Functions and functions of HART Protocol

In sensors, there is a type of protocol called HART protocol. So what is the specific purpose of this protocol? Now let's give a brief introduction. Then, we will discuss the functions and functions of this Protocol through several aspects.

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 communications with relatively low bandwidth and moderate response time. After more than 10 years of development, the HART technology has become very mature abroad and has become the industrial standard of global intelligent instruments.

According to the open Interconnection Model of the International Organization for Standardization (ISO), the HART protocol uses the OSI Standard physical layer, data link layer, and application layer. The HART Protocol specifies the physical form of transmission, message structure, data format, and a series of operation commands. It is a master-slave protocol. When the communication mode is "Q & A", a 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, up to 15 slave devices can be connected to a single cable line in a multi-point system.

The HART protocol uses a base-on-the-Bell202-Standard Frequency Shift Keying Signal. It overlays an audio digital signal with a amplitude of 20mA on a low-frequency 4-5mA analog signal for two-way digital communication. The data transmission rate is 1.2 Mbps. Because the average value of the 2's key-value pairs is 0, the analog signal sent to the control system is not affected, ensuring 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 configurations, 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, the current transitional period has a strong market competitiveness and achieved rapid development. HART specifies 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, and the provided functions can be implemented in many field devices, although not all). Such 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 the commands.

In a field device, these three types of commands are usually found at the same time.

The HART protocol adopts the unified device Description Language DDL. Field Equipment 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 master device uses DDL technology to understand the characteristic parameters of these devices without having to develop special interfaces for these devices. However, due to this analog digital hybrid 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 to meet the essential safety and explosion-proof requirements. It can form a dual-master device system consisting of a handheld programmer and a management system host.

2. Foundation Fieldbus, namely, FoudationFieldbus, FF. Based on the ISO/OSI Open System Interconnection model, it takes its physical layer, data link layer, and application layer as the corresponding layers of the FF communication model, and adds the user layer to the application layer.

The Foundation Fieldbus is divided into two types of communication rates: Low Speed H1 and high speed H2. The H1 transmission rate is 3125 Kbps, and the communication distance is up to 1900 m (the repeater can be extended). It supports bus power supply and intrinsically safe explosion-proof environments.

H2 transmission rate is 1 Mbps and 500 Mbps, and the communication distance is m and m. Physical transmission media can support cable, cable, and wireless transmission, protocols comply with IEC1158-2 standards. The transmission signals of the physical media are encoded in Manchester. The center location of each data transmission is either positive or negative. A positive hop changes to 0, and a 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" statuses of data based on the change polarity, and can precisely locate the data based on the data center location.