Concept, development, and command type of HART Protocol

HART protocol, in some important bus field, we usually hear its name. So what role does this Protocol play? How do I specify the message structure and its related command types? This article will answer your questions carefully.

1. HART (Highway Addressable Remote Transducer), an open communication protocol that can address Remote sensor express connect, it is a communication protocol between on-site intelligent instruments and control room equipment launched by Rosement in 1985. The HART device provides communication with relatively low bandwidth and moderate response time, after more than 10 years of development, HART technology has matured abroad and has become an industrial standard for global intelligent instruments ｡

The HART protocol is based on the open Interconnection Model of the International Standardization Organization, the OSI Standard physical layer, data link layer, and application layer are used. The HART Protocol specifies the physical form of transmission, message structure, data format, and a series of operation commands, is a master-slave protocol. When the communication mode is "Q &, one field device only responds to the request. The HART protocol allows two hosts in the system (for example, one for system control and the other for HART communication). If analog signals are not required, up to 15 slave devices can be connected to a single cable line in a multi-point system ｡

The HART protocol uses the binary Frequency Shift Keying Signal Based on the Bell202 standard, and overlays an audio digital signal with a amplitude of 20mA on the low-frequency 4-5mA analog signal for Bidirectional digital communication, the data transmission rate is 1.2Mbps. Because the average value of the key-1-bit-2 signal is 0, the analog signal sent to the control system is not affected, it ensures compatibility with the existing simulation system. The main variables and control information in the HART protocol communication are transmitted from 4-20mA, in addition, measurement, process parameters, device configuration, calibration, and diagnosis information are accessed through the HART protocol ｡

HART communication adopts a half-duplex communication method. It is a transitional product that implements digital signal communication over the existing analog signal transmission line, therefore, it has a strong market competitiveness in the current transitional period and has achieved rapid development. HART has defined a series of commands that work in command mode. It has three types of commands:

The first class is called General commands, which are commands that all devices understand and execute;

The second type is called general behavior commands. The functions provided can be implemented in many field devices (though not all). These Commands include the function libraries of the most commonly used field devices;

The third type is special device commands, which can be used in certain devices to implement special functions. These commands can be made available in the foundation and exclusive to the companies that develop such commands ｡

These three types of commands are usually found in a field device ｡

The HART protocol uses the unified device Description Language DDL. field device developers use this standard language to describe device features. 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 having to develop special interfaces for these devices, as a result, it is difficult to develop a communication interface chip that can meet the requirements of various companies. HART can use bus power supply to meet the essential safety and explosion-proof requirements, A dual-master device system consists of a handheld programmer and a management system host ｡

2. The Foundation Fieldbus, namely, FoudationFieldbus (FF), is based on the ISO/OSI Open System Interconnection Model and uses its physical layer, data link layer, and application layer as the corresponding layers of the FF communication model, the user layer is added to the application layer ｡

The Foundation's on-site bus is divided into two types of communication rates: Low Speed H1 and high speed H2. The transmission rate of H1 is 3125 Kbps, and the communication distance can reach 1900 m (can be extended by relay), which can support bus power supply, supports intrinsically safe and explosion-proof environments ｡

H2 transmission rates are 1 Mbps and Mbps, and the communication distance is M and M. Physical transmission media can support cable, optical cable, and wireless transmission, the protocol conforms to the IEC1158-2 standard. The transmission signal of its physical media is encoded in Manchester, and the center position of each data transmission is either positive or negative, the negative hop changes to 1, so that the serial data bit stream has sufficient positioning information, to maintain the time synchronization between the sender and receiver, the receiver can determine the "1" and "0" Status of the Data Based on the polarity of the jump, or precisely locate the data based on the data center location ｡