The difference between a PLC and a microcontroller is:
1. PLC is a relatively mature control system composed of single-chip microcomputers. It is a single-chip microcomputer application system that has been debugged and stabilized, and has strong versatility.
2. Microcontrollers can be used to construct a wide variety of application systems, and their application range is much wider. However, a "microcontroller" is just an integrated circuit; it must be combined with other components and software to form a system for it to be used.
3. From the perspective of engineering applications, for single projects or projects with very few repetitions, PLC is quick and convenient, has a high success rate and good reliability, but the cost is relatively high.
4. For large-scale supporting projects, using a microcontroller system has the advantages of low cost and high efficiency, but this requires considerable R&D strength and industry experience to ensure system stability.
In essence, a PLC is a pre-built microcontroller system (microcontrollers cover a wide range).
However, PLCs also have their own characteristics: PLCs widely use ladder diagrams instead of computer languages, which offers certain advantages for programming. You can think of ladder diagrams as a programming language, just like assembly language or other computer languages, only with a different scope of application! The usual practice is for the PLC software to convert your ladder diagram into C or assembly language (depending on the CPU used by the PLC), and then use an assembler or C compiler to compile it into machine code! The PLC simply runs the machine code. Ladder diagrams are merely designed to make the PLC easier for users to operate.
As mentioned above, the MCS-51 microcontroller can certainly be used to build a PLC. However, an 8-bit CPU is somewhat inadequate for some advanced applications, such as large-scale calculations (including floating-point operations) and embedded systems (UCOS can now be ported to MCS-51). Adding a DSP, however, is sufficient for most requirements. Furthermore, since ladder logic programming is still used, we can convert the ladder diagram to C51 and then compile it using KEIL's C51 compiler. We can also observe that different PLC models use different CPUs, which illustrates that a PLC is essentially a pre-built microcontroller system.
Therefore, it's certainly possible to develop a control system directly using a microcontroller. However, this requires a very high level of skill from the developer (not something that can be done by the average person), has a long development cycle, and is costly. (For larger systems, you need to conduct experiments, and the printed circuit board alone requires a considerable expense. You might say you can use a simulator or experimental board for development, but I must tell you that doing so only verifies the feasibility of the hardware and software; it doesn't mean it can be used in industrial control systems. This is because industrial control systems have extremely high requirements for interference resistance; stability is paramount, not performance. Therefore, your circuit board design must be constantly experimented with and improved.) Once you've solved the above problems, you'll find that you've already built a PLC. Of course, if you want others to be able to use it easily, you'll also need a user software, so you don't need to tell others about your circuitry. And you certainly wouldn't.
From this perspective, PLCs aren't actually that mysterious. Many PLCs are quite simple; their internal CPUs, aside from being faster, are less functional than ordinary microcontrollers. Typically, PLCs use a 16-bit or 32-bit CPU with one or two serial channels for external communication. An internal timer is usually sufficient; adding a watchdog timer to improve reliability solves the problem.
Furthermore, the key technology of a PLC lies in its internally embedded program that interprets ladder logic and its auxiliary communication program. The efficiency of the ladder logic interpreter determines the PLC's performance, while the communication program determines the ease with which the PLC exchanges information with the outside world. For simple applications, it typically operates as an independent controller, requiring no external information exchange; only the internal program that interprets ladder logic is needed. In fact, the main task in designing a PLC is developing the ladder logic interpreter program. Modern microcontrollers can completely replace PLCs. While older microcontrollers were inferior to PLCs in terms of stability and electromagnetic interference resistance, modern microcontrollers have achieved high stability and strong anti-interference capabilities, and have already replaced PLCs in some fields.
