I. Introduction A fully digital control system based on fieldbus replaces existing analog signal cables with a high-capacity fieldbus network, significantly reducing the cost and workload of field signal cable connections and improving signal transmission efficiency. In essence, a fieldbus control system is a platform integrating system programming, configuration, maintenance, and monitoring functions, with fieldbus-based intelligent I/O or intelligent sensors and intelligent instruments as the control body and a computer as the monitoring and command center. II. Performance Characteristics of Intelligent Sensors Conventional sensors can only function as sensing elements, requiring conversion instruments to detect changes in physical or chemical quantities. With the development of microelectronics technology, intelligent instruments have emerged. Intelligent instruments utilize very large-scale integrated circuits and embedded software to coordinate internal operations. In addition to performing nonlinear compensation, zero-point error correction, temperature compensation, and fault diagnosis of input signals, they can also control industrial processes, further distributing the functions of the control system. Intelligent sensors integrate all the functions of sensors and intelligent instruments, as well as some control functions, exhibiting high linearity and low temperature drift, reducing system complexity and simplifying system structure. The features are as follows: 1. A certain degree of artificial intelligence, combining hardware and software, enables learning functions and better reflects the role of the instrument in the control system. It can select appropriate solutions according to different measurement requirements, comprehensively process information, and predict system status. 2. Multi-sensitivity integrates previously scattered and independent single-sensitivity sensors into a multi-sensitivity sensor, capable of simultaneously measuring multiple physical and chemical quantities, comprehensively reflecting the overall information of the measured quantity. 3. High accuracy and wide measurement range: It can detect the influence of changes in the measured quantity on the characteristics of the sensing element at any time and complete various calculations, resulting in more accurate output signals. Its range ratio can reach 100:1, and up to 400:1, allowing a single intelligent sensor to handle a wide measurement range, especially suitable for control applications requiring a large range ratio. 4. Communication function: It can use a standardized bus interface for information exchange, which is one of the key characteristics of intelligent sensors. The emergence of intelligent sensors transforms complex signal processing from centralized to decentralized, ensuring data processing quality and improving anti-interference performance while reducing system costs. It has enabled sensors to evolve from single-function, single-detection devices to multi-functional and multi-variable detection devices, from passively converting signals to actively controlling and processing information, and from isolated components to systematized and networked devices. III. Fieldbus Architecture and Characteristics According to the IEC/ISA definition, a fieldbus is a digital, bidirectional, multi-branch communication network connecting intelligent field devices and automation systems. It is an interconnection network for the lowest-level field devices and field instruments in process automation, integrating field communication networks and control systems. Fieldbus brings the concepts of network communication and management to the control field, representing a future direction for the development of automation control architecture. Fieldbus is an architecture system based on the ISO OSI model, simplified according to actual needs. It generally includes a physical layer, a data link layer, an application layer, and a user layer. The physical layer connects upwards to the data link layer and downwards to the transmission medium. The physical layer specifies the transmission medium (twisted pair, wireless, and fiber optic), transmission rate, transmission distance, signal type, etc. During transmission, the physical layer encodes and modulates the data stream from the data link layer. During reception, it demodulates and decodes the received data using appropriate control information from the medium and sends it to the data link layer. The data link layer is responsible for executing bus communication rules and handling error detection, arbitration, scheduling, etc. The application layer provides a simple interface for end-user applications, defining how to read, write, interpret, and execute a message or command. The user layer is essentially application software for data or information queries, specifying standard function blocks, object dictionaries, and device descriptions, providing users with an intuitive and simple user interface. In addition to its advantages such as one-to-N structure, interchangeability, interoperability, distributed control functions, interconnected networks, and convenient maintenance, fieldbus also has the following characteristics: 1. Simple network architecture: Its structural model generally has only 4 layers. This simplified architecture offers advantages such as flexible design, intuitive execution, low cost, and good performance, while also ensuring communication speed. 2. Integrated automation functions: Field intelligent devices are treated as network nodes, and information transmission and communication between nodes and between nodes and the management layer are achieved through the fieldbus, facilitating the implementation of various complex integrated automation functions. 3. Strong Fault Tolerance: Fieldbus significantly improves the system's fault tolerance through fault detection methods such as error detection, self-verification, supervised timing, and shielded logic. 