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Thoughts on Several Issues Concerning the Application of Field Buses I. Introduction Field buses are an advanced industrial control technology that emerged in the early 1990s. Compared with DCS, they offer many advantages; they represent a new generation of control systems that are fully digital, fully decentralized, fully open, interoperable, and based on open networking architectures； It is a combination and integration of computer technology, communication technology, and control technology; it extends communication lines all the way to the production equipment at the production site, forming a field communication network used for interconnecting the equipment and instruments involved in process automation and manufacturing automation. This approach transforms the traditional three-layer DCS network structure into a two-layer structure consisting of workstations, field buses, and intelligent field instruments, thereby reducing costs, improving reliability, and creating an integrated control and management system. Since its inception, it has posed a significant challenge to DCS. At present, fieldbuses have become a hot topic in automation technology worldwide. Many giants in the industrial control sector, such as Honeywell and Fisher-Rosemount, are actively engaged in the research, development, and application of FCS technology, and have launched their own FCS products. According to incomplete statistics, there are currently over 40 different types of fieldbus products, which can be overwhelming for some users, even leading to confusion and uncertainty about which one to choose. From an application perspective, this paper will analyze the characteristics of five currently popular fieldbus technologies, and then discuss the issues of concern in engineering when implementing fieldbus systems. II. Characteristics of several fieldbuses 1. CAN CAN is short for Controller Area Network. It was developed by the German company Bosch and several semiconductor manufacturers; initially it was designed for the monitoring and control of vehicles, but it has since been extended to use in other industrial sectors. It is worth noting that CAN is currently the only fieldbus that has been approved as an international standard. It possesses the following basic characteristics: (1) The CAN protocol follows the ISO/OSI model, adopting its three-layer structure of physical layer, link layer, and application layer. (2) The communication speed of CAN is 5 Kbps per 10 km and 1 Mbps per 40 m, with a maximum of 110 nodes; the transmission medium can be twisted pair, fiber optic cable, etc. (3) CAN uses a short-frame structure for signal transmission, with 8 valid bytes per frame, which results in short transmission times and a lower probability of interference. Furthermore, when a CAN node encounters a serious error, CAN has the capability to automatically shut down that node, thereby breaking the connection to the bus and preventing other nodes on the bus as well as their communications from being affected; hence, it possesses strong interference resistance. (4) CAN uses several methods for sending and receiving data, including point-to-point, one-to-many, and global broadcasting, enabling fully distributed multi-machine systems without any master-slave hierarchy. (5) CAN uses a non-destructive bus arbitration technique. (6) CAN cannot support explosion-proof areas. Since its inception, CAN has gained increasing favor in the industrial sector thanks to its unique design philosophy, excellent functional features, and high reliability. It is now used in many industries, including automotive, robotics, building automation, textile machinery, medical devices, automated instruments, sensors, and more. Currently, there are several well-known large companies around the world that support the CAN protocol: Intel, Motorola, Honeywell, NEC, Siemens, Philips, and others. Its applications in measurement and control as well as smart instruments are shown in Figures 1 and 2. 2. LONWORKS LONWORKS is a local operating network; LON stands for Local Operating Network. It is a technology and product introduced by the American company Echelon in March 1991. Its main features are as follows: (1) LONWORKS uses the LONTALK communication protocol, which is based on the ISO/OSI reference model. This protocol provides all 7 layers of services defined by OSI, and this is a unique feature among all fieldbuses. (2) The communication rate is 78 Kbps/2700 m and 1.25 Mbps/130 m, with 32,000 nodes; the transmission medium can be twisted pair, coaxial cable, fiber optic cable, power cables, etc. (3) The core of LONWORKS is the Neuron chip, which contains 3 8-bit CPUs. The first CPU is a media access control processor that handles layers 1 and 2 of the LONTALK protocol. The second CPU is a network processor that implements the functions of layers 3 to 6 of LONTALK; the third CPU is an application processor that handles Layer 7 protocols of LONTALK, executing code written by users as well as operating system services called by such code. The programming language for Neuron chips is Neuron C, which is derived from ANSI C. LONWORKS provides a set of development platforms, LON BUILDER and Node Builder, priced at 100,000–300,000. (4) Lonworks can form typical network topologies such as bus, star, ring, and hybrid types; it allows for flexible combinations of network topologies, and different fieldbuses can be interconnected through gateways. See Figures 3 and 4. (5) The medium access protocol of the Lontalk protocol improves upon CSMA by adopting a new protocol called Predictive P-Persistent CSMA. (6) Lonworks can support explosion-proof areas. Since its introduction in 1991, LonWorks technology has developed rapidly; by 1996, there were 2,600 manufacturers using it, with over 2 million nodes installed. This far exceeds any other fieldbus for measurement and control applications. Its areas of use include industrial control, building automation, aviation/aerospace, and virtually all domains related to measurement and control. Of its total sales, 40% come from industrial control applications and 30% from building automation applications. (LonWORKS is not part of the IEC fieldbus standards.) 