Smart transmitters are very reliable and stable, but they often encounter communication problems. Since the unified communication protocol standard was not developed internationally when the smart transmitter was born, the communication protocols used by the transmitters produced by various manufacturers are not the same. Typical Rosemount 3051 uses the HART protocol, Honeywell's ST3000 uses the DE protocol, and Yokogawa's EJA series is the Brain protocol or the HART protocol.
HART (Highway Addressable Remote Transducer), an open communication protocol for addressing high-speed channels of remote sensors, was introduced in 1985 by ROSEMOUNT in the United States as a communication protocol between on-site smart meters and control room devices. After the handover to the HART Foundation in the early 1990s, the HART Foundation also has a large number of members, including Yokogawa and Honeywell, so Yokogawa's transmitters also have HART protocols, including EJA transmitters. The option "E" in the beginning is the HART protocol, and the "D" begins with the Brown protocol.
The HART protocol is based on the frequency shift keying (FSK) technology of the Bell 202 communication standard, which realizes digital communication by superimposing frequency signals on a current of 4 to 20 mA. 2 different frequencies: 1200 Hz and 2200H: represent "0" and "1", superimposed on the 4 ~ 20mA DC signal in the form of a sine wave, because the average value of these sine waves is zero, so no DC component is generated, no It will affect the 4~20rnA process signal, and it will complete the real synchronous communication without interrupting the transmission signal.
Some parameters of the HART protocol are on the instrument side, and some are in the upper computer or the handheld device. For different versions of the HART protocol, there may be restrictions in reading and writing. The corresponding software is required to support HART, if the version differs greatly. Instrumentation and Communicator must be supported by a corresponding upgrade package. That is, the HART protocol needs to be upgraded.
The BRAIN protocol is a protocol for Yokogawa's smart meters. It has this protocol in Yokogawa transmitters, vortex streets, electromagnetic flowmeters, and other instruments. The code on the output signal bar of the meter is “D”. The handheld intelligent terminal used with the BRAIN protocol is the BT200. With the support of the BRAIN protocol, the BT200 can set, change, display, print parameters, zero, etc. for the transmitter.
The parameters of BRAIN's protocol are written on the field instrument side, that is, in the table, using any BRAIN protocol host computer or handheld device, can read and write (if allowed to write) all the BRAIN protocol in the field table parameter. The BRAIN protocol software does not require an upgrade.
The DE protocol was developed by Honeywell and is primarily used in Honeywell transmitters. The DE protocol is a digital addition protocol that uses a low-frequency current pulse of 220 baud and the data is fed into the standard twisted pair in floating-point serial form. The signal uses two independent current pulses to communicate between the 4mA and 20mA loop currents, so the loop current value does not represent the measured value during digital communication. The digital signal and the analog signal are transmitted separately as the number of pulse currents represents "1" and "0". When the digital signal is transmitted, the analog signal must be interrupted. The HART communication frequency is high (about 1~2 kHz), and the time constant of the network is required to be no more than 0.65μs, so the maximum capacitance of the HART communication device and the network limits the maximum distance of communication, generally not more than 400m. The DE protocol communication frequency is lower, the maximum time constant of the network is 104μs, and the allowable cable distribution capacitance is much wider than the HART protocol. Therefore, the instrument communication with the DE communication protocol in the same device will be normal and the HART protocol will be used. Smart transmitters cannot communicate.
1.the concept of fieldbus
Fieldbus is a system that implements bidirectional serial multi-node digital communication between the computerized measurement and control devices at the production site. It is also called the underlying control network of open, digital, and multipoint communication.
Fieldbus technology puts dedicated microprocessors into traditional measurement and control instruments, enabling them to have digital computing and digital communication capabilities. Using a twisted pair cable that can be easily connected as a bus, multiple measurement and control instruments are connected into a network. System, and according to the open, standardized communication protocol, between the multiple computerized measurement and control equipment located in the field and between the field instrument and the remote monitoring computer, realize data transmission and information exchange, and form various automatic control to meet actual needs. system.
Fieldbus was developed internationally in the mid-1980s. With the continuous enhancement of microprocessor and computer functions and the reduction of prices, computer and computer network systems have developed rapidly. Fieldbus can realize information integration of the whole enterprise, implement integrated automation, form the underlying network of the factory, complete multi-point digital communication between on-site automation equipment, and realize information exchange between the underlying field devices and the production site and the outside world.
2. the development trend of fieldbus
In 1983, Honeywell introduced an intelligent instrumentation, which adds a computing function to the microprocessor chip based on the original analog meter. The digital signal is superimposed on the output 4-20mA DC signal to make the field and the control room. The analog signal is connected to a digital signal. Since then, the world's major companies have introduced a variety of smart meters. The emergence of smart meters laid the foundation for the birth of fieldbus.
