The difference between the HART protocol and the RS485 protocol is as follows:
1, different concepts
The HART protocol uses the FSK frequency shift keying signal based on the Bell202 standard to perform two-way digital communication with an audio digital signal with a magnitude of 0.5 mA superimposed on the low-frequency 4-20mA analog signal; The RS-485 protocol, a typical serial communication standard, is RS232 and RS485, which define voltage, impedance, etc., but do not define the software protocol.
2, different communication methods
HART communication uses a semi-duplex communication method, which is characterized by digital signal communication on existing analog signal transmission lines and is a transitional product during the transition from analog systems to digital systems. Therefore, in the current transition period, it has a strong market competitiveness and has achieved rapid development;
Electrical characteristics of RS-485: Logic "1" is represented by a voltage difference between two lines of +(2-6) V; The logic "0" is represented by a voltage difference of-(2-6) V. The interface signal level is lower than RS-232-C, so it is not easy to damage the chip of the interface circuit, and this level is compatible with TTL level and can be easily connected to the TTL circuit.
3, different norms
HART communication is based on the Bell 202 standard and uses frequency shift keying(FSK) to communicate at a rate of 1200 BPS. The signal frequencies representing 0 and 1 bit values are 2200 and 1200 Hz, respectively. The low level signal is superimposed on the analog measurement signal of 4 to 20 mA and will not cause any interference to the analog signal;
And ... The maximum transmission distance standard value of the RS-485 interface is 4,000 feet, which can actually reach 3,000 meters. In addition, the RS-232-C interface allows only one transceiver to be connected on the bus, that is, single-station capacity. The RS-485 interface allows up to 128 transceivers to be connected on the bus. That is, it has multiple stations, so that users can easily establish a device network using a single RS-485 interface.
Extended Data:
The HART protocol application layer defines the commands, responses, data types, and status reports supported by the protocol. At the application level, the public commands of the protocol are divided into four major categories:
General Command, which provides the required functionality for all devices on site
Commonly used commands provide functions common to many devices, but not all on-site devices have functional devices
A specific command that provides features specific to a particular site device, specified by the equipment manufacturer.
Series commands provide a standardized set of functions for specific measurement types of instruments that allow complete universal access without using device-specific instructions.
The RS-485 interface has good noise interference resistance. The above advantages such as long transmission distance and multi-station capabilities make it the preferred serial interface. The semi-duplex network composed of the RS485 interface generally requires only two connections. Therefore, the RS485 interface uses shielded twisted pair wire transmission.
The RS485 interface connector uses the DB-9 9-core plug holder, the DB-9(hole) interface with the smart terminal RS485, and the keyboard interface RS485 with the keyboard adopts the DB-9.