HART communication adopts half-duplex communication mode, which is characterized by realizing digital signal communication on the existing analog signal transmission line. It belongs to the transition product from analog system to digital system, so it has strong market competitiveness and has developed rapidly in the current transition period. HART specifies a series of commands that work in command mode. It has three commands. The first one is called general command, which is understood and executed by all devices. The second category is called general behavior command, which provides functions that can be implemented in many field devices (though not all). These commands include the function library of the most commonly used field devices; The third category is called special equipment command, which is convenient for work and realizes 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 adopts a 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 use bus to supply power, which can meet the requirements of intrinsic safety and explosion-proof, and can form a dual main system with handheld programmer and management system host as the main equipment.
2. Foundation Fieldbus, namely FoudationFieldbus (FF for short). Based on the ISO/OSI open system interconnection model, it regards its physical layer, data link layer and application layer as the corresponding layers of FF communication model, and adds a user layer to the application layer.
Foundation Fieldbus has two communication rates: low-speed H 1 and high-speed H2. The transmission rate of H 1 is 3 125Kbps, 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.