Overview of the HART Communication Protocol
HART (Highway Addressable Remote Transducer) is an open communication protocol introduced by the American company Rosemount in 1985 for data exchange between field smart instruments and control room equipment.
HART devices provide relatively low-bandwidth communication capability with moderate response times. After more than a decade of development, HART technology has become very mature abroad and has become the global industry standard for smart instruments.
Working Principle of the HART Protocol
FSK Signal Based on the Bell 202 Standard
The HART protocol uses an FSK frequency shift keying signal based on the Bell 202 standard. It superimposes an audio digital signal with an amplitude of 0.5mA on the low-frequency 4-20mA analog signal to achieve two-way digital communication.
Since the average value of the FSK signal is zero, it does not affect the magnitude of the analog signal transmitted to the control system, thereby ensuring compatibility with existing analog systems.
Coordination of Analog and Digital Signals
In HART protocol communication, the main variables and control information are transmitted by the 4-20mA signal. When needed, additional measurement, process parameters, device configuration, calibration, and diagnostic information are accessed through the HART protocol.
Half-Duplex Communication Mode
HART communication uses a half-duplex mode, characterized by realizing digital signal communication on existing analog signal transmission lines. It is a transitional product in the process of shifting from analog systems to digital systems, and therefore has strong market competitiveness during the current transition period and has developed rapidly.
HART Command Types
HART specifies a series of commands and operates according to command modes. It has three types of commands:
- Universal commands: Commands that all devices understand and execute.
- Common-practice commands: Functions that can be implemented in many field devices (though not all). This category includes the most commonly used field device function library.
- Device-specific commands: Facilitate the implementation of special functions in certain devices. Such commands can be openly used within the foundation and can also be proprietary to the company that developed them.
In a field device, all three types of commands can usually be found at the same time.
Device Description Language DDL
HART adopts 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 a device description dictionary.
Host devices use DDL technology to understand the characteristic parameters of these devices without having to develop dedicated interfaces for them. However, because of this mixed analog-digital signal system, it is difficult to develop a communication interface chip that can meet the requirements of all companies.
Core Advantages of the HART Protocol
- It can be powered by the bus and can meet intrinsic safety explosion-proof requirements.
- It can form a dual master device system with a handheld programmer and a management system host as the master devices.
- It is compatible with existing 4-20mA analog systems, protecting existing investment.
Quick Reference Table of Key HART Protocol Parameters
| Parameter | Specification |
|---|---|
| Modulation method | FSK frequency shift keying (Bell 202 standard) |
| Analog signal | 4-20mA |
| Digital signal amplitude | 0.5mA |
| Communication mode | Half-duplex |
| Command types | Universal, common-practice, device-specific |
| Device description | DDL (Device Description Language) |
Summary
As the global industry standard in the field of smart instruments, the HART protocol superimposes digital communication on the 4-20mA analog signal through FSK technology, balancing compatibility with analog systems and the flexibility of digital communication. Its three types of command systems and DDL device description language provide a solid foundation for the interoperability of field devices. For engineering technicians and procurement professionals, understanding the core mechanisms of the HART protocol helps make more reasonable instrument selection and system integration decisions during the transition period.