Foundation Fieldbus has two communication rates: low-speed H 1 and high-speed H2. The transmission rate of H 1 is 3 1.25Kbps, and the communication distance can reach 1900m (which can be extended by repeater), which can support bus power supply and intrinsically safe explosion-proof environment. The transmission rates of H2 are 1Mbps and 2 5Mbps, and the communication distances are 750m and 500m. The physical transmission medium can support twisted pair, optical cable and wireless transmission, and the protocol conforms to IEC 1 158-2 standard. The transmission signal of its physical medium adopts Manchester coding, and the central position of each transmission data is either positive or negative. The positive transition represents 0 and the negative transition represents 1, so that there is enough positioning information in the serial data bit stream to keep the time synchronization between the senders. The receiver can not only judge the "1" and "0" states of the data according to the polarity of the jump, but also accurately locate the data according to the center position.
In order to meet the needs of users, Honeywell, Ronan and other companies have developed special chips that can complete the physical layer and some data link layer protocols, and many instrument companies have also developed products that meet the FF protocol. 1 bus has passed the A test and β test, and the FF field bus factory test system composed of 13 devices provided by different manufacturers has been completed. 2 bus standard has also been formed.
1996 10 was organized and implemented by the Fieldbus Foundation at ISA96 exhibition held in Chicago, and more than 70 FF products from more than 40 manufacturers were displayed to the world. These FF exhibits distributed in different floors and exhibition halls were interconnected into a seven-segment Fieldbus demonstration system through eye-catching orange-red cables, and the field equipment of each booth could interoperate on site, showing the field of the Foundation. LonWorks is another powerful fieldbus technology, which was introduced by American Ecelon company, co-sponsored by them and Motorola and Toshiba Hitach company, and officially announced in 1990. It adopts all seven-layer communication protocols of ISO/OSI model, adopts object-oriented design method, and simplifies network communication design into parameter setting through network variables. Its communication rate ranges from 300bps to 15Mbps, and the direct communication distance can reach 2700m(78kbps, twisted pair). Support twisted pair, coaxial cable, optical fiber, radio frequency, infrared ray, power line and other communication media, and develop corresponding communication media.
LonTalk protocol adopted by LonWorks technology is encapsulated in a chip called Neuron. There are three 8-bit CPUs in the integrated chip; A media access control processor, which is used to complete the function of 1 ~ 2 layer in the open interconnection model and realize the control and processing of media access; The second is called network processor, which is used to complete the functions of layers 3-6, and is responsible for addressing, processing, background diagnosis, function path selection, software timing, network management, network communication control, sending and receiving data packets, etc. The third is the application processor, which executes operating system services and user code. There is also a buffer in the chip to store information and realize information transfer between CPUs, which is used as a network buffer and an application buffer. For example, the neuron integrated chip MC 143 120E2 produced by Motorola Company contains 2KRAM and 2KEEPROM.
The continuous popularization of LonWoeks technology has contributed to the low cost of neuron chips (about $5 ~ 9 per chip), which in turn promoted the popularization and application of LonWorks technology, forming a good cycle. According to relevant data of Ecelon company, by July 1996, 5 million neuron chips had been produced.
The technical strategy of LonWorks Company is to encourage OEM developers to use LonWorks technology and neuron chips to develop their own application products. More than 2,600 companies participated in LonWorks technology to varying degrees: more than 1000 companies launched LonWorks products, and further organized LonWark Interoperability Association to develop and promote LonWorks technologies and products. Widely used in building automation, home automation, security systems, office equipment, transportation equipment, industrial process control and other industries. In order to support the interconnection and interoperability between LonWorks and other protocols and networks, the company is developing various gateways to interconnect LonWorks with Ethernet, FF, Modbus, DeviceNet, Profibus, Serplex, etc. into a system.
In addition, LonWorks neuron chip also has unique advantages in developing intelligent communication interfaces and intelligent sensors. LonWorks technology is defined as the standard of building automation protocol BACnet by ASRE of American Association of HVAC Engineers. According to the news just received, the American Consumer Electronics Manufacturers Association has passed a resolution to formulate EIA-709 standard based on LonWorks technology.
In this way, LonWorks has established a series of complete development, manufacturing, promotion and application frameworks from protocol development, chip design, chip manufacturing, control module development and manufacturing, OEM control products, final control products, distribution and system integration, which has attracted tens of thousands of enterprises to participate in this work and greatly promoted the popularization and application of a technology. Profibus is the field bus of German national standard DIN 19245 and European standard prEN 50 170. The ISO/OSI model is also its reference model. Profibus series includes Profibus -Dp, Profibus -FMS and Profibus-PA. DP type is used for high-speed transmission between distributed peripherals, and is suitable for applications in the field of processing automation. FMS is the field information specification, which is suitable for general automation such as textile, building automation, programmable controller and low-voltage switch. And PA type is a bus type used for process automation, which conforms to IEC 1 158-2 standard. This technology is jointly launched by more than a dozen German companies and research institutions, mainly Siemens. It adopts the physical layer and data link layer of OSI model, which constitute a subset of the first part of its standard. The DP model hides 3 ~ 7 layers and adds direct data connection fitting as the user interface, while the FMS model only hides 3 ~ 6 layers and adopts the application layer as the second part of the standard. The standard of PA type is still being worked out, and its transmission technology conforms to IEC1158-2 (1) standard, which can realize bus power supply and intrinsically safe explosion-proof.
