1. Introduction In the field of industrial control, with the increasing intelligence of instruments and the deepening of industrial management automation, a large number of intelligent devices need to communicate with each other through networks to achieve functional autonomy of intelligent field devices, highly decentralized system structure, and integrated control. Fieldbus conforms to the development trend of intelligent field devices. It uses field devices with digital computing and digital communication capabilities as network nodes, and the bus as the link for digital communication between nodes, forming a digital, bidirectional, fully decentralized, multi-branch control network. The emergence of fieldbus adapts to the direction of industrial control systems towards decentralization, networking, and intelligence, and has prompted significant changes in the architecture and functional structure of current automation instruments, DCS, and programmable logic controllers (PLCs), leading to a generational upgrade in the field of industrial automation. From the perspective of information integration, with the development of information technologies such as computers, control, communication, and networks, the scope of information exchange has covered factories, enterprises, and even markets around the world. To achieve comprehensive automation in industrial enterprises, it is necessary to establish an integrated network platform that includes all levels from the industrial field device layer to the control layer and management layer, providing not only a channel for the vertical transmission of data information but also ensuring horizontal communication between devices. However, fieldbus technology has many shortcomings in information integration. There are too many existing fieldbus standards; the IEC international standard alone includes eight types, failing to unify into a single standard. The coexistence of multiple fieldbuses is a given. Different types of fieldbus devices have dedicated communication protocols, making them incompatible, unable to interoperate, collaborate, or achieve seamless information integration. To solve the integration problem of multiple fieldbuses in the same control system, some projects typically use a standard technology (such as OPC) to develop interfaces that can connect to the fieldbuses. However, the various fieldbus standards are not completely unified, requiring the development of numerous interfaces to meet the needs of different industrial control objects. Many companies have invested significant effort and resources to solve compatibility issues between products from different fieldbus manufacturers, with limited success. The development of switched Ethernet technology has improved Ethernet's real-time capabilities, making industrial Ethernet a deterministic network. Furthermore, the introduction of new fieldbus standards such as Ethernet/IP, HSE, PROFInet, and iLon provides a technological foundation for better solving the integration problem of multiple fieldbuses. In response to the current widespread coexistence of various fieldbuses, this paper proposes an integration scheme suitable for the current industrial applications of fieldbuses, utilizing switched Ethernet technology. Several typical fieldbus technologies for application in substation automation systems are also discussed. 2. Integration Trends of Multi-Fieldbus Systems 2.1 Current Status of Fieldbus Development Fieldbuses have gradually developed since the 1980s. According to the IEC definition, a fieldbus is a serial, digital, bidirectional, multi-branch communication network between field devices installed in the production process area and automatic control devices in the control room. Fieldbuses replace traditional analog signal transmission with digital communication, significantly reducing the number of connecting cables and terminals between instruments, thus lowering the hardware cost of the system. It can not only transmit normal measurement and control signals but also transmit additional information such as equipment status, alarms, and trends. It can even remotely program and maintain instruments, thus earning it the reputation of a computer local area network in the field of automation. Due to the enormous economic potential of fieldbus technology, many companies worldwide have invested significant human, material, and financial resources in comprehensive technical and application research. Currently, over 100 distinct fieldbuses have emerged, with more than 40 claiming to be open, sparking an international standard war in the fieldbus fieldbus market. Despite years of effort, the fieldbus standard adopted by the IEC in 2000 accommodates eight incompatible protocols: IEC 61158 (FF H1), ControlNet, Profibus, P-Net, Foundation Fieldbus, SwiftNet, WorldFip, and Interbus-S. The problem of interconnection and interoperability between heterogeneous fieldbuses remains unresolved. Users still face the challenge of selecting and integrating fieldbus systems, requiring considerable effort to resolve information exchange issues between different standard systems. Furthermore, the communication rates of traditional fieldbuses are generally low, failing to meet the real-time requirements of the exponentially increasing data communication volumes in industrial networks in certain situations. 2.2 Integration is an Inevitable Trend for Fieldbuses Due to the wide variety of fieldbuses and their extensive applications in different fields... Various fieldbuses represent years of R&D investment and market interests from different companies. Different buses have different technological focuses, unique characteristics, and corresponding application areas. Currently, no single fieldbus technology is universally applicable to all application areas. Although Industrial Ethernet, with its unparalleled advantages, has become a trend in field control due to its entry into the field control level, it is unlikely to completely replace fieldbuses as the sole standard for real-time control communication, at least for now. Therefore, the coexistence of multiple fieldbuses will continue for a considerable period. In systems composed of various types of fieldbuses, the different protocols prevent them from communicating with each other, severely hindering user choice. For users, if each intelligent product in a system requires its own