Abstract: By analyzing the shortcomings of distributed control systems (DCS), this paper proposes a scheme to integrate PROFIBUS into existing DCS systems. Based on the technical characteristics of DCS and PROFIBUS, the hierarchical architecture of the system is presented. A PROFIBUS-based DCS is designed, the network structure of the process I/O layer is described in detail, and the system's real-time performance, reliability, and economic efficiency are analyzed. This provides important reference for addressing the shortcomings of traditional DCS and realizing the comprehensive informatization of thermal power plants. Keywords: Fieldbus; Distributed Control System; Network Structure; Communication; Performance 0 Introduction Distributed Control Systems (DCS) are widely used in production process control due to their ease of operation and stability. Their main characteristics are centralized management and decentralized control. Although DCS achieves decentralized control functions, it remains a centralized control system physically, still employing a traditional centralized I/O system (see Figure 1). In Figure 1, the DCS signals use a one-to-one physical connection, with dedicated I/O processing modules and terminal blocks. This results in complex power plant cabling, long project cycles, high installation and commissioning costs, and difficult maintenance. Each DCS manufacturer usually uses its own defined dedicated network and protocol, which makes it difficult to interconnect devices or systems from different manufacturers. The entire DCS system becomes a closed "information island" [1]. In addition, the field instruments of DCS still use unidirectional transmission of 4-20mA analog signals, which are not only slow, low in accuracy, and susceptible to electromagnetic interference, but have also become a bottleneck that restricts the function of DCS. Fieldbus is a fully digital, bidirectional, serial communication network that connects field instruments with instruments in the control room. Its essential advantage is full digitalization, its substantial feature is complete decentralization, and its deeper advantage is its openness [2]. Fieldbus uses a distributed I/O system (see Figure 2), which can truly realize control functions and physical full decentralization. The PROFIBUS network in Figure 2 uses intelligent sensors, actuators and other field intelligent devices, and adopts digital signal transmission. Only one pair of wires is needed to transmit field signals to the process-level control computer. It breaks through the limitations of traditional 4-20mA analog transmission signals in terms of accuracy and function, and can remotely diagnose, debug and operate field devices in the control room. As functions are further distributed among field intelligent devices, risks are further dispersed, thereby improving the system's security and reliability. [align=center] Figure 1: Centralized I/O System Based on DCS Figure 2: Distributed I/O System Based on PROFIBUS[/align] 1. Distributed Control System Based on PROFIBUS Currently, there are various fieldbus standards internationally, such as PROFIBUS, FF, HART, and CAN. Among the many fieldbus standards, PROFIBUS is the most widely used and has been successfully applied in many fields in China. Among many DCS manufacturers, ABB Bailey's Industrial IT Symphony system is the most widely used. Therefore, choosing PROFIBUS bus and Symphony system to study distributed control systems based on fieldbus technology is representative. The PROFIBUS-based DCS adopts a hierarchical architecture, as shown in Figure 3. From bottom to top, the system can be divided into process I/O layer, process control layer, process management layer, and enterprise management layer, with corresponding networks being PROFIBUS-DP network, Control Way (CW), Control Network (Cnet), and Enterprise Management Network (MIS). [align=center]Figure 3 Structure of a PROFIBUS-based distributed control system[/align] The PROFIBUS-DP network uses RS485 transmission technology with a transmission rate of 9.6K bit/s to 12M bit/s. Token passing is used between master stations, and a master-slave mode is used between master and slave stations. The token passing procedure ensures that each master station obtains bus access rights within a precisely defined time. The maximum time for a token to circulate among all master stations is predetermined based on the number of master stations. The master-slave mode allows a master station to communicate with a slave station when it obtains a bus access token; each master station can cyclically query slave stations. The master station determines the data communication on the bus. When a master station obtains bus control, it can actively send information without external request. Slave stations are peripheral devices, such as valves, actuators, and input/output devices. They do not have bus control rights; they only acknowledge received information or send information to the master station when requested. The control channel uses a 1 Mbit/s serial redundant bus network, supporting 32 intelligent processing modules. The intelligent processing module includes communication module pairs INNIS and INNPM, and a bridge controller (BRC). Through these communication module pairs, the control channel can communicate with the control network. Each BRC can connect to 64 sub-modules via the I/O expansion bus (XB). This network layer provides a network interface to the PROFIBUS fieldbus—the Fieldbus Sub-Module (FBS). Therefore, each BRC can connect to 64 FBSs. Through the FBSs, communication between the PROFIBUS-DP network and the Symphony control channel is achieved. The control channel uses the IEEE 802.3 Ethernet protocol. The control network handles process management and operational data transmission, employing a 10 Mbit/s ring network structure, including a central ring and sub-rings, supporting up to 250 nodes. The communication protocol used is a multi-point, multi-target store-and-forward protocol. Since there is no communication controller, nodes on the network do not have a master-slave relationship; each node is an independent information forwarder with buffer registers. All nodes can send, forward, and receive data point-to-point at the same time, and information packaging technology is used, so it has high communication efficiency. The enterprise management network uses standard Ethernet to manage various data of the entire enterprise. 