3. HART HART stands for Highway Addressable Remote Transducer. It is a communication protocol developed by the American company Rosemount in 1986. It has the following characteristics: (1) The HART protocol is based on the ISO/OSI model, and its communication structure consists of three layers: the physical layer, the data link layer, and the application layer. (2) HART supports the simultaneous presence of analog and digital signals on the same line, allowing it to be used in combination with other analog devices. Its communication speed is 1200bps when an FSK digital signal is superimposed on a 4–20 mV ADC analog signal; with shielded twisted pair cable, the distance between a single device and another can reach 3000 meters ； The interconnection distance between multiple devices can reach 1500 meters. (3) HART supports point-to-point, master-slave response, and multi-point broadcast communication modes; the communication style is either \"question-and-answer\" or \"broadcast\". (4) HART can utilize bus power supply, meeting intrinsically safe explosion-proof requirements. Strictly speaking, HART does not belong to the category of fieldbuses; rather, it is a product that represents the transition from analog systems to digital systems. Products based on the HART protocol are still being manufactured worldwide, but there are many fieldbus products available at prices that are comparable to those of HART products, so its lifecycle will not be very long. 4. PROFIBUS PROFIBUS is short for Process Fieldbus. It is the German standard for fieldbuses, established in 1987 by the Federal Ministry of Research and Technology of Germany in accordance with the ISO/OSI reference model; it officially became the German fieldbus standard (DIN 19245) in 1991 ； It also became the European standard (EN50170) in 1998. It has the following characteristics: (1) PROFIBUS consists of three parts: a. PROFIBUS—FMS (Fieldbus Message Specification, the specification for fieldbus messages). This section is responsible for establishing communication between the controller and the field devices, as well as for the exchange of information among controllers; therefore, it focuses on system functionality rather than system response time, and is suitable for handling cyclic and aperiodic communication tasks that involve large data exchanges at moderate transmission speeds. b. PROFIBUS—DP (Decentralized Periphery) is an optimized communication module suitable for communication between automatic control systems and peripheral devices in applications with strict timing requirements. c. PROFIBUS—PA (Process Automation), used in process automation; it implements the communication protocols specified in IEC 1158-2, and is applied in applications with high safety requirements as well as at stations powered by the bus. (2) PROFIBUS follows the ISO/OSI model, and its communication model consists of three layers: the physical layer, the data link layer, and the application layer. (3) The communication speed of PROFIBUS is: a. PROFIBUS—FMS: When using RS—485, it ranges from 9.6k to 500k bauds; the distance covered is from 1.6km to 4.8km, and up to 122 nodes can be connected ； When using FSK (Frequency Shift Keying), up to 32 nodes can be used, with a range of up to 5 km; the medium can be twisted pair or optical fiber. b. PROFIBUS—DP: RS-485 twisted pair or optical cable, at 9.6K-12M baud ； Maximum distance: 100m at 12M baud ； 1. At 1.5M baud, it is 200m; the distance can be increased using repeaters, with a maximum of 126 stations. c. The communication speed of PROFIBUS-PA is 31.25 kbps, with a maximum distance of ≤1.9 km. The number of instruments that can be connected to each segment is ≤32; however, this figure depends on the power consumption of the instruments connected to the bus as well as the maximum current allowed on the bus. As long as the power supplied to the bus does not exceed the specified maximum voltage and current values, intrinsic safety when operating in hazardous areas can be ensured. (4) Bus access protocol: All three series of PROFIBUS use the same bus access protocol. The data link layer employs a mixed-media access method, whereby master stations communicate with each other using a token-based system, while master stations and slave stations communicate in a master-slave manner. (5) The three PROFIBUS series can be integrated easily; DP and FMS use the same transmission technology and a unified bus access protocol, allowing these two systems to operate simultaneously on the same wire ； PA and DP can be easily integrated together using a segmented coupler, as shown in Figure 5. (6) Safety: PROFIBUS-PA uses bus-powered operation and meets explosion-proof requirements. Since its introduction in 1989, PROFIBUS has become a model of open systems thanks to its strict specifications and comprehensive functions. To date, more than 500 manufacturers around the world have joined the PROFIBUS User Association, offering nearly a thousand PROFIBUS products. The renowned Siemens company alone provides over 100 PROFIBUS products. By 1999, it was estimated that 350,000 sets of PROFIBUS equipment had been successfully installed in more than 50,000 industrial sites worldwide. 