What is fieldbus?
Fieldbus, developed internationally in the late 1980s and early 1990s, is a communication network for interconnecting intelligent field devices in fields such as process automation, manufacturing automation, and building automation. As the foundation of factory digital communication networks, it connects production process sites and control equipment with higher control and management levels. It is not only a basic network but also an open, new type of fully distributed control system. This integrated technology, primarily based on intelligent sensing, control, computer, and digital communication technologies, has attracted worldwide attention, becoming a hot topic in automation technology development and leading to profound changes in automation system architecture and equipment. Fieldbus devices operate at the lowest level of process equipment. As a factory-level basic communication network, it requires simple protocols, strong fault tolerance, good security, and low cost. It also needs a certain degree of time determinism and high real-time performance, as well as stable network load, mostly short frame transmission, and frequent information exchange. Due to these characteristics, fieldbus systems, from network structure to communication technology, exhibit features different from higher-level high-speed data communication networks.
Currently, there are over 40 types of fieldbuses internationally, but no single fieldbus can cover all applications. Based on the size of the data transmitted, they can be categorized into three types: sensor buses (sensorbus), which transmit data bitwise; device buses (devicebus), which transmit data bytewise; and fieldbuses, which transmit data streams. Let's learn about the main types of buses together.
I. FF Fieldbus
The FF Fieldbus Foundation was jointly established in June 1994 by WORLDFIPNA (North America, excluding Europe) and the ISP Foundation. It is an international organization whose goal is to establish a single, open, and interoperable international fieldbus standard. This organization provides strong support to the IEC fieldbus standard drafting working group. Currently, it has over 100 member companies, including major process control product and system manufacturers worldwide. In April 1997, the organization established the China Instrument and Control Society Fieldbus Professional Committee (CFC) in China, dedicated to promoting the application of this technology in China. FF was established relatively late, and its product launches and full application of this technology in engineering came later than PROFIBUS and WORLDFIP. However, because FF was founded in September 1992 through the merger of Fisher Rosemount's ISP (Interoperable System Protocol) and WORLDFIPNA, it possesses considerable strength: currently, FF plays a pivotal role in the development of the IEC fieldbus standard.
2. LonWorks
LonWorks fieldbus, introduced by Eschram in 1992, is a local operating network initially used for building automation but quickly expanded to industrial field networks. LonWorks technology provides a complete, open, and readily available solution for designing and implementing interoperable control networks. The core of LonWorks technology is the NeuronChip. This chip contains three microprocessors: a MAC processor for media access control; a network processor for OSI layers 3-6; and an application processor for user field control applications. Data is transferred between them via shared memory. The control unit requires data acquisition and control functions; therefore, the NeuronChip has 11 I/O ports. These I/O ports can be flexibly configured to interface with peripheral devices , such as RS232, parallel ports, timers/counters, interval processing, and bit I/O, depending on the requirements.
The LON bus integrates multiple functions of modern fieldbuses while also possessing some characteristics of local area networks (LANs), making it widely used in aerospace, agricultural control, computer/peripheral equipment, diagnostics/monitoring, electronic measurement equipment, testing equipment, medical and health, military/defense, office equipment systems, robotics, security, confidentiality, sports/entertainment, telephone communications, transportation equipment, and other fields. Its versatility indicates that it is not a bus designed for a specific field, but rather a network technology that can integrate control systems from different fields into a more complex system based on LONWORKS.
III. Profibus
Profibus is a fieldbus standard based on the German national standard DIN19245 and the European standard prEN50170. The ISO/OSI model is also its reference model. The Profibus series consists of Profibus-DP, Profibus-FMS, and Profibus-PA. The DP type is used for high-speed transmission between distributed peripherals and is suitable for applications in manufacturing automation. FMS stands for Field Information Specification and is suitable for general automation such as textiles, building automation, programmable logic controllers (PLCs), and low-voltage switches. The PA type is a bus type used for process automation and conforms to the IEC1158-2 standard. This technology was jointly developed by Siemens and more than a dozen German companies and research institutes. It adopts the physical layer and data link layer of the OSI model, which form a subset of its first part of the standard. The DP type omits layers 3-7 and adds a direct data connection fitting as a user interface, while the FMS type only omits layers 3-6 and adopts the application layer, forming the second part of the standard. The standard for PA type is still under development. Its transmission technology complies with the IEC1158-2(1) standard and can achieve bus power supply and intrinsically safe explosion protection.
