1. Introduction Programmable Controller (PC) was commonly abbreviated as PC in the 1970s and 80s. With the rise of personal computers in the 1990s, it was also commonly abbreviated as PC. Furthermore, the concept of "programmable" encompassed a broad range of topics, so AB (Allied Power Systems) first officially named it Programmable Logic Controller (PLC). For convenience, it is still commonly abbreviated as PLC. Some refer to systems composed of PLCs as PCS (Programmable Logic Controller System), emphasizing that PLC manufacturers provide complete systems. 2. PLC Development and Market Situation 2.1 PLC Development History In 1968, GM (General Motors) proposed replacing relay control devices. The following year, Digital Automation Systems developed the first generation of programmable controllers, meeting the requirements of GM's assembly lines. With the development of integrated circuit and computer technologies, fifth-generation PLC products are now available. In manufacturing industries characterized by altering geometry and mechanical properties, and process industries characterized by physical and chemical changes that transform raw materials into products, besides feedback control primarily based on continuous quantities, there is a significant amount of open-loop sequential control, especially in manufacturing, primarily based on switching quantities. This sequential action is performed according to logical conditions and time sequences. Additionally, there are controls that perform interlocking protection actions based on logical relationships, independent of sequence and timing. Furthermore, there is a large amount of discrete data acquisition and monitoring, primarily based on state quantities such as switching quantities, pulse quantities, timers, counters, and analog quantity over-limit alarms. Due to these control and monitoring requirements, PLCs have developed into products primarily used to replace relay circuits and perform sequential control. Over many years of production practice, a three-way balance has gradually formed between PLCs, DCSs, and IPCs, as shown in Table 1. Other single-loop intelligent regulators also hold a certain percentage of the market share. [align=center]Table 1 Comparison of PLC, DCS, and IPC Sales in the International Market[/align] The period from the 1980s to the mid-1990s was the fastest period of PLC development, with an annual growth rate consistently maintained at 30-40%. Due to advancements in PLC's analog processing capabilities and networking functions, it has squeezed out a portion of the DCS (process control) market and gradually monopolized industries such as wastewater treatment. However, the emergence of Industrial PCs (IPCs), especially the development of fieldbus technology in recent years, has also led to IPCs and FCSs encroaching on the PLC market. Therefore, the growth rate of PLCs has generally slowed down in recent years. Currently, there are over 200 manufacturers worldwide producing more than 300 varieties of PLC products, mainly used in industries such as automotive (23%), grain processing (16.4%), chemical/pharmaceutical (14.6%), metal/mining (11.5%), and pulp/paper (11.3%). 2.2 PLC Market Situation There are approximately 30 PLC manufacturers in China, but none have formed a large-scale production capacity or branded products. Some are produced through imitation, assembly of supplied parts, or "OEM" manufacturing. Therefore, it can be said that PLCs have not yet formed a manufacturing industry in my country. As a product with no cutting-edge technical difficulties in principle, technology, or process, it is possible to form a manufacturing industry with effort. In terms of PLC application, my country is very active. In recent years, about 100,000 new PLC products have been invested annually, with annual sales of RMB 3 billion. The application industries are also very wide. However, compared with other countries, the application in mechanical processing and production lines still needs to be increased. The following PLC products are popular in the Chinese market: (1) Schneider Electric, including the products introduced by Tianjin Instrument Factory in the early days from Modicon, currently has Quantum, Premium, Momentum and other products; (2) Rockwell Automation (including AB) PLC products, currently have SLC, Micro Logix, Control Logix and other products; (3) Siemens products, currently have SIMATIC S7-400/300/200 series products; (4) GE products; products from Japanese companies such as Omron, Mitsubishi, Fuji, and Panasonic. The market potential of PLC is huge. Not only in my country, but even in industrially developed Japan, surveys show that the proportion of mechatronics products supporting PLC accounts for 42%, and the proportion of relay and contactor