HART(Highway Addressable Remote Transducer), an open communication protocol that can address the high-speed channel of remote sensors, is a communication protocol between field intelligent instruments and control room equipment introduced by Rosement Corporation in 1985. HART devices provide 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 for global smart meters.
HART protocol uses FSK frequency shift keying signals based on Bell202 standards, and superimposes audio digital signals with an amplitude of 0.5mA on low-frequency 4-20mA analog signals for bidirectional digital communication, with a data transmission rate of 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, additional measurements, process parameters, equipment configuration, calibration and diagnostic information are accessed by 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, which is a transitional product in the transition process from analog system to digital system, so it has a strong market competitiveness in the current transition period and has been developed rapidly. HART specifies a series of commands that work in a commanded manner. It has three types of commands. The first type is called general commands, which are commands that all devices understand and execute; the second type is called general behavior commands, and the functions provided can be used in many field devices (although not all) Implement this type of command includes the function library of the most commonly used field devices; the third type is called special device commands, which are convenient for working to implement special functions in some devices, this type of command is either open for use within the Foundation or unique to the company that developed the command. These three types of commands can usually be found simultaneously in a field device.
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, and the main device uses DDL technology to understand the characteristic parameters of these devices without having to develop a dedicated interface for these devices. 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, can meet the requirements of intrinsically safe explosion-proof, and can be composed of handheld programmer and management system host as the main device of the dual master system.