The emergence of smart meters provides conditions for the digitization of on-site signals, but the communication standards provided by different manufacturers are not uniform, which constrains the development of the underlying network. Fieldbus requires different vendors to comply with the same manufacturing standards, and the formation of an open interconnect network is a trend in fieldbus.
3.the characteristics and advantages of the fieldbus
The fieldbus system breaks the control loop requirements adopted by the traditional control system, and the devices are connected one-to-one separately. The control module and the input and output modules in the original DCS system are put into the field device, and the field device has the communication capability, so the control system function can be directly completed on site without relying on the computer or control instrument in the control room. Complete decentralized control.
The fieldbus control system is both an open communication network and a fully distributed control system. It connects the intelligent devices as network nodes into an automated network system, and realizes integrated automation functions of basic control, compensation calculation, parameter modification, alarm, display, monitoring and optimization. It is a comprehensive technology with intelligent sensors, control, computer, digital communication and network as the main content.
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The fieldbus system has the following technical features:
1) The system is open and interoperable
The communication protocol complies with the same standards, and information exchange can be realized between devices. Users can form products of different vendors into open and interconnected systems according to their own needs. Information can be exchanged between systems and between devices. Devices with similar performance from different manufacturers can be interchanged.
2) System function autonomy
The system distributes the functions of sensing measurement, compensation calculation, engineering quantity processing and control to the field device. The field device can complete the basic functions of automatic control and can diagnose the operation status of the device at any time.
3) The system is dispersive
The fieldbus constitutes a fully decentralized control system structure that simplifies the system structure and improves reliability.
4) The system has adaptability to the environment
The field bus supports twisted pair, coaxial cable, optical cable, radio frequency, infrared, power line, etc. It has strong anti-interference ability, can realize power supply and communication by two-wire system, and can meet the requirements of safety explosion protection.
Due to the simplified structure of the field bus, functional units such as signal conditioning and conversion isolation of the DCS system and their complicated wiring are no longer needed, saving hardware quantity and investment. Simple wiring design saves installation costs. The device has self-diagnosis and simple fault handling capabilities, reducing maintenance effort. The interchangeability, intelligence and digitization of the equipment improve the accuracy and reliability of the system. It also has the advantages of simple design and easy reconstruction.
4.Several influential fieldbus technologies
1) Foundation Fieldbus
Foundation Fieldbus (FF) was founded in 1994 by Fisher-Rosemount and Honeywell. It is based on the ISO/OSI open system interconnection model, taking its physical layer, data link layer and application layer as the corresponding layers of the FF communication model, and adding a user layer on the application layer. The FOUNDATION fieldbus is divided into two communication rates, H1 and H2. The H1 has a transfer rate of 31.25 kbps and supports bus-powered and intrinsically safe explosion-proof environments. Supports twisted pair, fiber optic cable and wireless transmission, protocol symbol IEC1158-2 standard. The transmission signal is encoded by Manzant.
It was launched by Echelon Corporation of the United States. It uses the entire 7-layer communication protocol of the ISO/OSI model and adopts an object-oriented design method to simplify the network communication design to parameter settings through network variables. It supports a variety of communication media such as twisted pair, coaxial cable, optical cable and infrared, and has developed intrinsically safe explosion-proof products, which is known as a universal control network. Products using LonWorks technology and neuron chips are widely used in building automation, home automation, security systems, office equipment, transportation, industrial process control and other industries.
PROFIBUS is the fieldbus standard for the German standard (DIN19245) and the European standard (EN50170). It consists of PROFIBUS-DP, PROFIBUS-FMS and PROFIBUS-PA. DP is used to spread high-speed data transmission between peripherals and is suitable for processing automation. FMS is suitable for textile, building automation, programmable controllers, low voltage switches, etc. The bus type used by PA for process automation is subject to the IEC1158-2 standard.
CAN is the abbreviation for controlling the local area network. It is launched by the German company BOSCH and is widely used in the field of discrete control. CAN signal transmission adopts short frame structure, short transmission time, automatic shutdown function and strong anti-interference ability.
HART was first developed by Rosemount. It is characterized by the realization of digital signal communication on the existing analog signal transmission line, which belongs to the transition product of the analog system to the digital system. Because it uses analog and digital signal mixing, it is difficult to develop a universal communication interface chip. HART can be powered by the bus to meet the requirements of intrinsic safety and explosion protection, and can be used in a dual-master system with the handheld programmer and the management system host as the master device.
Fieldbus is one of the hotspots in the development of technology in the field of automation today, and is known as the computer LAN in the field of automation. Its emergence marks the beginning of a new era in the field of industrial control technology and will have an important impact on the development of this field.
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