control is still 24%. Therefore, there are still many applications requiring PLCs, especially in China. From the perspective of technological innovation, large and medium-sized enterprises in China still need to vigorously develop CIMS (Computer Integrated Manufacturing System), and FMS (Flexible Manufacturing System) needs to be formed in machinery manufacturing plants. PLCs are the foundation, so the PLC market is vast. PLCs have the advantages of stability and reliability, low price, complete functions, flexible and convenient application, and convenient operation and maintenance. This is the fundamental reason why they can maintain their market share for a long time. We will focus on several issues below and study their development trends. 3 PLC Hardware and Software 3.1 Classification of PLCs In the 1990s, PLCs were already divided into micro, small, medium, large, and giant types. Based on the number of I/O points, PLCs can be categorized as follows: * Micro PLC: 32 I/O * Small PLC: 256 I/O * Medium PLC: 1024 I/O * Large PLC: 4096 I/O * Mega PLC: 8195 I/O In recent years, large systems supporting 300 loops and 65,000 I/O points have emerged, corresponding to medium-sized PLCs and above. These systems all utilize 16-bit to 32-bit CPUs. Micro and small PLCs, originally using 8-bit CPUs, are now sometimes modified to use 16-bit to 32-bit CPUs to meet communication requirements. Since PLCs with fewer than 64 I/O points account for 47% of total PLC sales, and those with 64 to 256 I/O points account for 31%, totaling 78% of total PLC sales, micro and small PLCs deserve further research. 3.2 PLC Hardware The hardware of the PLC controller itself adopts a modular structure, and the structures of products from different manufacturers are largely similar. Taking the Omron C200HE from Japan as an example, it features a bus-based module frame structure. The CPU module is mounted on the basic frame (CPU motherboard), while other slots hold I/O modules. If there are multiple I/O modules, the CPU motherboard can be connected to an I/O expansion motherboard via I/O expansion cables, and I/O modules can be mounted on it. Another method is to equip it with remote I/O slave stations. This demonstrates that PLC manufacturers are developing all hardware components for users, facilitating selection and configuration into PLCs of varying sizes. This openness in hardware configuration brings significant convenience to manufacturers, distributors (agents), system integrators, and end users, greatly benefiting the marketing supply chain – a highly successful approach. In addition to general DI/DO and AD/DA modules, PLC I/O modules have evolved to include a series of special-function I/O modules. This has opened up possibilities for PLC applications across various industries, such as ASCII/BASIC modules for barcode recognition, PID modules for feedback control, high-speed counting modules for operation control and machining, single-axis position control modules, dual-axis position control modules, cam positioner modules, and RFID interface modules. Further development is expected in these areas. Furthermore, improvements will be made in input/output components, input/output electrical isolation, and ground isolation in environments with strong interference. The CPU and memory in a PLC work together to complete control functions. Unlike DCS systems that handle parameters such as temperature, pressure, and flow, PLCs employ a fast cyclic scanning cycle, typically 0.1–0.2 seconds, with faster systems using 50 ms or less. It is a digital sampling control system. 3.3 To implement control strategies and replace relays, PLC software uses a set of control algorithm function blocks (or subroutines) to allow users to create ladder diagrams for control systems similar to relay circuits. This set, called the instruction system, is stored in ROM and can be called by the user when developing application programs. The instruction system can be broadly divided into two categories: basic instructions and extended instructions. A typical PLC instruction system includes: basic instructions, timer/counter instructions, shift instructions, transfer instructions, comparison instructions, conversion instructions, BCD arithmetic instructions, binary arithmetic instructions, increment/decrement instructions, logical operation instructions, and special arithmetic instructions. These instructions are mostly similar to assembly language. Furthermore, PLCs provide ample internal resources such as timers, counters, internal relays, registers, and storage areas, greatly facilitating programming. 