5. FF FF is the abbreviation for the Fieldbus Foundation. It is the only internationally recognized impartial, non-commercial standardization organization that is not affiliated with any particular company. Its goal is to establish a single international standard for fieldbuses, without any patent licensing requirements, so that it can be used by anyone. It has the following characteristics: (1) FF follows the ISO/OSI model; its communication model consists of four layers: the physical layer, the data link layer, the application layer, and the user layer. An additional layer (the user layer) is added on top of the OSI model. (2) Communication rate: FF defines two types of buses with different rates – the low-speed bus H1, which has a baud rate of 31.25 kbps and a transmission distance of 0.2 km to 1.9 km (depending on the medium) ； The high-speed bus H2 has a baud rate of 1.0 Mbps/750 m or 2.5 Mbps/500 m. The transmission medium can be twisted pair, coaxial cable, fiber optic, or radio. (3) Topology structure: H1: Supports bus or tree topology ； H2 supports bus topology. (4) Medium access: A technology that combines token granting with query-based communication is used, allowing multiple master stations to exist within a single network. During initialization, only one station is allowed to be in the speaking mode. Once the speaking mode is activated, the master station queries the slave stations, and a special frame structure is used to transmit the speaking mode to another master station. (5) Safety: The low-speed bus H1 is powered by an intrinsically safe bus, meeting intrinsically safe explosion-proof requirements. Established in 1994, FF has seen over 120 member companies from around the world join it, including renowned manufacturers of field equipment and suppliers of control systems. The IEC has approved the physical layer standards for FF. In April 1995, the Automation Research Institute of China’s Ministry of Metallurgy joined FF, and in April 1996, Huakong Technology Co., Ltd. also joined FF. III. Discussion on Application Issues It can be seen that there is no fieldbus in industry that can cover all application areas; each type of bus has its own characteristics. Given that the international standards for fieldbuses have not yet been established, and with numerous fieldbuses coexisting, a user who intends to use products based on fieldbus technology must choose appropriate products by considering their own requirements as well as the characteristics of various fieldbuses. So how should one make a choice? Before deciding on which fieldbus product to use, it is generally necessary to clarify the following aspects: 1. The scale, that is, the number of nodes that need to be connected via a fieldbus network. The scale affects the choice of fieldbus; for example, CAN can support up to 110 devices, while LONWORKS can handle as many as 32,000 nodes. PROFIBUS also has a node count ranging from several dozen to over a hundred. 2. Environmental conditions: These include the proximity of node locations, on-site safety and explosion prevention requirements, and the electromagnetic environment. Different environmental conditions also influence the choice of fieldbus. Firstly, the distance between nodes determines the length of the communication lines, and buses with different capabilities handle this distance range differently, which varies from several dozen meters to 10 kilometers. The on-site safety and explosion-proof requirements are a very important criterion. Based on the analysis above, all except the CAN bus can meet these safety and explosion-proof requirements; considering current trends, it is best to choose PROFIBUS-PA or FF H1. The quality of the electromagnetic environment at the site determines the communication medium to be used in building the network; if electromagnetic interference is severe, fiber optics are the best choice as the transmission medium. 3. Signal transmission conditions: This includes whether the transmitted signal is analog or digital, the amount of information, and the requirements regarding real-time performance. Different transmission signal conditions impose varying requirements on on-site capabilities; if analog and digital signals coexist, HART can be used ； If the amount of data to be transmitted is very large and real-time performance is not critical, PROFIBUS—FMS could be considered as an option ； If the amount of information is large and real-time performance is important, PROFIBUS—FMS and PROFIBUS—DP can be considered for constructing the system. 4. Status of field devices: This refers to either the technical upgrading of existing old devices or the use of new intelligent instruments that meet the requirements of field buses. This situation exists in many organizations across the country. Since many of the instruments originally used were of Type II or Type III, and they still perform well today, there is a desire to utilize advanced fieldbus technology to enhance the overall level of automation in production. This essentially means establishing a fieldbus network while making full use of the existing older equipment. Fortunately, the remote intelligent I/O solutions available on the market today can meet such requirements. For example, Beijing Zhongji Pufa Industry and Trade Development Co., Ltd. in China is capable of producing such products. With these I/O devices, the analog 4–20mA or 0–5V signals from traditional instruments can be converted into communication signals compatible with fieldbus systems, thereby enabling connection and communication with fieldbus networks. This approach represents a viable way for some older enterprises with limited financial resources to carry out technological upgrades. In short, as users, we always want to choose products with the best performance-to-price ratio. However, due to the wide variety of individual needs, it is often difficult for a single product to meet all requirements. Therefore, when making a selection, we can utilize gateway technology to integrate different fieldbuses together, thereby meeting our own needs. Additionally, if there are several products that can meet one’s requirements and their performance-to-price ratios are similar, then the factor to consider should be service. If a particular product has more users and offers better service (including installation, training, maintenance, etc.), it should be given priority over other options. Under the current circumstances, PROFIBUS and FF have a greater advantage in this regard.