IV. CAN Bus
Controller Area Network (CAN) was first developed by Bosch in 1985 for building in-vehicle networks. Prior to this, automakers used point-to-point cabling systems to connect in-vehicle electronic devices. However, as the number of in-vehicle electronic devices increased, the wiring required for this system also increased, making the system bulky and expensive. Therefore, manufacturers began using in-vehicle networks to replace point-to-point cabling systems to reduce wiring costs, complexity, and system weight. Against this backdrop, CAN emerged as a highly integrated serial bus system for building intelligent device networks, becoming the standard for in-vehicle networks. Consequently, CAN rapidly gained popularity in the automotive industry and became an international standard (ISO 11898) in 1993. After 1994, several higher-level CAN protocol standards were developed, such as CANopen and DeviceNet. These new protocols have also been widely accepted in other markets and are now part of industrial communication standards.
CAN originated in the automotive industry, so its most common application is in-vehicle electronic networks. However, in the past two decades, more and more industries have recognized the reliability and advantages of CAN, applying the CAN bus to many other scenarios. For example, CAN networks are used in trams, subways, light rail, and long-distance trains. Various CAN-based networks can be found in these vehicles, such as connecting door units, brake controllers, and passenger counting units. CAN applications are also found in the aerospace field, such as flight status sensors, navigation systems, and cockpit computers. Furthermore, CAN buses are also present in aerospace applications, such as flight data analysis and aircraft engine control systems (fuel systems, pumps, linear actuators, etc.).
V. Devicenet
DeviceNet is an open, low-cost, high-performance communication network based on CAN (Controller Area Network) technology that emerged in the mid-1990s and conforms to global industry standards. It was initially developed and applied by Rockwell Automation in the United States.
DeviceNet inherits many features from CAN, a well-designed communication bus primarily used for real-time control data transmission. Key features of DeviceNet include: short frame transmission (maximum 8 bytes per frame); non-destructive bit-by-bit arbitration; a network capacity of up to 64 nodes; data transmission baud rates of 125kb/s, 250kb/s, and 500kb/s; point-to-point, multi-master, or master/slave communication modes; and adoption of CAN's physical and data link layer protocols.
VI. HART Bus
HART stands for Addressable Remote Sensor High-Speed Channel Protocol. It was developed in the 1980s as many instrument users desired a digital communication standard compatible with 4-20mA analog signals. In 1986, Losmond, a subsidiary of Emerson, launched this standard.
The HART protocol uses FSK (Frequency Shift Keying) signals based on the Bell202 standard . It superimposes a 0.5mA audio digital signal onto a low-frequency 4-20mA analog signal for bidirectional digital communication, achieving a data transmission rate of 1.2kbps . Since the average value of the FSK signal is 0, it does not affect the magnitude of the analog signal transmitted to the control system, ensuring compatibility with existing analog systems. In HART protocol communication, the main variables and control information are transmitted via the 4-20mA signal. When needed, additional measurement, process parameters, equipment configuration, calibration, and diagnostic information are accessed through the HART protocol.
7. CClink
CC-Link, or Control and Communication Link System, is an open fieldbus jointly launched by Mitsubishi and several other companies. It features large data capacity, selectable communication speeds, and is a composite, open, and highly adaptable network system capable of accommodating different ranges from higher management layer networks to lower sensor layer networks.
8. WorldFIP
WorldFIP is a component of European standards. WorldFIP is one of the three components (Volume 3) of the European standard EN50170. It was developed based on the French standard FIP-C46-601/C46-607, adopting the IEC physical layer international standard (1158-2), and consists of three communication layers. A significant feature of WorldFIP is that it provides a single fieldbus with a single physical layer for all industrial and process control. Lower-level control systems, manufacturing systems, and drive systems can all be directly connected to the WorldFIP bus at the control level, eliminating the need to use a mix of RS485 and other low-speed buses to connect lower-level devices and achieve the same functionality.
9. Interbus
INTERBUS, as one of the IEC 61158 standards, is widely used in manufacturing and machining industries to connect signals from sensors/actuators to computer control stations. It is an open serial bus system. The INTERBUS bus was introduced in 1984, and its main technology developer was the German company PhoenixContact. INTERBUSClub is a global organization of INTERBUS device manufacturers and users, currently with independent Club organizations in 17 countries and regions, and more than 500 members.
The INTERBUS bus includes a remote bus network and a local bus network. Both networks transmit the same signals but at different voltage levels. The remote bus network is used for long-distance data transmission using RS-485. This network is not powered locally and communicates in full-duplex mode at a rate of 500 kbps. The local bus network connects to the remote network, and the BK module on the bus terminal (BT) on the local network is responsible for converting remote network data into local network data.