dedicated communication card or controller, the system's configurability and flexibility will be poor, and the cost of upgrades and modifications will be substantial. With the development of Industrial Ethernet technology, various fieldbuses have successively launched new-generation fieldbus technologies and products bundled with Ethernet, aiming to establish control systems that enable multiple fieldbuses to work collaboratively. For example, Rockwell introduced Ethernet/IP, composed of Ethernet, ControlNet, and DeviceNet layer buses; FF abandoned its original high-speed bus H2 and began developing the HSE standard in 1998; Siemens also launched the PROFInet solution. These new fieldbus technologies all adopt the IEEE 802.3 physical layer and data link layer standards and the TCP/IP protocol suite, using standard Ethernet transmission media and connection devices, and are compatible with previous generation fieldbus systems and even DCS. It is clear that integrating multiple fieldbuses to collaboratively complete the measurement and control tasks of industrial enterprises is a crucial strategy for automation system suppliers to seize market share. Only in this way can they adapt to the current polymorphism of field measurement and control equipment and the diverse needs of users, maximizing the protection of user interests. 3. Multi-Fieldbus Integration Solutions 3.1 Switched Ethernet Technology As is well known, Ethernet has advantages such as full openness, low cost, high bandwidth, high stability and reliability, wide application, and abundant shared resources. Its application to industrial networks has become a research hotspot in the field of industrial control both domestically and internationally. However, traditional Ethernet uses the CSMA/CD MAC layer protocol, and each node employs the Binary Exponential Back-Off (BEB) algorithm to handle collisions. This inherently results in uncertain collision delays, hindering its effective application in industrial real-time control systems. With the rapid development of IT technology, Ethernet transmission speeds have increased from 10 Mbit/s to 100 Mbit/s, and now to 1 Gbit/s, significantly reducing collision times. The development of switched Ethernet technology offers hope for a complete solution to the nondeterministic nature of Ethernet communication. Switched Ethernet increases the throughput and bandwidth of each segment through segment miniaturization, providing each node with a dedicated point-to-point link. Architecturally, it is identical to a simple point-to-point connection, with each device having a dedicated channel connecting to other devices. Therefore, there is no need to compete for underlying transmission channels, greatly reducing the likelihood of collisions between different devices and effectively resolving the deterministic problem of network transmission. 3.2 Multi-Fieldbus Integrated System Introducing fieldbuses into industrial networks establishes a highly reliable, open data communication line in the industrial field to enable data exchange between various intelligent devices and between intelligent devices and monitoring units. A major reason why fieldbus technology cannot be unified is the use of different network technologies. Since each fieldbus has gained support from many manufacturers and has numerous users, it is impossible to unify fieldbuses using a single network technology. Introducing switched Ethernet technology into industrial control systems and integrating various existing fieldbuses can form a multi-fieldbus integrated system as shown in Figure 1. The fieldbuses in this system all adopt a new technology that bundles Ethernet standards, enabling not only the interconnection of various intelligent devices but also data communication between intelligent devices and upper-level monitoring units. The Ethernet application services provided by various fieldbuses are shown in Table 1. The communication of the entire system is built on a hybrid communication protocol of Ethernet, TCP/IP, and fieldbuses. Various new fieldbus technologies are used to achieve the interconnection of high-speed Ethernet with relatively low-speed fieldbuses, enabling interconnection and interoperability between upper-level monitoring units and field intelligent devices. The system's information transmission mechanism is as follows: When the upper-level monitoring unit needs to send information to field devices, it first sends the information to the corresponding new fieldbus Ethernet application service (such as the PROFInet proxy mechanism) based on Ethernet and TCP/IP protocols. Then, the service sends the information to the corresponding intelligent device according to the fieldbus protocol. The process of field devices sending information to the upper-level monitoring unit is the reverse. The intelligent devices in the system are mainly used for data acquisition, control output, and implementing some simple control algorithms. The system's control functions can be arranged on any computer in the network, or simultaneously on several computers, truly achieving full distribution and redundancy of control functions, greatly improving system reliability, and enabling the implementation of various complex control algorithms. Computers can utilize the high flexibility of software design to virtually implement multiple standard fieldbus stations, playing a core role in the entire system. This allows different fieldbuses to be integrated into a distributed control system, alleviating the pressure of standardization and interoperability between different fieldbuses and protecting the existing investments of manufacturers and users. When users need to expand the system scale, any type of fieldbus can be used to achieve system integration of multiple fieldbuses. Because Ethernet facilitates seamless connection with the Internet, the system also supports remote access via the Internet. 