1. The characteristics of fieldbus are mainly manifested in the following aspects:
(1) Digital signals completely replace traditional analog signals. Digital signals completely replace the 4-20mA analog signals of traditional DCS, and bidirectional signal transmission is enabled. Multiple devices can usually be connected to a single twisted pair or cable, thus greatly reducing the amount of cables, terminals, trunking, and cable trays used. At the same time, the communication bus extends to field sensors, transmitters, controllers, and servo mechanisms, allowing operators in the control room to monitor, diagnose, verify, and tune parameters of field devices online from the main control system, saving on hardware quantity and investment.
(2) Fieldbus achieves complete structural decentralization. The fieldbus has only two levels in structure: field devices and operation management station. It integrates the I/O control station of the traditional DCS into the field intelligent devices, eliminates the I/O modules, and the field instruments are all equipped with microprocessors. The output results are sent directly to the adjacent regulating valves without going through the main control system in the control room, thus achieving complete structural decentralization.
(3) Bus network systems are open and give users the power to integrate systems. Users can combine products from different suppliers into systems of varying sizes according to their needs and considerations. Field instruments from different manufacturers can be used to replace field instruments from another manufacturer that have failed.
2. Current Fieldbus Standards: Fieldbus technology originated in the late 1980s as a network communication technology. After more than a decade of development, several representative fieldbus standards and product series have emerged internationally. Among the more popular ones are:
(1) Foundation Fieldbus: During the research and development of fieldbus standards, various corporate groups or organizations emerged. Through continuous competition, by 1994, two major camps had basically formed internationally: the ISP protocol and the World FIP protocol, developed in conjunction with 150 European companies. These two groups merged in 1994 to form the Fieldbus Foundation (FF), dedicated to developing a unified international fieldbus protocol. FF's protocol conforms to the IEC 1158-2 standard, also known as the SP50 standard.
(2) Profibus Fieldbus is a fieldbus standard jointly developed by the German National Standard and the European National Standard. It adopts the physical layer and data link layer of the OSI model. The Fieldbus Information Specification (FMS) type only omits layers three to six of the OSI standard and adopts the application layer.
(3) The LonWork (Local Operating Network) fieldbus was officially launched in 1990. It adopts all 7 layers of the ISO/OSI model and uses an object-oriented design method. It simplifies network communication design to parameter settings through network variables. Its maximum transmission rate is 1.5Mbps, the transmission distance is 2700m, and the transmission medium can be twisted pair, optical fiber, radio frequency, infrared and power line, etc.
(4) Control Area Network (CAN): The control network is used for data communication between measurement and actuation components inside the vehicle. The CAN architecture model adopts layers 1, 2, and 7 of the ISO/OSI model, namely the physical layer, data link layer, and application layer. It is also worth mentioning the Highway Addressable Remote Transducer (HART) protocol, the earliest fieldbus protocol compatible with 4-20mA analog signals and modulated digital signals. Its digital communication uses modulation/demodulation, making it a transitional product in the transition from analog to digital systems. Therefore, it has strong competitiveness during the current transition period and has experienced rapid development.
3. The Future of Fieldbus: Fieldbus will represent a revolutionary leap forward in industrial control development. Reports on fieldbuses are abundant, with a key focus being whether a globally unified fieldbus standard will emerge. Different fieldbuses have different functions and applicable scenarios, making the idea of relying on a single fieldbus for universal application unrealistic. Of course, on the other hand, many fieldbuses are very similar, technically very close, and will eventually converge in a competitive environment. However, fierce commercial competition is the main reason for this. Automation manufacturers that have a significant impact on fieldbuses are unwilling to relinquish their existing fieldbuses in order to maintain their position and interests; therefore, the competition for fieldbus standards will intensify for a considerable period.
