Introduction to 485, MODBUS Communication, and HART Communication
RS485 and MODBUS communication are commonly used concepts in industrial control systems, but they have differences. Below is a brief introduction.
RS485 is a physical interface, simply put, hardware.
MODBUS is an international standard communication protocol used for exchanging data between devices from different manufacturers (usually for industrial purposes); Industrial sites often transmit data between two devices through the MODBUS protocol: the MODBUS protocol is divided into three modes: MODBUSRTU, MODBUSASCII, and later developed MODBUSTCP. The physical hardware interfaces used for the first two (MODBUSRTU, MODBUSASCII) are all serial communication ports (RS232, RS422, RS485).
MODBUSTCP uses Ethernet network to transmit data. The hardware interface of this mode is the Ethernet port.
We categorize industrial networks into three types: RS485 networks, HART networks, and fieldbus networks.
HART Network
HART is a transitional bus standard proposed by Emerson, which mainly superimposes digital signals on 4-20mA current signals. The physical layer uses BELL202 frequency shift keying technology to achieve some functions of intelligent instruments. However, this protocol is not a truly open standard and requires joining his foundation to obtain the protocol, which incurs a portion of the cost. The technology is mainly monopolized by several large foreign companies, and in the past two years, some domestic companies have been doing it again, but it has not yet reached the level of foreign companies. Nowadays, a large number of smart meters have HART communication function. It is commonly used for range calibration of HART devices.
Fieldbus network
Fieldbus technology is one of the hot topics in the development of automation technology today, known as the computer local area network in the automation field. Its emergence marks the beginning of a new era in automation control technology. Fieldbus is a digital, serial, and multi station communication network that connects instruments set up in the control field with control equipment set up in the control room. Its key feature is the ability to support bidirectional, multi node, bus based all digital communication. In recent years, fieldbus technology has become a hot topic in the development of automation and instrumentation internationally. Its emergence is a revolutionary change in the traditional control system structure, and it is the direction of intelligent, digital, information-based, networked, and decentralized automation systems, forming a new type of network integrated fully distributed control system - Fieldbus Control System (FCS).
However, the various standard parallel memories of fieldbus have not yet formed a truly unified standard. In addition, there are relatively few types of instruments available for fieldbus, and the price is also relatively high. From the perspective of end users, most of them are still in a wait-and-see state and want to wait until the technology matures before considering it. Currently, there are few implementations.