4. Windows-based Programming Language Standard—IEC 61131-3 Due to differences in instruction sets among PLC manufacturers and varying user requirements for programming methods, the IEC has recently developed the Windows-based programming language standard IEC 61131-3 (the international standard for programmable logic controllers, IEC 1131, was issued by the IEC in 1993). It specifies five programming languages: Instruction List (IL), Ladder Diagram (LD), Sequential Function Chart (SFC), Function Block Diagram (FBD), and Structured Text (ST). This encompasses both text-based programming (IL, ST) and graphical programming (LD, FBD), while SFC can be used in both types of programming languages. The IEC Technical Committee (TC65) recently initiated the IEC 61499 project, extending IEC 61131-3. This standard addresses the architecture of modular distributed systems interconnected through communication networks, improving upon IEC 61131-3. This represents a high-level standardization of programmable logic control devices based on digital technology towards openness, a major trend in PLC development. 5. PLC Networking and Development Trends A system where one or more PLCs are connected to a PC, with the PC acting as both a programmer and a human-machine interface (HMI) operator station, was a new trend in the 1990s. This created business opportunities for system integration, and simultaneously, programming software, HMI software (monitoring software or configuration software), and software interfaces (or driver software) also developed. In recent years, PLC manufacturers have gradually added various communication interfaces to the existing CPU modules that provide physical layer RS232/422/485 interfaces, and have also provided complete communication networks. Due to the rapid development of data communication technology and the strong demand for openness from users, fieldbus technology and Ethernet technology have also developed in tandem. Therefore, the openness of PLC-based PCS systems is slightly better than that of DCS systems. Currently, Rockwell Automation has formed a multi-layered architecture, namely EtherNet, ControlNet, DeviceNet, and ASIC fieldbuses (the original DH+ network is also compatible). In addition to the Profibus-DP and Profibus-FMS communication networks, Siemens proposed the S7 Routing network, a two-layer structure of Profibus-DP and Industrial Enternet. Network development is ongoing, and my country should actively participate in it. In 2001, my country's machinery industry became a new highlight of industrial development, with total output value increasing by 17.15% year-on-year. Automobile production ranked among the top 10 in the world, and machine tool production ranked 5th. Machinery industry profits increased by 33.35%, accounting for more than 60% of the total increase in industrial profits. Exports also saw encouraging growth. Now, the machinery industry is proposing to implement networking. PLC professionals should have a clear understanding of this and focus on the openness of networking, the performance/price ratio of network structure, and the reliability, security, and advancement of the network. Uplinking to the internet is a problem, while downlinking is a problem of fieldbus. Furthermore, whether existing networks can use Ethernet ("e-network to the end"), whether they adopt client/server, browser/server, producer/consumer, or OPC interface software all require further clarification. PLCs can be integrated with intelligent MCC motor control centers, NC/CNC numerical control equipment配套 with CNC machine tools, and other operation control systems, electrical control equipment, frequency converters, and soft starters to form a system. PLCs need to collaborate with DCS, acting as remote I/O stations for DCS; they also need to collaborate with IPCs, serving as I/O components for software PLCs in addition to being used as human-machine interfaces. Furthermore, the relationship between PLCs and emergency shutdown systems (ESD), automated warehouses, robots, CAD/CAM, etc., must be well-managed. In short, PLCs must be compatible with various new technologies to truly become "computers." The application areas of PLCs are broad, and many fields urgently need to be explored, such as customs vehicle certification (Shenzhen Yantian) and automated drug vending (several traditional Chinese medicine stores), which already have examples in my country. Furthermore, PLCs can be used in discrete event systems, such as highway traffic flow (vehicle counting, passenger counting, and dwell time measurement), logistics systems, flexible manufacturing systems (agile manufacturing systems), and all non-standard on-demand service systems, for modeling, countermeasures, and optimization. The future of PLCs is bright, and all pessimistic arguments are untenable. As for technological advancements, the integration of PLCs with other technologies, and even their eventual disappearance, that will still take time!