Communication principle of HART protocol
HART（Highway Addressable Remote Transducer），The High-Speed Addressable Remote Transducer (HART) protocol is a communication protocol introduced by Rosemount Inc. in 1985 for use between field smart instruments and control room equipment. HART devices provide communication with relatively low bandwidth and moderate response time. After more than 30 years of development, HART technology has become highly mature both domestically and internationally, and has become the industrial standard for smart instruments worldwide.
I.Transmission principle of HART protocol
The HART protocol utilizes FSK (Frequency Shift Keying) signals based on the Bell 202 standard, superimposing an audio digital signal with an amplitude of 0.5 mA on a low-frequency 4-20 mA analog signal for bidirectional digital communication, with a data transmission rate of 1.2 Mbps.
Although the HART communication protocol has a ±0.5mA signal modulated on top of a 4~20mA signal, the average value of the sine signal is 0, so it does not affect the average value of the 4~20mA analog signal transmitted to the control system. It can realize digital signal communication on existing analog signal transmission lines, ensuring compatibility with existing analog systems. It is a transitional product in the process of transitioning from analog systems to digital systems, and therefore has strong market competitiveness in the current transitional period and has developed rapidly. Because 1200Hz represents logic "1", it determines that the communication transmission rate of HART is 1200bps.
In HART protocol communication, the main variables and control information are transmitted through 4-20mA analog signals. When necessary, additional measurement, process parameters, device configuration, calibration, and diagnostic information are accessed through the HART protocol.Characteristics of HART protocol
II.HART Network
The HART network can operate in two configurations: point-to-point mode and multi-drop mode.
In the peer-to-peer network depicted in the figure below, traditional 4-20 mA current signals are utilized for process control and are unaffected by HART signals. Configuration parameters are transmitted digitally via the HART protocol.
The HART multi-drop communication network is used in situations where the distance between devices is relatively wide. As shown in the figure below, only two wires are required for communication with the master device. If necessary, IS shielding and auxiliary power supply can be configured to support up to 15 devices. All polling addresses of the devices are set to values greater than zero, and the device current is limited to a typical value of 4mA.
III.Characteristics of HART protocol
The HART protocol, also known as the hybrid protocol, is so named because it integrates both analog and digital communication. It supports both single-variable communication of 4-20 mA analog signals and the transmission of additional information in the form of digital signals. The digital information is modulated using FSK and carried on the standard 4-20 mA current loop. By employing filtering techniques, the digital signal can be separated from the analog signal, ensuring that it does not interfere with the transmission of the analog signal.
Therefore, some characteristics of the HART protocol are as follows:
- Includes both analog and digital communication
- Compatible with traditional analog instrument solutions
- Supports multivariable field devices
- Flexible data access through two hosts
- De facto development standard
- Backward compatible
- The only protocol supporting both analog and digital, unlike other fieldbuses which are inherently digital
- Point-to-point or multi-drop operation
- Response time of approximately 0.5 seconds