RS485 and MODBUS differences and contacts

- Jul 24, 2019-

RS485 is a physical interface, simply hardware. 

MODBUS is an international standard communication protocol for the exchange of data (generally for industrial use) of equipment between different manufacturers;

In general, two devices transmit data through the MODBUS protocol: the earliest is to use RS232C as a hardware interface, (that is, the serial communication port on ordinary computers (serial port);

The MODBUS protocol is divided into three modes, MODBUS RTU, MODBUS ASCII, and modBUS TCP, which was later developed: The first two (MODBUS RTU, MODBUS ASCII) physical hardware interfaces are serial (Serial) communication ports (RS232, RS422, RS485).

MODBUS TCP, on the other hand, is to keep pace with the current trend of today's world, everything can be connected to the Ethernet network or the Internet, transmitting data. So then modBUS TCP mode, the hardware interface of this mode is The Ethernet port, that is, the network port commonly used on our computers.

We put industrial networks into three categories: RS485, HART and Fieldbus. 

HART Network: HART is presented by Emerson an excessive bus standard, he is mainly in 4 to 20 mAh current signal above the overlay of digital signals, the physical layer using BELL202 frequency shift keying technology to achieve some of the functions of intelligent instruments, but this protocol is not a true sense of open standards, To join his foundation to get an agreement, it will cost a portion of the cost to join the foundation. Technology is mainly monopolized by several large foreign companies, in the last two years there are domestic companies to do, but has not yet reached the level of foreign companies. A large part of today's smart meters are equipped with HART round cards, all with HART communication functions. But from the domestic point of view has not really used its part of the function, the most is only the use of hand-drilled to set its parameters, did not play the HART intelligent instrument should function, no network for equipment monitoring. In the long run, because of hart communication rate low network difficulties and other reasons, HART instrument procurement volume will decline, but because HART meters have more than 10 years of history is now in the installation of a very large number, for some system integrators still have a lot of room to use.

Fieldbus Network: Fieldbus technology is one of the hot spots in the field of automation technology development, known as the computer LAN in the field of automation, its emergence marks the beginning of a new era of automation control technology. Fieldbus is a digital, serial and multi-station communication network that connects the instruments set up in the control field and the control equipment set up in the control room. The key sign is the ability to support two-way, multi-node, bus-type all-digital communication. Field bus technology has become a hot spot in the development of automation and instrumentation in the world in recent years, and its emergence is a revolutionary change in the traditional control system structure, which is moving towards intelligent, digital, information, network, decentralized, and forming a new type of integrated network-wide distributed control system. Fieldbus Control System FCS. But now the field bus of various standards exist in parallel and has its own field of existence, has not yet formed a truly unified standard, the key is not to see when to form a unified standard, technology is not mature enough. In addition, the fieldbus instrument types are less room to choose from, the price is also high, from the end-user perspective is mostly still in a wait-and-see state, all want to wait until the technology matures after consideration, now implementation of less.

RS485 network: RS485/MODBUS is now popular a network ingress, which is characterized by simple and convenient implementation, and now supportRS485 meters and a lot, especially in the oil industry RS485/MODBUS is simply a world, and now the instrument dealers have turned to support RS485/ MODBUS, FOR THE SIMPLE REASON THAT LIKE THE ORIGINAL HART METER, IT IS VERY DIFFICULT AND EXPENSIVE TO BUY A SWITCHPORT, AND THE RS485'S CONVERSION INTERFACE IS MUCH CHEAPER AND VARIED. At least in the low-end market RS485/MODBUS will also be the main networking method, in the last two or three years will not change.