2 PROFIBUS-DP network structure Since fieldbus technology is suitable for distributed field-controlled equipment systems with communication interfaces, it does not have a significant advantage for single-machine control systems with centralized I/O. Moreover, the variety of field intelligent devices and software systems currently developed is not large, and the price is relatively high. Therefore, different solutions should be selected according to different characteristics. (1) For some relatively concentrated analog parameters, such as boiler tube wall temperature, metal temperature, generator stator coil temperature, and rotor bearing temperature, distributed I/O or remote intelligent I/O can be used. If fieldbus technology is used for these concentrated measurement points, not only will the technical advantages of fieldbus not be brought into play, but the cost will also increase. (2) For equipment with single parameter adjustment loops and widely distributed and relatively independent measurement points, intelligent transmitters or intelligent actuators are used to connect to the fieldbus. Depending on whether the field devices have PROFIBUS interfaces, there are three network structures in the system. (1) Bus interface type: The field devices do not have PROFIBUS interfaces, and distributed I/O is used as the bus interface to connect with the field devices. (2) Single bus type: The field devices are all intelligent devices with PROFIBUS interfaces. This is an ideal situation where fieldbus technology can be used to achieve a completely distributed structure and fully enjoy the benefits brought by this advanced technology. New projects may have this condition, but currently, the equipment cost of this solution will be relatively high. (3) Hybrid type: Some field devices have PROFIBUS interfaces, and in this case, a hybrid approach of using PROFIBUS + distributed I/O should be adopted. Whether it is the renovation of old equipment or the construction of new projects, there may not be many situations where all field devices with PROFIBUS interfaces are used. Distributed I/O can be used as a general fieldbus interface, which is a flexible integration solution. Therefore, the PROFIBUS-DP network adopts a hybrid network structure. As shown in Figure 3, the PROFIBUS-DP network is a multi-master and multi-slave structure. Field devices without PROFIBUS interfaces are connected to the PROFIBUS-DP bus through Siemens' distributed I/O module ET200. Intelligent devices with PROFIBUS interfaces can be directly connected to the PROFIBUS-DP network. Two types of Class I master stations DPM1 are designed in the network. They are integrated with the DCS in two different ways. (1) Class I master stations are composed of PLCs or other controllers. The monitoring station is connected to the PROFIBUS bus through a PROFIBUS network card and serves as a Class II master station. This network structure is used for distributed auxiliary control systems, such as water networks and coal conveying systems in thermal power plants. The DCS monitoring station can perform remote programming, parameter setting and online monitoring functions in the central control room, thereby realizing unmanned operation. (2) Class I master stations are composed of a rugged PC (COMPACT COMPUTER) + PROFIBUS interface + SOFTPLC structure. This network structure is used in other independent control systems, such as the electrical control system ECS of thermal power plants. It is a network structure that integrates monitoring stations and a type of master station. It can complete functions such as programming, fault alarm, equipment graphic monitoring screen design, database establishment, online graphic monitoring of equipment status, data storage and statistics, and reporting. 3 Performance Analysis As an industrial control communication network, DCS particularly emphasizes real-time performance and reliability. On the basis of meeting the real-time performance and reliability of the system, the economy of the system should be fully considered. 