Porfibus supports a variety of transmission modes, such as master-slave system, pure master station system and multi-master and multi-slave hybrid system. The master station has control over the bus and can send information on its own initiative. For multi-master station system, information is transmitted between master stations through tokens, and the station that obtains the token can have the bus control right within a predetermined time. * * * The maximum time for the token to circulate in each master station is specified in advance. According to the communication specification of Profibus, tokens are transmitted in the uplink direction between master stations in the order of address numbers. After the master station obtains the control right, it can send or request information to the slave station in a master-slave way to realize point-to-point communication. The master station can broadcast to all stations (no response is required) or selectively broadcast to a group of stations.
The transmission rate of Profibus is 96 ~ 12kbps. The maximum transmission distance is 1000m at 12 kbps and 400m at 15Mbps, which can be extended to 10km with repeater. Its transmission medium can be twisted pair or optical cable, and at most 127 stations can be connected. CAN is the abbreviation of Control Area Network, which was first introduced by BOSCH company in Germany, and is used for data communication between internal measurement and executive parts of automobiles. Its bus specification has been formulated as an international standard by ISO International Standards Organization, supported by Motorola, Intel, Philips, Siemens, NEC and other companies, and has been widely used in the field of discrete control.
CAN protocol is also an open system interconnection model based on international standards organization, but its model structure is only three layers, only taking the physical layer at the bottom of OSI, the data link layer and the application layer at the top. Its signal transmission medium is twisted pair, the highest communication rate can reach 1Mbps/40m, the farthest direct transmission distance can reach 1km/kbps, and the maximum number of connectable devices can reach 1 10.
The signal transmission of CAN adopts short frame structure, and the number of effective bytes in each frame is 8, so the transmission time is short and the interference probability is low. When a node has a serious fault, it has the function of automatic shutdown, cutting off the connection between the node and the bus, so that other nodes on the bus and their communication are not affected, and the anti-interference ability is strong.
CAN supports multi-master mode, and any node on the network can actively send information to other nodes at any time, and supports point-to-point, point-to-multipoint and global broadcasting to receive/send data. It adopts bus arbitration technology. When several nodes transmit information on the network at the same time, the node with high priority can continue to transmit data, while the node with low priority actively stops transmitting, thus avoiding bus conflicts.
Many companies have developed and produced communication chips conforming to CAN protocol, such as Intel's 82 52 7, Motorola's MC68HC05X4 and Philips' 82 C2 50. There is also a CAN bus interface card inserted in PC, which has simple interface, convenient programming and cheap development system. HART is the abbreviation of addressable remote transmission highway. It was first developed by Rosemout Company and supported by more than 80 famous instrument companies. 1993, HART communication foundation was established. This open communication protocol, called addressable remote sensing high-speed channel, is characterized by digital communication on the existing analog signal transmission line, which belongs to the transitional product of industrial process control in the process of transforming analog system into digital system, so it has strong market competitiveness and has been well developed in the current transitional period.
HART communication model consists of three layers: physical layer, data link layer and application layer. The physical layer adopts FSK (frequency shift keying) technology, and a frequency signal is superimposed on the 4 ~ 20mA analog signal, and the frequency signal adopts Bell202 international standard. The data transmission rate is 1200bps, the signal frequency of logic 0 is 2200Hz, and the signal transmission frequency of logic 1 is 1200Hz.
The data link layer is used to establish the HART information format according to the HART communication protocol rules. Its information structure includes starting code, address of display terminal and field device, number of bytes, field device status and communication status, data, parity, etc. Its data byte structure is 1 start bit, 8 data bits, 1 parity bit and 1 stop bit. The function of application layer is to realize HART instruction, that is, to convert communication state into corresponding information. It provides a series of commands; Work in a commanding manner. It has three kinds of commands, the first is called general command, which all devices understand and execute; The second category is called general behavior command, which provides many functions that can be realized by field devices (though not all), including the function library of the most commonly used field devices; The third category is called special equipment command, in order to realize special functions in some equipment. Such orders can be used publicly in foundations and can be unique to the company that developed them. These three types of commands can usually appear in field devices at the same time. HART supports point-to-point master-slave response and multicast. The data update rate is 2 ~ 3 times/second when working in the commitment mode and 3 ~ 4 times/second when working in the broadcast mode. It can also support two communication master devices. The number of devices that can be hung on the bus is as high as 15, each field device can have 256 variables, and each message can contain at most 4 variables. The maximum transmission distance is 3000m, and HART adopts unified device description language DDL. Field equipment developers use this standard language to describe equipment characteristics, and the HART Foundation is responsible for registering and managing these equipment descriptions and compiling them into the equipment description dictionary. The master device uses DDL technology to understand the characteristic parameters of these devices without developing special interfaces for these devices. However, due to this analog-digital mixed signal system, it is difficult to develop communication interface chips that can meet the requirements of various companies. HART can supply power through the bus, which can meet the requirements of intrinsically safe and explosion-proof. Although RS-485 can't be called fieldbus, as the originator of fieldbus, many devices continue to use this communication protocol. Using RS-485 communication, the equipment is simple, the cost is low, and it still has certain vitality. OPTO-22 command set based on RS-485 has also been widely used in many systems.