Abstract : This paper introduces fieldbus technology and its characteristics, explains the main fieldbuses, and elaborates on the impact of fieldbus technology on computer control systems. Keywords : Fieldbus control, intelligent instrument, computer control system 1. Introduction Since the advent of computer control systems, sensors, transmitters, and actuators in analog instrument systems have been widely used in engineering practice. Their signal transmission generally uses 4-20mA current signals, requiring a pair of transmission lines to transmit one analog signal unidirectionally to each transmitter or actuator. This transmission method uses many wires, resulting in a large workload for on-site installation and commissioning, high investment, low transmission accuracy and anti-interference capability, and inconvenient maintenance. Control room personnel cannot understand the actual situation of the field instruments, cannot adjust their parameters, or diagnose faults. Therefore, the analog transmitters and actuators at the lowest level become the weakest link in the computer control system, the so-called bottleneck in the development of DCS systems. Fieldbus emerged under these circumstances. 2. Fieldbus Technology and Its Characteristics Fieldbus technology, developed in the late 1980s, is an advanced field industrial control technology. It integrates digital communication, computer, automatic control, network, and intelligent instrument technologies, fundamentally overcoming the limitations of traditional point-to-point analog or digital-to-analog signal control. It forms a fully distributed, fully digital, intelligent, bidirectional, interconnected, multi-variable, and multi-connector communication and control system. Fieldbus is a digital, bidirectional, multi-branch communication network connecting intelligent field devices and automation systems, based on intelligent instruments. Intelligent instruments distributed across various industrial sites are connected via a digital fieldbus and, together with controllers and monitors in the control room, constitute a Fieldbus Control System (FCS). By adhering to certain international standards, fieldbus products from different manufacturers can be integrated into the same FCS, achieving interchangeability and interoperability. FCS further delegates the control functions of traditional DCS to field intelligent instruments, which then perform data acquisition, data processing, control calculations, and data output. Data from field instruments (including acquired and diagnostic data) is transmitted to control equipment in the control room via the fieldbus. The control equipment in the control room monitors the operating status of each field instrument, saves data uploaded by each intelligent instrument, and performs advanced control functions that some field instruments cannot perform. Furthermore, the FCS can connect to the enterprise's upper-level management network via a gateway, allowing enterprise managers to access first-hand information and provide a basis for decision-making. Therefore, the fieldbus has the following prominent characteristics: 2.1 Openness: The Fieldbus Control System (FCS) adopts an open communication protocol, allowing interconnection and information exchange between devices from different manufacturers that adhere to the same communication standard. Users can flexibly select fieldbus products from different manufacturers to form an actual control system, achieving optimal system integration. 2.2 Interoperability: Interoperability means that control devices from different manufacturers can not only communicate with each other but also be uniformly configured to achieve the same control strategy and "plug and play." Devices with the same performance from different manufacturers can be interchanged. 2.3 Flexible Network Topology: The fieldbus control system can form different network topologies according to complex field conditions, such as tree, star, bus, and hierarchical network structures. 2.4 High Decentralization of System Structure: Field devices are inherently intelligent and possess basic independent automatic control functions. This fundamentally changes the centralized-distributed architecture of DCS, forming a completely new distributed control system. It achieves thorough decentralization of control functions, improves the reliability of the control system, and simplifies its structure. Even after the fieldbus is disconnected from the upper-level network, the underlying devices can still maintain independent normal operation, significantly enhancing their intelligence. 2.5 High Intelligence of Field Devices: Traditional DCS uses relatively centralized control stations, which consist of CPU units and input/output units. The fieldbus control system completely distributes the control station functions of the DCS to field control devices. Basic automatic control functions, such as data acquisition and compensation, PID calculation and control, equipment self-verification, and self-diagnosis, can be achieved solely through fieldbus devices. System operators can remotely monitor from the control room, set or adjust the operating parameters of field devices, and locate and diagnose faults using the self-diagnostic functions of the field devices. 2.6 High Environmental Adaptability: The fieldbus is specifically designed for industrial environments. It can transmit data using twisted-pair cables, coaxial cables, optical fibers, power lines, and wireless methods, exhibiting strong anti-interference capabilities. Commonly used data transmission lines are inexpensive twisted-pair cables, which allow field devices to be powered via data communication lines and also meet intrinsic safety and explosion-proof requirements. Due to the many advantages of fieldbuses, several influential fieldbuses have emerged in the past decade, such as Foundation Fieldbus (FF), Profibus, CAN, LONworks, and HART, and have been widely used. The following is a brief introduction to the first two fieldbuses. 