3.1 Real-time performance Real-time performance is a very important indicator of industrial control networks. To ensure the real-time performance of the entire communication network, three time constraints must be met: (1) Limit the upper limit of the time for each node to obtain communication rights each time. If this value is exceeded, the communication rights should be released immediately regardless of whether the current communication task is completed. (2) Ensure that every station on the communication subnet has the opportunity to obtain communication rights within a certain fixed time period. (3) For urgent tasks, when their real-time requirements temporarily become very high, priority service should be given. Stations with high real-time requirements should also be given higher priority. The commonly used protocols for industrial control networks are token bus, token ring network and CSMA/CD. Comparing the advantages and disadvantages of these three networks, token bus is the most suitable for industrial process communication networks, followed by token ring network, and CSMA/CD Ethernet is the worst [3]. PROFIBUS uses a token bus network and has a high communication rate, which can significantly improve the communication performance of DCS. The response time requirement of industrial control networks is sometimes hundreds of times faster than that of ordinary local area networks. Since the digital communication network of PROFIBUS is used in the process control layer, the control operation cycle can be greatly shortened. In a distributed system with 32 stations, PROFIBUS-DP transmits 512 bit/s input and 512 bit/s output to all stations, and only 1 millisecond is needed at 12M bit/s. FCS devices can perform PID regulation operations up to 10-20 times/s, while the current operation cycle of DCS is generally 4-5 times/s [2], which greatly improves the system regulation characteristics. In a multi-master PROFIBUS-DP network, the more nodes there are, the worse the real-time performance becomes [4]. 3.2 Reliability Reliability is another important indicator of industrial control networks. Generally, three criteria are followed: (1) The system operation is not affected by faults. That is, if any device in the system fails, it should not cause the failure of the entire system or other functions except for the functions involved in the failed device. Redundant design and multi-level operation can minimize the impact of faults. (2) The system is not prone to failure. That is, start from the reliability of the field control layer devices of the system. (3) The fault is quickly eliminated. That is, start from fault diagnosis, online monitoring and easy maintenance. At present, the reliability of DCS is between 99.99% and 99.9999%[5], which has high reliability. Since the system absorbs the advantages of mature, stable and reliable PLC and DCS technology, and adopts PROFIBUS fieldbus in the process control layer of DCS, it achieves thorough decentralization in functions and equipment, which can significantly improve the "system operation is not affected by faults" criterion among the three reliability criteria. Therefore, the reliability of the system can be further improved. 3.3 Economy (1) PROFIBUS adopts a 1-to-N wiring structure, and one pair of transmission lines can connect N devices. The thermal system in the main building of the thermal power plant is a special large-scale system with dense I/O points and three-dimensional layout. According to the current DCS standard design, the thermal cables of the 300MW unit need about 360km. Compared with the centralized control method, it can save 30% to 40% of the control cables and save expensive thermocouple compensation wires. For the auxiliary system outside the main building, the use of FCS can save about 60% of the cables [2]. (2) It can reduce the number of cabinets and connecting cables between electronic equipment, reduce the area of ​​electronic rooms, and the cable interlayer will gradually disappear, laying the foundation for the final realization of the cockpit-style control room. (3) It can simplify the system and reduce the design, installation, commissioning and maintenance costs. Since the system absorbs the advantages of DCS, it has the following advantages that FCS cannot replace at present: (1) PLC and DCS can form a large-scale control system, which FCS is currently lacking in. (2) PLC and DCS have comprehensive functions and can have continuous, sequential, analog and digital control, but FCS is not very comprehensive, especially in terms of digital. 4. Conclusion Fieldbus technology is a mature and reliable technology. With its expanding application scope, an open, fully digital era is inevitable. The application of PROFIBUS bus technology in industrial control proves that using a fully distributed I/O system instead of a centralized I/O system can not only significantly reduce costs but also play a vital role in achieving comprehensive information automation, including basic automation, management automation, and decision-making automation. The authors' innovations are: 1. A PROFIBUS-based distributed control system was designed, and the working mechanism and communication principle of its hierarchical architecture were analyzed, which can significantly improve the real-time performance and reliability of the distributed control system. 2. The process I/O layer—PROFIBUS-DP network structure—of the designed distributed control system is discussed, which solves the shortcomings of using centralized I/O in distributed control systems, generates significant economic benefits, and provides important guidance for the application of fieldbus in distributed control systems. References: [1] Liang Geng, Bai Yan, Li Wen. Full automation and information sharing of thermal power plants based on fieldbus control system [J]. China Electric Power, 2004, 37-1:36-39. [2] Yan Yuping. Fieldbus control system - the cornerstone of modern control, management and decision-making of thermal power plants [J]. China Electric Power, 2003, 36-3:54-58. [3] Huang Tianshu, Li Ming. Research and analysis of DCS communication network [J]. Computer Engineering, 2003, 29-7:124-126. [4] Liu Liangwen, Dong Ming, Zhao Hongzhou. Performance analysis of PROFIBUS-DP fieldbus [J]. Microcomputer Information, 2006, 22-9:46-48. [5] Yin Jiang, Feng Jiangtao. Distributed control system of power plant [M]. Beijing: China Electric Power Press, 2006.162-163.