Introduction to the hart protocol

HART (Highway Addressable Remote Transducer), an open communication protocol for addressable remote sensor high-speed channels, is a communication protocol for on-site intelligent instruments and control room equipment launched by Rosement in 1985. HART devices provide communication with relatively low bandwidth and moderate response time. After more than 10 years of development, HARTHART (Highway Addressable Remote Transducer), an open communication protocol for addressable remote sensor high-speed channels, was launched by Rosement in the United States in 1985 A communication protocol used between field intelligent instruments and control room equipment. HART devices provide communication with relatively low bandwidth and moderate response time. After more than 10 years of development, HART technology has become very mature abroad and has become the global industry standard for smart meters.　　HART protocol adopts FSK frequency shift keying signal based on Bell202 standard, superimposes audio digital signal with amplitude of 0.5mA on low frequency 4-20mA analog signal for two-way digital communication, data transmission rate is 1.2Mbps. Since the average value of the FSK signal is 0, it does not affect the size of the analog signal transmitted to the control system, ensuring compatibility with the existing analog system. In the HART protocol communication, the main variables and control information are transmitted by 4-20mA. If necessary, other measurement, process parameters, device configuration, calibration, and diagnostic information are accessed through the HART protocol. HART communication uses a half-duplex communication method, which is characterized by the realization of digital signal communication on the existing analog signal transmission line. It is a transitional product during the transition from analog system to digital system, so it has a strong advantage in the current transition period. Market competitiveness has developed rapidly. HART stipulates a series of commands, which work according to the command mode. It has three types of commands, the first is called general commands, which are understood and executed by all devices; the second is called general behavior commands, and the functions provided can be implemented in many field devices (though not all) , This type of command includes the function library of the most commonly used field devices; the third type is called special device commands, in order to work in some devices to achieve special functions, such commands can be used in the foundation, but also Unique to the company that developed this command. These three types of commands can usually be found in a field device at the same time.　　HART uses a unified device description language DDL. Field device developers use this standard language to describe device characteristics. The HART Foundation is responsible for registering and managing these device descriptions and compiling them into device description dictionaries. The main device uses DDL technology to understand the characteristics of these devices without having to provide these devices. Develop a dedicated interface. However, due to this analog-digital mixed-signal system, it is difficult to develop a communication interface chip that can meet the requirements of various companies. HART can use bus power supply, which can meet the requirements of intrinsic safety and explosion-proof, and can form a dual master device system with a handheld programmer and a management system host as the master device.