Abstract: This article introduces the concept of fieldbus, its background and latest developments, summarizes the essence and advantages of fieldbus, compares several mainstream fieldbuses, and provides a brief introduction to the relationship between industrial Ethernet and fieldbus. This article is intended for beginners interested in control systems and fieldbuses, serving as introductory material. Fieldbus is a hot topic in process control theory and a technological focus in related fields, attracting widespread attention worldwide. Before discussing fieldbus, let's review the development of process control and the background of fieldbus emergence. This may help us understand the essence and development of fieldbus. The development of control systems and the emergence of fieldbus: With the rapid development of science and technology, the field of process control has undergone tremendous changes in the past two centuries. The emergence of the 5-13 psi pneumatic signal standard (PCS, Pneumatic Control System) more than 150 years ago marked the initial formation of control theory, but the concept of a control room did not yet exist at that time. In the 1950s, with the proposal and widespread application of analog process control systems based on 0-10mA or 4-20mA analog current signals, the era of electrical automatic control arrived. The establishment of the three major cybernetics laid the foundation for modern control, and the model of setting up a control room and separating control functions has been used ever since. In the 1970s, with the intervention of digital computers, a "centralized control" central control computer system emerged, but the signal transmission system still mostly used 4-20mA analog signals. Soon, people also discovered that the system had the disadvantages of easy runaway and low reliability associated with "centralized control," and it was quickly developed into a distributed control system (DCS). Distributed control systems (DCS) have become widely used due to the widespread application of microprocessors and the improved reliability of computers. Distributed control, characterized by multiple computers, intelligent instruments, and intelligent components, is its most prominent feature, with digital transmission signals gradually replacing analog transmission signals. With the rapid development and widespread application of microprocessors, it has become possible to extend digital communication networks to industrial process sites, resulting in intelligent devices that use integrated circuits instead of conventional electronic circuits, with microprocessors at their core, to perform functions such as information acquisition, display, processing, transmission, and optimized control. Communication and control between devices require higher precision, operability, reliability, and maintainability. This led to the emergence of fieldbus. The essence and advantages of fieldbus: The concept of fieldbus was formally proposed in 1984. The IEC (International Electrotechnical Commission) defines fieldbus as: a bidirectional, serial, multi-node digital communication technology applied in the production field, enabling bidirectional communication between field devices and between field devices and control devices. Different organizations and individuals may have different definitions of fieldbus, but generally, the essence of fieldbus is recognized in the following six aspects: Field communication network: A field communication network used for interconnecting field devices or field instruments in process automation and manufacturing automation. Field device interconnection: Using different transmission media to associate different field devices or field instruments according to actual needs. Interoperability: Users can choose products from different manufacturers or models to form the required control loops according to their own needs, thus freely integrating FCS. Distributed function blocks: FCS eliminates the input/output units and control stations of DCS, distributing the function blocks of DCS control stations to field instruments, thus forming virtual control stations and completely realizing distributed control. Communication line power supply: Communication line power supply allows field instruments to draw power directly from the communication line. This method provides low-power field instruments for intrinsically safe environments, and is accompanied by safety barriers. Open interconnection network: Fieldbus is an open interconnection network, which can interconnect with networks at the same layer or different layers, and can also realize network database sharing. From the above, we can see that Fieldbus embodies the characteristics of distribution, openness, interconnectivity, and high reliability, which are precisely the shortcomings of DCS systems. DCS typically transmits signals one-to-one, using analog signals with low accuracy and susceptibility to interference. Operators in the control room often find it difficult to adjust parameters and predict faults on analog instruments, leaving them in a state of "out of control." Many instrument manufacturers define their own standards, resulting in poor interchangeability, and the functions of the instruments are relatively limited, failing to meet modern requirements. Moreover, almost all control functions reside in the control station. Fieldbus, on the other hand, adopts a one-to-many bidirectional signal transmission, using high-precision and highly reliable digital signals. Equipment is always under the remote monitoring and control of the operator. Users can freely choose different brands and types of equipment for interconnection as needed. Intelligent instruments have rich functions such as communication, control, and computation, and control functions are distributed among various intelligent instruments. Therefore, we can see the significant progress that FCS represents compared to DCS. The aforementioned characteristics of FCS give it significant advantages in design, installation, commissioning, and normal production: Firstly, because the intelligent devices distributed at the front end can perform complex tasks, separate controllers and computing units are no longer needed, saving hardware investment and floor space; FCS wiring is relatively simple, and multiple devices can be connected to a single transmission line, greatly reducing installation costs; because field control devices often have self-diagnostic functions and can send fault information to the control room, maintenance work is reduced; simultaneously, users have a high degree of autonomy in system integration and can flexibly choose suitable manufacturers' products; the overall system reliability and accuracy are also greatly improved. All of this helps users reduce installation, use, and maintenance costs, ultimately increasing profits. The current state of fieldbus technology is complex. Due to the competition for interests among various countries and companies, although the International Electrotechnical Commission/International Organization for Standardization (IEC/ISA) began developing fieldbus standards as early as 1984, a unified standard has not yet been completed. Many companies have also launched their own fieldbus technologies, but their openness and interoperability are still difficult to unify. The current fieldbus market is characterized by the following: Coexistence of multiple fieldbuses. There are approximately forty types of fieldbuses in the world, such as FIP from France, ERA from the UK, ProfiBus from Siemens (Germany), FINT from Norway, LONWorks from Echelon, InterBus from PhenixContact, CAN from RoberBosch, HART from Rosemounr, Dupline from CarloGarazzi, P-net from ProcessData (Denmark), F-Mux from PeterHans, as well as ASI (Actratur Sensor Interface), MODBus, SDS, Arcnet, FieldBus Foundation (FF), WorldFIP, BitBus, DeviceNet and ControlNet from the US, etc. These fieldbuses are mostly used in process automation, pharmaceuticals, manufacturing, transportation, defense, aerospace, agriculture, and building applications. Approximately ten different types of fieldbuses account for about 80% of the market. Each type of bus has its own application area. For example, FF and PROFIBUS-PA are suitable for process control in industries such as petroleum, chemical, pharmaceutical, and metallurgy; LonWrks, PROFIBUS-FMS, and DeviceNet are suitable for building, transportation, and agriculture; DeviceNet and PROFIBUS-DP are suitable for manufacturing. However, these classifications are not absolute; each fieldbus strives to expand its application areas, leading to cross-application. Each fieldbus also has its own international organizations and supporting background. Most fieldbuses are backed by one or more large multinational corporations and have established corresponding international organizations to expand their influence and gain more market share. For example, PROFIBUS is primarily supported by Siemens and has established the PROFIBUS International User Organization WorldFIP; Alstom is the main backer and has also established the WorldFIP International User Organization.