What Is The Difference Of RS485 And MODbus

- Apr 03, 2018-

The relation and difference of RS485 and MODBUS

 RS485 and MODBUS often write together,but what is  their difference and relation?

RS485 is a physical interface, simply hardware.

MODBUS is an international standard communication protocol for exchanging data between devices from different vendors (usually for industrial use);

The so-called agreement can also be understood as the "language" which is above mentioned. It is simply software.

Under normal circumstances, two devices transmit data through the MODBUS protocol:

The earliest use RS232C as a hardware interface, (that is ordinary computer serial communication port -serial port);also use RS422,

There are also commonly use RS485,this interface has a long transmission distance and is used in many industrial fields.

MODBUS protocol is divided into MODBUS RTU, MODBUS ASCII and later developed MODBUS TCP three modes:

The physical hardware interfaces used in the first two (MODBUS RTU, MODBUS ASCII) are serial communication ports (RS232, RS422, RS485).

And MODBUS TCP is in order to comply with the current development trend of the world, and anything can be connected with Ethernet network or the Internet to transmit data. So MODBUS TCP mode, the hardware interface of this mode is the Ethernet port, which is the common network port on our computer.

We attribute the industrial network to three types: RS485 network, HART network and fieldbus network.

HART network: HART is a transitional bus standard proposed by Emerson. He mainly superimposes digital signals on current signals of 4-20 mA. The physical layer uses BELL202 frequency shift keying technology to achieve the functions of some smart meters. However, this agreement is not a truly open standard. To join his foundation, one must get an agreement and join a fee for a part of the foundation. The technology is mainly monopolized by several large foreign companies. In the past two years, companies in the country have done so, but they have not yet reached the level of foreign companies. A large number of smart meters now have HART round cards and all have HART communication capabilities. However, from the domestic point of view, it has not really utilized this part of its functions. At most, it has only used the communicator to set parameters, it has not played the HART smart meter's proper function, and it has not been networked to monitor the equipment. In the long run, due to the low HART communication speed and networking difficulties, the procurement volume of HART instruments will decline. However, because HART instruments have a history of more than 10 years, the number of HART instruments is now very large. For some system integrators, There is still a lot of space available.

Fieldbus network: Fieldbus technology is one of the hotspots in the field of automation in today's automation industry. It is known as a computer local area network in the field of automation. Its appearance marks the beginning of a new era of automation control technology. Fieldbus is a digital, serial, multi-station communication network that connects instruments that are set up on the control site to control devices that are set up in the control room. The key sign is that it can support bidirectional, multi-node, bus-type all-digital communications. In recent years, fieldbus technology has become a hot spot in the development of automation and instrumentation in the world. Its appearance is a revolutionary change in the structure of traditional control systems. It is an intelligent, digital, information, network, and decentralized automation system. The direction of the move, the formation of a new type of network integrated full distributed control system --- fieldbus control system FCS (Fieldbus Control System). However, various standards for fieldbus now exist in parallel and have their own areas of survival. There is no truly unified standard. The key is not to see when they can form a unified standard and the technology is not mature enough. In addition, the types of instrumentation on the field bus are still relatively small and there is little room for selection, and the price is also high. From the perspective of end-users, most of them are still in a wait-and-see mode. They all want to wait until after the technology is mature and consider it to be implemented.

RS485 network: RS485/MODBUS is a popular cloth network mode, which is characterized by simple and convenient implementation, and now supports  RS485 instruments more and more, especially in the oil industry RS485/MODBUS is simply dominate the world, the current instrument Businesses have also turned to support RS485/MODBUS. The reason is very simple. Like the original HART instrument, it is very difficult and expensive to buy a switch port. The RS485 switch interface is much cheaper and more varied. At least in the low-end market, RS485/MODBUS will also be the most important networking method and will not change in the next two to three years.

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