4. Enhanced Anti-interference Capability and Measurement/Control Accuracy: Field intelligent devices can process signals locally and exchange information with the main control system using digital communication. This not only provides strong anti-interference capabilities but also greatly improves accuracy and reliability. These characteristics of fieldbus not only ensure its complete adaptability to the current industrial requirements for digital communication and traditional control but also make it possible to implement complex, advanced, and optimized control functions at different levels. IV. Fieldbus Control System (FCS) With the continuous development of complex process industries, industrial process control has placed newer and higher demands on the acquisition, transmission, and data conversion of large amounts of field signals, as well as on accuracy, reliability, and integrated control. Existing DCS systems can no longer meet these requirements; moreover, existing DCS systems have drawbacks such as incomplete decentralized control, relatively concentrated faults, incomplete system openness, and high costs. Therefore, through the integration of digital communication technology, sensor technology, and microprocessor technology, the traditional hybrid system of digital and analog signals has been transformed into a fully digital signal system, resulting in a new generation of control systems, FCS. 1. Smart sensors and fieldbus are two important components of FCS. FCS uses fieldbus to establish a highly reliable data communication line in the control field, enabling data communication between smart sensors and between smart sensors and the main control unit, turning individual, distributed smart sensors into network nodes. Data processing in smart sensors helps reduce the workload of the main control station, enabling large-scale information processing locally, reducing information back-and-forth between field instruments and the main control station, and lowering the requirements for network data communication capacity. Data preprocessed by smart sensors is collected on the host via fieldbus for more advanced processing (mainly system configuration, optimization, management, diagnosis, fault tolerance, etc.), allowing the system to analyze and judge the controlled object from a surface-to-point and then from point-to-surface perspective, improving the system's reliability and tolerance. In this way, FCS connects various smart sensors into a network system and control system that can communicate with each other and jointly complete control tasks, better reflecting the "centralized information, decentralized control" function of DCS, and improving the accuracy, real-time performance, and speed of signal transmission. Based on fieldbus technology, with a microprocessor at its core and digital communication as its transmission method, fieldbus intelligent sensors, compared to general intelligent sensors, require the following functions: Sharing a single bus for information transmission, possessing multiple calculation, data processing, and control functions, thereby reducing the burden on the host computer. Replacing 4-20mA analog signal transmission, realizing the digitization of transmitted signals, and enhancing signal anti-interference capabilities. Adopting a unified networking protocol, becoming a node in the FCS, realizing information exchange between sensors and actuators. The system can perform verification, configuration, and testing, thereby improving system reliability. Standardized interfaces, possessing "plug-and-play" characteristics. Fieldbus intelligent sensors are the mainstream instruments for future industrial process control systems. Together with fieldbus, they form two important parts of the FCS, bringing revolutionary changes to traditional control system structures and methods. However, the development of international fieldbus standards has been slow, and the lack of unified fieldbus standards has affected the application of fieldbus intelligent sensors. Globally, several popular fieldbuses each have their own advantages and characteristics, making it difficult to unify them into a single fieldbus standard for various reasons. Firstly, there are technical reasons. Existing fieldbuses each have their own protocol specifications and industry standards, making unification a complex technical challenge. Secondly, there are commercial interests at play. Each fieldbus is closely linked to its underlying development company, and each company, aiming for greater market share, wants its technology to be adopted in the international fieldbus standard. Thirdly, there are organizational reasons. Fieldbus standardization requires a unified international organization. For many years, users have urgently needed a unified international standard for fieldbuses to achieve interoperability and interchangeability of field devices. In this context, Foundation Fieldbus (FF) emerged. FF's mission is to develop a unified fieldbus standard and promote its application. Currently, FF includes 95% of the world's instrumentation and control system manufacturers and has developed the low-speed H1 standard (31.25 kb/s), with the high-speed H2 standard under development. The finalization of a unified fieldbus standard will undoubtedly promote the application of FCS (Fieldbus Control System), bringing significant changes to user control strategies and system development. 2. The Impact of FCS on DCS: Traditional DCS systems consist of various workstations connected via a local area network. Operator stations and information management stations handle system configuration, monitoring, and operation management, while field control stations collect and control production process information. The main problems with DCS are poor openness, insufficient decentralization, and the need for extensive cable transmission of signals. FCS overcomes the shortcomings of closed systems where communication relies on dedicated networks. It transforms closed, dedicated solutions into open, universal, and standardized solutions, changing the distributed system structure into a new fully distributed structure. It completely decentralizes basic and independent functional blocks from DCS control stations to field intelligent instruments, creating virtual control stations and better embodying the essence of DCS principles. In reality, the development of industrial process control can be broadly divided into several stages: pneumatic instrument control, electric instrument control, computer-centralized control, and DCS. Electric instrument control, primarily using type 2 meters, is essentially a simple closed-loop control. Its greatest