4. Multi-Fieldbus Integration in Substation Automation Systems 4.1 System Architecture Substation automation systems are applications of automation, computer, and communication technologies in the substation field. They typically employ a hierarchical distributed structure, divided into process level, bay level, and substation level. The process level primarily handles field data acquisition and provides I/O interfaces; protection and control units constitute the distributed bay level; and the substation level mainly includes the central control unit, common equipment, local monitoring systems, and is responsible for exchanging information with the remote dispatch center. Due to the rapid development of computer, network communication, and power measurement and control technologies, the technologies applied to substation automation cannot be standardized. Although fieldbuses have seen considerable successful applications in substation automation, they include various incompatible fieldbus standards such as LonWorks, CAN, and Profibus. Therefore, it is necessary to integrate various fieldbus systems to enable them to work collaboratively. With the development of switched Ethernet technology, fieldbus control networks and Ethernet-based data communication networks are gradually converging. The International Electrotechnical Commission (IEC) has developed the IEC 61850 substation automation system standard, proposing an open, fully distributed, and interoperable industrial control Ethernet network for fully digital communication between the bay level and process level within the substation. Figure 2 shows a substation multi-fieldbus integrated automation system based on switched Ethernet technology. To meet the requirements of large information exchange volume and real-time data operation in substation automation systems, this integrated system adopts a two-level network structure consisting of a monitoring network and a waveform recording network. The monitoring network transmits various control information, while the waveform recording network transmits power system fault waveform recording information. Since the backbone network uses 10 Mb/s or higher bandwidth full-duplex switched Ethernet, it can fully meet the needs of numerous network nodes and large data traffic. Existing fieldbus systems (such as LonWorks, CAN, Profibus, etc.) are connected to the backbone network through corresponding Ethernet services, thereby realizing information interaction between intelligent field devices in different fieldbus systems. The application of interactive Ethernet technology not only realizes the integration of different types of fieldbuses but also improves the openness and scalability of the network platform, enabling the entire substation automation system to meet the needs of medium and low voltage substations developing towards high voltage and ultra-high voltage substations. 4.2 Several Typical Fieldbus Technologies 4.2.1 i.LON 1000 in LonWorks Systems A key device in the new generation of LonWorks systems is the i.LON 1000, which replaces traditional gateway devices and serves as a bridge connecting LonWorks and Ethernet. It packages LonWorks messages, encapsulates them using the TCP/IP protocol, and then sends them over the network. When the data packet arrives at the LonWorks network segment, the TCP/IP encapsulation is discarded, and the LonWorks data packet is repositioned on the network. From a computer network perspective, the i.LON 1000 can be considered a typical IP host, seamlessly connecting the control network and the data network. Its built-in network server allows web browsers to easily obtain monitoring information, such as network variables like voltage and current. The i.LON 1000 makes system installation, monitoring, fault diagnosis, and maintenance easier, ensuring complete connectivity between nodes on the network segment. 4.2.2 Ethernet/IP Ethernet/IP is an open industrial network that supports both I/O and data exchange. It uses Ethernet switches to achieve point-to-point connections between devices and can simultaneously support 10 Mb/s and 100 Mb/s commercial Ethernet products, facilitating high-speed transmission of large amounts of data. The Ethernet/IP protocol consists of Ethernet, TCP/IP, and CIP, and is compatible with the DeviceNet protocol used by the CAN bus, enabling information exchange between the CAN bus and Ethernet. Figure 3 shows the application architecture combining Ethernet/IP and the CAN bus. 4.2.3 PROFInet for PROFIbus Systems PROFInet specifies open and transparent communication between PROFIbus and Ethernet. It is based on standard Ethernet connection media and uses TCP/UDP/IP protocols plus application-layer RPC/DCOM to complete communication and network addressing between nodes. PROFInet adopts an advanced object model (COM/DCOM), making the entire structure clear, simple, and easy to manage, and can simultaneously connect traditional PROFIbus systems and new intelligent field devices. Traditional PROFIBUS devices can communicate with COM objects on PROFInet through a proxy interface, and calls between COM objects can be achieved through the OLE automation interface. 5. Conclusion Due to the development and competition of various fieldbus technologies, the penetration and application of various mainstream computer technologies in industrial control, and the continuity and inheritance of automation technology development, industrial field control networks will inevitably face a long-term situation of multiple fieldbuses coexisting. Although the promotion and application of Ethernet technology in industrial automation is unstoppable and its application scope is rapidly expanding, Ethernet will not dominate the market at present. Therefore, how to better solve the interoperability problems between different types of fieldbuses in industrial enterprises and enable them to work together is of great practical significance for realizing information integration in industrial automation systems. To protect users' original investments while providing a high-bandwidth data channel for high-speed Ethernet, this paper proposes a multi-fieldbus integration solution based on switched Ethernet technology. This system can not only effectively solve the current problems of fully distributed, fully open, and multi-fieldbus system integration in control systems, but also, through the lower-level integration of Ethernet and fieldbuses, make the control system architecture more flattened. By exploring the Ethernet application services provided by several typical fieldbuses, it can be seen that applying the multi-fieldbus integration scheme proposed in this paper to substation automation systems can not only achieve better system integration of fieldbuses such as LonWorks, CAN, and Profibus, but also conform to the current application status of fieldbuses in substation automation systems.