The development journey from microcontrollers to PLCs and back to (more advanced) microcontrollers/embedded systems in automation control is driven by the balance and rebalancing of cost, reliability, usability, and performance at different historical periods.
This journey can be clearly divided into three stages:
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First Stage: Early Origins – The Pioneer Era of Microcontrollers (MCUs)
· Time: Early 1970s and before
· Technical Status:
· The prototype of the microcontroller (Microcontroller Unit, MCU) emerged. Early computer control was based on simple-function microprocessors (CPUs) or bit-slice computers, requiring external memory, I/O interfaces, timers, and other chips, leading to complex circuit designs.
· Engineers had to use assembly language or early high-level languages (like the newly born C language) to write low-level hardware driver code, which required a high level of electronic and software expertise.
· Pain Points:
1. High development difficulty: Requires deep electronic hardware knowledge, with tight coupling between software and hardware.
2. Poor reliability: Weak anti-interference capability, complex industrial environments (such as electromagnetic interference, high temperatures, dust) easily lead to system crashes or malfunctions.
3. Difficult maintenance: Each project is “customized,” and once functionality needs to be modified, it may require redesigning the circuit and rewriting the code, making it maintainable only by the original developers.
Summary: Microcontrollers provided the possibility of flexibility and low cost, but the technology level and development model at that time could not meet the stringent requirements for reliability and usability in industrial control.
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Second Stage: Development and Maturity – The Rise and Dominance of PLCs
· Time: Late 1970s – 2000s
· Birth Opportunity: In 1968, General Motors (GM) proposed the famous “GM Ten Points” bidding requirements to address the enormous costs of redesigning and rewiring relay control cabinets every time the production line changed models. The core ideas were:
1. Easy to program and debug on-site.
2. Easy to maintain and replace modules.
3. Reliability higher than relay systems.
4. Ability to communicate with central data systems.
5. And more…
· Based on these requirements, the Programmable Logic Controller (PLC) was born.
· Technical Essence:
· A PLC is essentially a highly customized, hardened computer system designed for industrial environments. Its core is one (or more) high-performance microcontrollers or microprocessors.
· However, it does not present complex electronic circuits but rather:
· A robust hardware enclosure: Resistant to electromagnetic interference, wide temperature operation, shock and dust proof.
· Modular design: Power supply, CPU, digital/analog I/O, communication modules, etc., are pluggable, making maintenance and expansion easy.
· Dedicated programming language: Created the Ladder Diagram programming language, which greatly caters to the habits of electrical engineers (it looks like traditional relay control circuit diagrams), significantly lowering the programming threshold.
· Real-time reliable operating system: Designed specifically for scanning cycles (Input – Process – Output), ensuring deterministic response times and extremely high reliability.
· Impact:
· PLC perfectly solved all the pain points of the first stage, especially reliability and usability.
· It quickly became the absolute leader in the field of industrial automation control, ubiquitous from automotive manufacturing, chemical industry to assembly line packaging.
· This stage is a classic case of “specialization defeating generalization.” PLCs traded off some extreme cost and flexibility (due to higher hardware costs) for unparalleled reliability and development efficiency.
Summary: PLCs are not a replacement for microcontrollers but rather a “black box” solution for the industrial field built on microcontroller technology through engineering, standardization, and software innovation.
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Third Stage: Integration and Return – The “Revenge” of Microcontrollers/Embedded Systems
· Time: 2000s to present
· Technical Driving Forces:
1. Leap in microcontroller performance: The processing power, integration (SoC), peripheral richness (integrated ADC, DAC, high-speed communication interfaces), and reliability of microcontrollers have vastly improved. A modern 32-bit ARM Cortex-M core MCU’s performance far exceeds that of early PLC CPUs.
2. Improvement of development tools: Mature real-time operating systems (RTOS like FreeRTOS, Zephyr), rich software libraries, hardware abstraction layers (HAL), and graphical configuration tools (like STM32CubeMX) have greatly reduced the difficulty of developing reliable embedded systems.
3. Emergence of new demands: The explosive growth of the Internet of Things (IoT), intelligent edge computing, miniaturization, and cost-sensitive application scenarios (like smart homes, consumer electronics, small devices). These areas often do not require all the powerful functions of PLCs but are extremely sensitive to cost, size, and power consumption.
· Current Status:
· High-end: PLCs continue to develop towards large, complex, and highly reliable applications (like process control, large production lines), with many core CPUs being high-performance x86 or ARM processors.
· Low-end penetration/integration:
· Soft PLC: The runtime software of PLCs is ported to general industrial computers (IPC) based on Intel or ARM, where the hardware is general-purpose, and the software provides the programming and reliability environment of PLCs.
· Embedded PLC/Intelligent Controllers: Many manufacturers are beginning to offer compact, low-cost PLCs based on high-performance microcontrollers (like ARM Cortex-M series). At the same time, engineers can also choose to develop customized controllers directly using microcontrollers + RTOS, achieving lower costs and more flexible designs than standard PLCs while meeting the required reliability.
· IoT Gateways: Many IoT gateways are fundamentally microcontrollers, responsible for data collection and network transmission, while also executing local logic control.
Summary: This is not a simple return but a spiral ascent. Modern microcontroller solutions, with their powerful performance and modern development tools, are eroding the low-end and specific application markets of traditional PLCs. The boundaries between the two are becoming blurred, forming a situation where “the inside of PLCs is microcontrollers, and the outside of microcontrollers can mimic PLCs.”
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Summary Comparison
Features Early Microcontrollers Traditional PLCs Modern Microcontrollers/Embedded Systems
Core Simple MCUs/MPUs Specialized MCU/MPU systems High-performance, highly integrated MCUs/MPUs
Reliability Low Extremely high High (can reach near PLC levels through design)
Development Difficulty Extremely high (hardware + low-level software) Low (graphical logic language) Medium (depends on modern toolchains)
Cost Low chip cost, high overall cost High hardware cost, low overall cost Extremely low cost (especially in large quantities)
Flexibility High Low Extremely high
Applicable Scenarios Consumer electronics, simple control Complex industrial environments, mainstream automation IoT, edge computing, consumer, customized devices
This development journey is a classic case of technological evolution from general to specialized, and then to a high-level integration of specialization and generalization. The success of PLCs lies in their engineering approach that perfectly packages complex technology, making it user-friendly and reliable. The “return” of microcontrollers is attributed to the maturity of their own technology, allowing them to maintain low costs and high flexibility while also meeting considerable reliability requirements. Today, the two are no longer in a simple replacement relationship but coexist and permeate each other in different niche markets based on application scenarios, cost budgets, development resources, and reliability requirements.