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The difference between HART protocol and RS485 protocol

The difference between HART protocol and RS485 protocol

Views: Release Time:2022-03-29

The difference between HART protocol and RS485 protocol is as follows: 


The HART protocol uses the FSK frequency shift keying signal based on the Bell202 standard, and superimposes an audio digital signal with an amplitude of 0.5mA on the low-frequency 4-20mA analog signal for two-way digital communication; while the RS-485 protocol, the serial communication standard is RS232 And RS485, they define the voltage, impedance, etc., but do not define the software protocol. 

2.Communication mode

HART communication adopts a half-duplex communication method, which is characterized by the realization of digital signal communication on the existing analog signal transmission line. It is a transitional product in the process of transition from analog system to digital system.Therefore, it has strong market competitiveness in the current transitional period and has achieved rapid development; 

Electrical characteristics of RS-485: Logic "1" is represented by the voltage difference between the two wires as +(2-6)V; logic "0" is represented by the voltage difference between the two wires as -(2-6)V. 


HART communication is based on the Bell 202 standard, adopts frequency shift keying (FSK), and communicates at a rate of 1200bps. The signal frequencies representing 0 and 1 bit values are 2200 and 1200 Hz, respectively. This low-level signal is superimposed on the 4-20mA analog measurement signal and will not cause any interference to the analog signal;

The standard value of the maximum transmission distance of the RS-485 interface is 4000 feet, which is actually up to 3000 meters. In addition, the RS-232-C interface only allows one transceiver to be connected on the bus, that is, single station capability. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is, it has multi-station capability, so that users can use a single RS-485 interface to easily establish a device network.


Extended information 

The HART protocol application layer defines the commands, responses, data types and status reports supported by the protocol.

At the application layer, the common commands of the protocol are divided into four categories:

General Commands, providing functions that must be implemented in all field devices.

Commonly used commands provide functions common to many devices, but not all field devices have functional devices.

Specific commands provide specific functions of a specific field device and are specified by the device manufacturer.

The series of commands provide a set of standardized functions for specific measurement types of instruments, allowing complete universal access without using device-specific commands.

The RS-485 interface has good anti-noise interference, long transmission distance and multi-station capability, etc., making it the preferred serial interface. The half-duplex network composed of RS485 interface generally only needs two wires, so The RS485 interface adopts shielded twisted pair transmission.

The RS485 interface connector adopts a 9-pin plug socket of DB-9, the RS485 interface with the intelligent terminal adopts DB-9 (hole), and the keyboard interface RS485 connected with the keyboard adopts DB-9. 

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