How does HART communication work

The open communication protocol of Hart (highway addressable remote transducer) is a communication protocol between field intelligent instruments and control room equipment launched by rosement company in 1985. Hart device provides communication with relatively low bandwidth and moderate response time. After more than 10 years of development, Hart technology has been very mature abroad and has become the industrial standard of global intelligent instruments.

Transmission principle of hart communicator

The HART protocol adopts FSK frequency shift keying signal based on be11202 standard, superimposes audio digital signal with amplitude of 0.5mA on low-frequency 4-20mA analog signal for two-way digital communication, and the data transmission rate is 1.2mbps. Since the average value of FSK signal is O, it does not affect the size of analog signal transmitted to the control system and ensures the compatibility with the existing analog system. In HART protocol communication, the main variables and control information are transmitted by 4-20mA analog signal. If necessary, other measurement, process parameters, equipment configuration, calibration and diagnosis information are accessed through HART protocol.

Three types of commands for hart communicator

HART communication adopts half duplex communication mode, which is characterized by the realization of digital signal communication on the existing analog signal transmission line. It is a transitional product in the transformation from analog system to digital system. Therefore, it has strong market competitiveness and has developed rapidly in the current transition period. Hart specifies a series of commands and works in command mode. It has three types of commands. The first type is called general command, which is understood and executed by all devices; The second type is called general behavior command, and the functions provided can be realized in many field devices (although not all). This kind of command includes the function library of the most commonly used field devices; The third type is called special equipment command, which is convenient for work to realize special functions in some equipment. This kind of command can not only be used in the foundation, but also be unique to the company developing this command. These three types of commands can usually be found in a field device at the same time.

Advantages of hart communicator

1. Hart adopts a Unified Device Description Language DDL, which is used by field device developers to describe device characteristics. The Hart foundation is responsible for registering and managing these device descriptions and compiling them into a device description dictionary. The main device uses DDL technology to understand the characteristic parameters of these devices without developing special interfaces for these devices. However, due to this analog-to-digital mixed signal system, it is difficult to develop a communication interface chip that can meet the requirements of various companies. Hart can be powered by bus, which can meet the requirements of intrinsic safety and explosion-proof, and can form a dual master equipment system with handheld programmer and management system host as the main design standard.

2. HART protocol can transmit digital information in both directions. Analog signals can only transmit information in one direction, either from the device to the host (input) or from the host to the device (output). HART protocol can transmit digital information in both directions. This breaks through the way that traditional instruments can only receive control information from the host - such as valve control, which can also transmit the information status of the valve to the host. Similarly, in the past, only process variables could be sent to the monitoring computer. Now, information such as configuration settings can also be received.

3. A large amount of information is transmitted. Traditional analog and discrete devices can only communicate in the way of single process variable - so it is difficult to find a simple way to determine whether the transmitted information is valid. Using Hart can not only obtain process index, but also obtain other types of information. Each Hart device includes 35-40 standard information items, such as equipment status, diagnostic alarm, process variable, unit, loop current, percentage range, manufacturer and equipment label. Other information also includes: the host computer queries the Hart device digitally and informs whether the device is set correctly and whether it operates normally. This function can eliminate most of the daily inspection work and help to find process faults before they lead to major problems.

4. Support multivariable processing. In digital communication mode, a pair of cables can handle multiple variables. For example, a transmitter can process inputs from multiple sensors. Four process variables can be processed in one message. In any field instrument, the HART protocol supports 256 process variables.

5. Foundation management. When using Hart, users are not subject to a specific supplier or local "standard". This is because Hart technology does not belong to a company, nor is it controlled by a national or standard entity. In fact, the technology is managed by the unrestricted non-profit Hart communications foundation.

6. Widely used in process industry, Hart is a widely used protocol in the world. Nearly hundreds of products using Hart technology can be obtained from different suppliers. The complete range of products means that Hart products can meet the needs of almost all process applications, and the most suitable one can be selected from multiple dealers.

7. Full interoperability full interoperability means that Hart compatible products and master systems from any vendor can work together. Compatible with HART devices, some main systems use unified instructions, and further systems use device description to understand all Hart information.