3. Introduction to Major Fieldbuses 3.1 Foundation Fieldbus (FF) Foundation Fieldbus is a fieldbus developed after fierce competition among several international fieldbus providers, launched by the Fieldbus Foundation. Unlike proprietary network bus protocols, FF is not affiliated with any single company or country. Its bus architecture is based on the ISO OSI model, which includes the physical layer, data link layer, and application layer, with the addition of a user layer. FF has the support of almost all major instrument manufacturers worldwide and adheres to IEC protocol planning, being largely consistent with IEC's international fieldbus standards and drafts. Coupled with its technological advantages, it is highly likely to become a major international standard in the future. FF provides two physical layer standards: H1 and H2. H1 is a low-speed bus for process control, with a transmission rate of 31.25 Kbps and transmission distances of 200m, 450m, 1200m, and 1900m (extended by repeaters). It can be powered by the bus and supports intrinsically safe and non-intrinsically safe bus devices. H2 is a high-speed bus with a transmission rate of 1 Mbps (750m transmission distance) or 2.5 bps (500m transmission distance). Both H1 and H2 can have up to 32 nodes per segment, which can be increased to 240 with repeaters. H1 and H2 can be interconnected via bridges. The key features of FF are device interoperability, improved process data, earlier predictive maintenance, and reliable security. 3.2 Process Fieldbus Profibus Profibus, proposed and strongly promoted by Siemens, has become the German national standard DIN19245 and the European standard EN50170. It is an open and independent bus standard used as a communication network in mechanical manufacturing, industrial process control, and intelligent buildings. Profibus comprises three series: Profibus-PA, Profibus-DP, and Profibus-FMS. Profibus-PA (Process Automation) is used for low-speed data transmission in process automation. Its basic characteristics are similar to the FF H1 bus, providing bus power supply and intrinsic safety, and is supported by application-specific integrated circuits (ASICs) and software. Profibus-DP is compatible with Profibus-PA, with basic characteristics similar to the FF H2 bus, enabling high-speed transmission. It is suitable for high-speed data transmission between distributed external devices and automated control equipment, and is used to connect Profibus-PA and process automation. Profibus-FMS is suitable for medium-speed data transmission in general automation, mainly used for sensors, actuators, electrical drives, PLCs, textiles, and building automation. The latter two series adopt the RS485 communication standard, with transmission rates from 9.6kbps to 12Mbps and transmission distances from 1200m to 100m (depending on the transmission rate). The basic media access control methods are token-based between master stations, master-slave between master and slave stations, and a hybrid method combining these two methods. PROFIBUS is a relatively mature bus with wide applications in engineering. 4. The Impact of FCS on Computer Control Systems Traditional computer control systems generally adopt a DCS (Distributed Control System) architecture. In a DCS, point-to-point connections are required for field signals, and I/O terminals are placed in a control cabinet along with the PLC or automation instruments, rather than in the field. This necessitates laying a large number of signal transmission cables, resulting in complex wiring that is both material and time-consuming. Signals are prone to attenuation and interference, and maintenance is inconvenient. DCS typically consists of operator stations, control stations, etc., leading to a complex structure and high cost. Furthermore, DCS is not an open system, exhibiting poor interoperability and difficulty in data sharing. PC-based FCS, however, completely overcomes these shortcomings. (1) In FCS, with the help of fieldbus technology, all I/O modules are placed in the industrial field, and all signals are converted into standard digital signals in the field through distributed intelligent I/O modules. Only one cable is needed to connect all field substations, and the field signals are transmitted to the control room monitoring equipment very simply, which reduces costs and facilitates installation and maintenance. At the same time, digital data transmission gives the system a high transmission speed and strong anti-interference capability. (2) FCS is open. In FCS, both software and hardware follow the same standards, with good interchangeability and easy updates. The program design adopts five international standard programming languages ​​of IEC1131-3. The programming and development tools are completely open, and the rich software and hardware resources of the PC can also be utilized. (3) The system efficiency is greatly improved. In FCS, one PC can complete the PLC and NC/CNC tasks that originally required two devices. Under the multi-tasking Windows NT operating system, the soft PLC in the PC can execute up to a dozen PLC tasks at the same time, which improves efficiency and reduces costs. Furthermore, the PLC on the PC has online debugging and simulation functions, greatly improving the programming environment. In FCS, the basic system structure consists of: an industrial PC or commercial PC, a fieldbus master interface card, fieldbus input/output modules, a PLC or NC/CNC real-time multi-task control software package, configuration software, and application software. The main functions of the host computer include system configuration, database configuration, historical database configuration, graphical configuration, control algorithm configuration, data report configuration, real-time data display, historical data display, graphical display, parameter list, data printing output, data input and parameter modification, control operation and adjustment, alarm handling, fault handling, communication control, and human-machine interface, truly achieving the control requirements of centralized control, decentralized hazards, data sharing, and complete openness. 5. Conclusion As can be seen from the preceding discussion, the key technologies of FCS are intelligent instrument technology and fieldbus technology. Intelligent instruments not only have advantages such as high accuracy and self-diagnosis, but also have control functions, and will inevitably replace traditional 4-20mA analog instruments. The fieldbus, connecting intelligent instruments in the field, is an open, digital, multi-connector, bidirectional serial data transmission path. It is a product of the combination of computer technology, automatic control technology, and communication technology. Combined with the rich hardware and software resources of PCs, it overcomes the shortcomings of traditional control systems and greatly improves the flexibility and efficiency of control systems, forming a completely new control system, ushering in a new era of automatic control, and becoming an inevitable trend in the development of automatic control.