characteristic is that the risks are completely distributed across individual loops, while its greatest weakness is the difficulty in achieving complex, advanced, and optimized control. Therefore, centralized computer control emerged in the 1960s, enabling complex, advanced, and optimized control. However, this centralized control failed to inherit the characteristic of complete hazard dispersion inherent in the Type III electrical system. Consequently, the DCS system appeared in the 1970s, combining the advantages of the previous two systems: achieving complex control and distributing hazards across controllers. However, it still couldn't completely disperse hazards or provide full openness. In the 1980s, the FCS system was proposed—a system that could inherit all the functions of the DCS, provide full system openness, and achieve complete hazard dispersion across all loops (similar to the Type III electrical system). Because the FCS has significantly superior features to the DCS, it is destined to replace the DCS. However, currently, in most process industries in my country, the DCS is the mainstream control system, and type III electrical instruments are the dominant instruments. Furthermore, due to differing perceptions and the fact that fieldbus intelligent instruments are not yet the dominant products, the widespread adoption of FCS is not yet possible. From the perspective of utilizing existing resources, the disappearance or complete replacement of DCS systems in the short term is unreasonable. We should focus on existing DCS systems and fully explore the potential of existing equipment (e.g., installing gateways between DCS and FCS to enable information transmission), ensuring investment without waste. Furthermore, DCS is a continuously evolving control system, inevitably requiring the adoption of fieldbus technology to upgrade itself, enabling it to connect with fieldbus smart sensors (smart instruments) and local FCS systems. Currently, all these factors contribute to the coexistence of FCS and DCS. It is estimated that it will take approximately 10 years for FCS to truly become dominant. V. Fieldbus Enterprise Networks Fieldbus, as the future direction of control system development, with its openness and networking advantages, makes its integration with the Internet possible. Enterprise intranet (Interconnect Networks) is a product of this combination. As an application of Internet technology within enterprises, the Intranet provides a comprehensive technical solution for internal management and information exchange, becoming a crucial facility for connecting various departments within an enterprise. As Intranets become increasingly integrated into enterprises, the future of enterprise information management will be a distributed management model based on fieldbus control networks. Following the emergence of Intranets, Extranets (external enterprise networks) have also arisen, enabling enterprises to share information. This significantly expands the scope of network technology, from managing and controlling a single production process to unifying all production processes across the entire enterprise, and further to sharing external information relevant to the enterprise. Because Intranets utilize Internet technology, they offer excellent openness and support multiple network protocols and standards, allowing for seamless integration within the enterprise's existing network environment. Distributed network control systems, combining fieldbus technology and Intranets, connect highly dispersed intelligent sensors, transmitters, actuators, and other intelligent instruments at the industrial field front end to the control unit or management agency via a fieldbus network, forming a local area network (LAN) control system. This distributed LAN system saves on transmission lines, enhances the scalability of the entire system, offers longer transmission distances and stronger anti-interference capabilities, and enables fully distributed, masterless operation without a host computer, providing a novel solution for industrial control and enterprise management decision-making. Meanwhile, the combination of fieldbus technology and Intranet technology has greatly promoted the development of control technology and accelerated the pace of the information age. Using fieldbus enterprise networks brings many benefits to enterprise management and control: First, through the enterprise network, information sharing within the enterprise and between the enterprise and external entities is strengthened, improving resource utilization. Second, publishing relevant enterprise information on the Intranet, replacing paper documents with electronic documents, saves manpower and improves work efficiency. Third, the Intranet uses Internet technology, making development tools readily available, shortening software development cycles, and allowing direct use of some excellent Intranet software, saving development costs. Finally, using fieldbus enterprise networks makes it easier to achieve the integration of industrial process management decision-making and control—CIPS (Computer Integrated Process System). VI. Conclusion The key to FCS is fieldbus technology and fieldbus intelligent sensors (fieldbus intelligent instruments). As electrical instruments are transformed into intelligent instruments, fieldbus can be conceived; with the maturity of digital communication technology, fieldbus can emerge, thus distinguishing FCS from DCS. It can be said that FCS is the third major revolution in industrial process control systems, namely, analog instrument distributed control (represented by electrical instruments). Distributed Control System (DCS) – a fully open, fully decentralized, and fully centralized control system (FCS). FCS was gradually implemented abroad starting in the mid-1990s; while in China, some FCS trials began in 1998, but these were still imperfect and incomplete. Currently, we are entering the implementation phase of FCS, which facilitates the true development and implementation of CIPS. It is a completely reliable, fully decentralized, open system, easy to install, operate, and maintain, saving investment, manpower, and resources. Based on current development trends, FCS will replace DCS and dominate industrial process control systems within the next 10 years.