The difference between RS485 and MODBUS

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RS485 and MODBUS are the concepts which are often used in the industrial control system, but they are different, the following is a brief introduction.

RS485 is a physical interface, which is simply hardware. MODBUS is an international standard communication protocol, be used to exchange data between devices from different manufacturers (generally for industrial purposes), MODBUS protocol is used to transmit data between two devices in the industrial field. MODBUS protocol is divided into MODBUSRTU, MODBUSASCII and MODBUSTCP. The physical hardware interfaces used by the first two (MODBUSRTU, MODBUSASCII) are Serial communication interfaces (RS232, RS422, RS485).

MODBUSTCP uses Ethernet networks to transmit data.  The hardware interface of this mode is Ethernet interface.

We classify the industrial network into three categories: RS485 network, HART network and Fieldbus network.

HART network is a transitional bus standard, which is presented by Emerson, superimposed on the digital signal on the 4 ~ 20 mA current signal, the physical layer adopt BELL202 frequency shift keying technology to realize the functions of some intelligent instruments,

However, this protocol is not an open standard in the real sense. If want to get this protocol, need to join HART foundation and pay the membership fees. The HART technology have been monopolized by the international famous companies, china domestic also has company to do the HART technology in recent several years, but haven’t reach to the level of international famous brand companies. Now a large number of smart instruments are equipped with basic function of HART communication. HART equipment is generally used for range setting, engineering unit setting.

Fieldbus network: Fieldbus technology is one of the hot topics in the automation field today, it is known as the computer LAN in the field of automation, its appearance marks the beginning of a new era of automation control technology. Fieldbus is a digital, serial and multi-station communication network which connects the field instruments to the control equipment in the control room. Its key mark is that it can support all-digital communication of bidirectional, multi-node, bus-organization. In recent years, Fieldbus technology has become the focus of automation and instruments development in the world, its appearance make the traditional control system structure occur a revolutionary change, make the automatic control system stride toward the direction of intelligent, digital, information, network, decentralized, form a new network integration fully distributed control system, Fieldbus Control System (FCS).

However, Fieldbus has several kinds of standards, has not formed a truly unified standard. In addition, only a few types of instruments which has Fieldbus protocol, the room for choice is small, and the price is also high, most end user are still in a wait-and-see state, they don’t consider Fieldbus instruments until the Fieldbus technology is mature.

RS485 network: RS485/MODBUS is a popular communication mode nowadays, the characteristic is that implementation is simple and convenient. And now most instrument manufacturer support RS485/Modbus instrumentation, because RS485 converters are much cheaper and varied types. At least in the low-end market, RS485/MODBUS will still be the most important networking mode, will not change in the next two or three years.

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