate the lens window on the chip to erase the written content. Other electrically erasable programmable read-only memories include 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 loaded initially needs to be modified and debugged, so it is called the user debug program. It is stored in the random access memory (RAM) that can be read and written randomly to facilitate user modification and debugging.
The modified and debugged program is called the user executable program. Since no further modification or debugging is needed, the user executable program is permanently stored in the EPROM for long-term use.
3) Data storage
During PLC operation, intermediate result data (such as status data of input/output elements, preset and current values of timers and counters, etc.) and configuration data (such as input/output configuration, setting input filtering, pulse capture, output table configuration, defining memory area hold range, analog potentiometer settings, high-speed counter configuration, high-speed pulse output configuration, communication configuration, etc.) need to be generated or called. This type 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 high-density, low-power semiconductor memory that can be powered by a lithium battery as a backup power source. In the event of a power outage, the contents of the RAM can be preserved by powering the lithium battery.
3. Interface
Input/output interfaces are the interface circuits that connect a PLC to control, detection, and execution components in the industrial field. PLC input interfaces include DC input, AC input, and AC/DC input; output interfaces include transistor output, thyristor output, and relay output. Transistor and thyristor outputs are contactless output circuits; transistor outputs are used for high-frequency, low-power loads, while thyristor outputs are used for high-frequency, high-power loads. Relay outputs are contactless output circuits used for low-frequency loads.
Various control signals, such as limit switches, operation buttons, selector switches, and other digital or analog signals output by sensors, are input to the PLC from field control or detection components. These signals are then converted into signals that the CPU can receive and process through the input interface circuit. The output interface circuit converts the low-voltage control signals sent by the CPU into high-voltage signals required by the field to drive the actuators of the controlled equipment, such as solenoid valves and contactors.
1) Input Interface
The input interface is used to receive and acquire two types of input signals: one type is the switching input signal from buttons, selector switches, limit switches, relay contacts, etc.; the other type is the continuously changing analog input signal provided by potentiometers, tachogenerators, and various converters.
Taking the DC input interface circuit as an example, R1 is a current-limiting and voltage-dividing resistor, and R2 and C form a filter circuit. The filtered input signal is coupled to the internal circuit via optocoupler T. When the input button SB is turned on, optocoupler T conducts, and the DC input signal is converted into a 5V standard signal level (TTL) that the PLC can process. Simultaneously, the LED input indicator lights up, indicating that the signal is on. Microcomputer input interface circuits generally consist of registers, gating circuits, and interrupt request logic circuits, all integrated on a single chip. AC input and AC/DC input interface circuits are similar to the DC input interface circuit.
The filter circuit eliminates input contact bounce, and the optocoupler circuit prevents strong electrical interference from the field from entering the PLC. Because the input electrical signal and the PLC's internal circuitry are coupled optically, they are completely electrically isolated, giving the input interface anti-interference capabilities. The field input signal is converted to 5V TTL after optocoupler connection and sent to the input data register, then transmitted to the CPU via the data bus.
2) Output interface
The output interface circuit outputs control signals to various actuators of the controlled object. Common actuators include contactors, solenoid valves, regulating valves (analog signals), speed control devices (analog signals), indicator lights, digital displays, and alarm devices. The output interface circuit generally consists of a microcomputer output interface circuit and a power amplifier circuit. Similar to the input interface circuit, the internal circuitry and the output interface circuitry are electrically isolated from each other using optocouplers for interference suppression.
The microcomputer output interface circuit generally integrates the output data register, gating circuit and interrupt request logic circuit on the chip. The CPU sends the output signal to the output data register through the data bus. The power amplifier circuit is used to amplify the output signal of the microcomputer to meet the requirements of industrial control.
3) Other interfaces
If the number of I/O ports in the main unit is insufficient, it can be expanded by connecting to an I/O expansion unit (without a CPU) via an I/O expansion interface cable. PLCs are also often configured with interfaces for connecting various peripheral devices, enabling functions such as serial communication and EPROM writing via cables.
4. Programmer
The function of a programmer is to download user-written programs to the PLC's user program memory, and to use the programmer to check, modify, and debug user programs, monitor the execution process of user programs, and display PLC status, internal device and system parameters, etc.
There are two types of programmers: simple programmers and graphical programmers. Simple programmers are small and portable, but can only perform online programming in statement format, making them suitable for programming and on-site debugging of small PLCs. Graphical programmers can perform programming in statement format, ladder diagram format, and also offline programming.
Currently, most PLC manufacturers have developed computer-aided PLC programming support software. When a personal computer is equipped with PLC programming support software, it can be used as a graphical programmer to edit and modify user programs. It can also realize bidirectional transmission of user programs and monitor the PLC's operating status through the communication interface between the personal computer and the PLC.
5. Power supply
The PLC power supply converts externally supplied AC power into DC power for the CPU, memory, and other components, serving as the central energy source for the entire PLC. Most PLCs utilize high-quality switching power supplies with good operational stability and strong anti-interference capabilities. Many PLC power supplies can also provide external 24V regulated DC power to supply electrical components connected to the input interfaces, thus simplifying peripheral configuration.
