Differences between MCU and PLC

Comparison between MCU and PLC

To understand the similarities and differences between MCU and PLC, we must first clarify what is MCU and what is PLC. In this regard, it may be helpful for us to briefly review the development history of computers. According to the original definition of computer experts, the computer system consists of five parts: control unit (CU), arithmetic operation unit (ALU), memory, input device (Input), and output device (Output). The CU or ALU of early computers (transistor or integrated circuit, excluding vacuum tubes) consisted of one or more circuit boards. The CU and ALU were separate. With the improvement of integration, the CU and ALU were combined to form a central processing unit (CPU). Then, the CPU was integrated into a single integrated circuit to produce an MPU or MCU, and MPUs such as Intel4004, 8008, 8080, 8085, 8086, 8088, and Z80 appeared. Since then, the development of MPU has produced two branches. One branch is developing towards high performance, high speed and large capacity. Typical chips include Intel80186, 286, 386, 486, 586, P2, P3, P4, etc., with speeds ranging from 4.7MHz to the current 3.2GHz. The other branch is developing towards multi-functions, integrating all memories (ROM, PROM, EPROM, EEPROM, FLASH ROM, SRAM, etc.) and input/output interfaces (Timer/Counter, PWM, ADC/DAC, UART, IIC, SPI, RTC, PCA, FPGA, etc.) into one integrated circuit to become SOC (System On a Chip). In my humble opinion, this is the single-chip computer widely used today, referred to as a single-chip microcomputer. This branch can be said to have a wide variety of varieties, with bit widths ranging from 8 bits to 32 bits, pin counts ranging from 6 to hundreds, operating frequencies ranging from tens of KHz to hundreds of MHz, and architectures including both CISC and RISC, which are countless. Commonly used ones include MCS-51 series, MCS-96 series, PIC series, AVR series, ARM7/9 series, TMS320 series, MSP430 series, many MOTOROLA microcontrollers and so on.

At this point, we can divide the development of computer core processors into three stages: board-level CPU, chip-level MPU and SOC.

What is PLC? The full name of PLC is Programmable Logic Controller. When it was first introduced to China, it was abbreviated as PC. Later, IBM-PC was widely used, and PC became synonymous with personal computer, and then it was changed to PLC. PLC also has another meaning, Power Line Carrier.

PLC is a product, but this product is a bit special. Before the control program is downloaded, it does not have any control function, that is, a PLC without an application is useless. PLC is actually a universal control platform designed for industrial environments. It must be secondary developed to achieve the ultimate control purpose. Therefore, it also requires the cooperation of program editing/debugging software.

PLC is an intelligent product. What solution should be used for its core controller? Board-level CPU is definitely not an option. MPU also needs several integrated circuits. For example, Z80 MPU requires Z80MPU, PIO, CTC, SIO, EPROM, SRAM, etc. These integrated circuits are installed on a circuit board. This is an early single-board computer. This solution is too large and not suitable for modern requirements. Therefore, it is most appropriate to use a single-chip microcomputer as the core controller of PLC.

From this we can conclude that:

1.PLC is a product based on a single-chip microcomputer, which is an integrated circuit. The two are not comparable. [page]

2. Single-chip microcomputers can form a variety of application systems, from micro, small to medium and large. PLC is a special case of single-chip microcomputer application system.

3. PLCs from different manufacturers have the same working principle, similar functions and indicators, certain interchangeability, guaranteed quality, and programming software is moving towards standardization. This is the basis for PLCs to be widely used. However, single-chip microcomputer application systems are like the Eight Immortals crossing the sea, each showing their magical powers, with a wide range of functions and uneven quality, making it difficult to learn, use and maintain.

Finally, from an engineering perspective, let’s talk about the selection of PLC and single-chip microcomputer systems;

1. For single projects or projects with very few repetitions, adopting PLC solution is a wise and quick way, with high success rate, good reliability, and less trouble, but the cost is higher.

2. For large-scale supporting projects, the use of single-chip microcomputer systems has the advantages of low cost and high efficiency, but this requires considerable R&D strength and industry experience to operate efficiently. The best method is to embed the single-chip microcomputer system into the PLC function, which can greatly simplify the development time of the single-chip microcomputer system, ensure performance, and ensure efficiency.

The Difference Between PLC and MCU

Seeing netizens discussing the difference between PLC and MCU, I would like to say a few words: PLC is actually a ready-made single-chip (MCU has a wide range of) system.

You can think of the ladder diagram of PLC as a programming language like assembly and other computer languages, but the scope of use is different! And the usual practice is that the PLC software converts your ladder diagram into C or assembly language (determined by the CPU used by the PLC), and then compiles it into machine code using the assembly or C compilation system! PLC runs only a few machine codes. The ladder diagram just makes it easier for users to use.

Similarly, MCS-51 microcontrollers can also be used for PLC production, but the 8-bit CPU is not capable of some advanced applications such as: large-scale calculations (including floating-point calculations), embedded systems (UCOS can now be ported to MCS-51), etc. An industrial system used by our company is made using MCS-51 microcontrollers, but with the addition of DSP, it can already meet our requirements (our equipment is slow and mainly logic control, but the number of points is not small, 128 points of I/O!!), and it also uses ladder diagram programming. We convert our ladder diagram into C51 and then use KEIL's C51 to compile it. Have you noticed that different models of PLCs use different CPUs?!

Of course, you can also use a single-chip microcomputer to directly develop a control system, but the requirements for developers are quite high (not something that ordinary people can do), the development cycle is long, and the cost is high (for some larger systems, you need to do experiments, and printed circuit boards require a considerable amount of money. You can say that you use simulators and experimental boards to develop, but I want to tell you that by doing so, you only verify the feasibility of hardware and software, and it does not mean that it can be used in industrial control systems, because industrial control systems have very high requirements for anti-interference, stability first, not performance first, so your circuit board design must be constantly experimented and improved). When you solve the above problems, you will find that you have made a PLC. Of course, if you need others to use it easily, you also need a set of software, so you don’t need to tell others about your circuit (you can’t tell others either).

The above are just some of my humble opinions. Please correct me if I am wrong. But please don’t hit me or scold me! I just want to discuss with you all!!!

Many people think that PLC is very mysterious, but in fact, PLC is very simple. Its internal CPU is fast, but its other functions are not as good as ordinary single-chip microcomputers. Usually PLC uses a 16-bit or 32-bit CPU, with 1 or 2 serial channels to communicate with the outside world, and an internal timer is sufficient. If you want to improve reliability, adding a watchdog timer is enough.

The key technology of PLC lies in the fact that it has a program that can interpret the ladder diagram language and an auxiliary communication program. The efficiency of the ladder diagram language interpretation program determines the performance of the PLC, and the communication program determines the difficulty of the PLC exchanging information with the outside world. For simple applications, it usually operates as an independent controller, without exchanging information with the outside world, and only needs to have a program that can interpret the ladder diagram language internally. In fact, the main work of designing a PLC is to develop a program that interprets the ladder diagram language.