New control strategies and network structures based on fieldbus technology will have a revolutionary impact on existing instrumentation and control systems. This paper analyzes the impact of fieldbus technology on traditional distributed control systems (DCS) based on its essential characteristics, and, combined with the network structure characteristics of DCS, presents three methods for integrating fieldbus into DCS. Industrial control has evolved from early local and centralized control to today's distributed control systems (DCS). Over the past 20 years, the process industry has invested heavily in DCS systems and related instrumentation, and the application results of DCS systems have been well-received by users. The 4-20mA signal is the most fundamental characteristic of the interconnection between DCS systems and field devices, representing a significant advancement in control system and instrumentation development. However, digitization and networking have become the main directions of control network development. It is recognized that traditional analog signals can only provide raw measurement and control information, and the ability of intelligent transmitters to add information on top of 4-20mA signals is limited by their low communication rate. Therefore, digitizing and networking the entire process control system mechanism is an inevitable trend. Fieldbus provides a fully digital, bidirectional, multi-node communication link between intelligent field devices and automation systems. The emergence of fieldbus has promoted the digitization and networking of field devices and made field control functions more powerful. This improvement has brought openness to process control systems, making the system a control network with integrated capabilities of measurement, control, execution, and process diagnostics. 1. The Impact of Fieldbus on Traditional DCS The impact of fieldbus on traditional DCS stems from its inherently superior technical features compared to DCS systems. According to the definitions of the International Electrotechnical Commission (IEC) and the Fieldbus Foundation (FF), fieldbus technology has the following five main characteristics: ① Digital signals completely replace 4-20mA analog signals; ② Basic process control, alarm, and calculation functions are completely distributed and completed in the field; ③ Equipment adds non-control information, such as self-diagnostic information, configuration information, and compensation information; ④ Achieves unified field management and control; ⑤ Truly realizes system openness and interoperability. Fieldbus technology is not only a communication technology; it actually integrates the essence of intelligent instrumentation, computer networks, and Open Systems Interconnection (OSI) technologies. All these characteristics give Fieldbus Control Systems (FCS) based on fieldbus technology significant advantages over traditional DCS systems: ① Greatly simplified system structure and significantly reduced cost; ② Enhanced autonomy of field devices and overall improved system performance; ③ Improved reliability and accuracy of signal transmission; ④ Truly realized fully distributed and fully digital control network; ⑤ Users always retain system integration rights. These advantages can be verified by comparing the network structures of DCS and fieldbus systems. 2. Fieldbus integration into DCS systems is the current trend in control network development. Although users welcome improvements in the structure of control systems, they do not want to make major changes to their existing instrumentation systems. Currently, in the early stages of fieldbus development, most users prefer to gradually add to and replace their existing instrumentation systems; on the other hand, the disappearance or complete replacement of DCS systems and their instruments is unreasonable in terms of cost or manpower. The most feasible solution at present is to consider how to make fieldbus work as collaboratively as possible with traditional DCS systems. This integration solution allows for flexible system configuration to be applied to a wider range of practical applications. 3. Integration of Fieldbus into the DCS System I/O Bus In the DCS architecture, it can be broadly divided into three layers from top to bottom: the management layer, the monitoring and operation layer, and the I/O measurement and control layer. On the I/O bus of the I/O measurement and control layer, the DCS controller and various I/O cards are connected. The I/O cards are used to connect field 4-20mA devices, discrete signals, or PIC signals, while the DCS controller is responsible for field control. The key to integrating fieldbus into the DCS system's I/O bus is to connect a fieldbus interface card to the DCS's I/O bus. This maps the data information from the fieldbus system to the corresponding data information on the original DCS I/O bus, such as basic measurement values, alarm values, or process setpoints. This makes the information from the fieldbus seen by the DCS controller appear as if it came from a traditional DCS device card. This achieves fieldbus technology integration on the I/O bus. This solution is mainly suitable for smaller-scale applications where the DCS system is already installed and running stably, and the fieldbus is being introduced to the system for the first time; this solution can also be applied to PLC systems. The advantage of this approach is its relatively simple structure, but the disadvantage is that the integration scale is limited by the fieldbus interface card. Fisher-Rosemount's DeltaV DCS system uses this integration approach. The DeltaV system has a dedicated interface card for this function—the Fieldbus H1 communication module (31.25 kbit/s)—in its I/O cards, successfully integrating fieldbus technology into the DeltaV system. This allows for connection to fieldbus instruments, significantly reducing installation, operation, and maintenance costs. The same controller is compatible with both H1 and traditional I/O modules, facilitating the transition from traditional control modes to fieldbus control modes. The DeltaV system's Fieldbus H1 communication module has a specific driver to map fieldbus system data to corresponding information on the I/O bus. 4. Integration of Fieldbus into the DCS System Network Layer Besides integration on the I/O bus, the fieldbus system can also be integrated at a higher layer—the DCS network layer. In this approach, the fieldbus interface card is not connected to the DCS I/O bus, but rather to the upper-layer LAN of the DCS. In this approach, fieldbus control execution information, measurement, and the control functions of field instruments can all be viewed and modified at the DCS operator station. Its advantages include the ability to offload control and calculation functions previously performed by the DCS host computer to field instruments, with relevant parameters and data readily available on the DCS operator interface. Another advantage is that it requires no modifications to the DCS control station, minimizing impact on the existing system. This integration approach has also been applied in practical engineering. For example, Smar's 302 series fieldbus products can integrate fieldbus functions at the DCS system network layer. 5. Parallel Integration of Fieldbus with DCS System via Gateway: If a DCS system and a fieldbus system operate in parallel within a factory, they can be connected via a gateway. A gateway is installed to facilitate information transfer between the DCS system and the high-speed fieldbus network. In this structure, DCS system information can be accessed and displayed on the new operator interface. A large number of H1 low-speed buses can be installed using a network bridge. The fieldbus interface unit provides functions such as control coordination, alarm management, and short-term trend collection. The parallel integration of fieldbus and DCS completes the integration and unification of the entire plant's control and information systems, and can achieve interconnection between Infranet and Internet through a Web server. The advantages of this approach are: enriching the network's information content and facilitating the comprehensive advantages of data and control information; furthermore, in this integration scheme, the fieldbus system and the DCS system integrated through a gateway are independent of each other. 6. Conclusion In summary, we believe that fieldbus systems will be widely used in process industrial control. By making some necessary modifications to the process control system and introducing fieldbus technology into the DCS, users will reap significant benefits. On the other hand, even if most continuous control loops are completed by the fieldbus system, the DCS system will still play an important role in many control applications with high real-time requirements. At present, the coexistence of fieldbus and DCS systems will give users more choices to achieve more rational control systems.