Introduction to MODBUS and HART Communication

RS485 and MODBUS communication are commonly used concepts in industrial control systems, but they are different. Below is a brief introduction.

RS485 is a physical interface, simply put, it is hardware.

MODBUS is an internationally standardized communication protocol used for device data exchange between different manufacturers (generally for industrial purposes); in industrial environments, data is often transmitted between two devices via the MODBUS protocol: MODBUS protocol is divided into three modes: MODBUS RTU, MODBUS ASCII, and the later developed MODBUS TCP. The first two modes (MODBUS RTU, MODBUS ASCII) use serial communication interfaces (RS232, RS422, RS485) for physical hardware.

MODBUS TCP, on the other hand, uses Ethernet networks to transmit data. The hardware interface for 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. It mainly superimposes digital signals on a 4-20 mA current signal, using BELL202 frequency shift keying technology at the physical layer to achieve some functions of smart instruments. However, this protocol is not an open standard in the true sense; one must join its foundation to obtain the protocol, which requires a fee. The technology is mainly monopolized by several large foreign companies, although some domestic companies have started to develop it in recent years, they have not yet reached the level of foreign companies. A significant portion of smart instruments now have HART communication capabilities and are commonly used for range calibration of HART devices.

Fieldbus Network: Fieldbus technology is one of the hot topics in today’s automation field, hailed as the local area network of automation. Its emergence marks the beginning of a new era in automation control technology. Fieldbus is a digital, serial, multi-station communication network that connects instruments located in the control field with control devices located in the control room. Its key feature is the ability to support bidirectional, multi-node, bus-based full digital communication. In recent years, fieldbus technology has become a hot topic in international automation and instrumentation development. Its emergence has brought revolutionary changes to the traditional control system structure, moving self-control systems towards intelligence, digitization, informatization, networking, and decentralization, forming a new type of network-integrated fully distributed control system—Fieldbus Control System (FCS).

However, various standards for fieldbus technology coexist, and a truly unified standard has not yet been formed. Additionally, the types of instruments available for fieldbus are still relatively few, with limited options and higher prices. From the end-user’s perspective, most are still in a wait-and-see state, hoping to consider it after the technology matures, so current implementations are rare.

RS485 Network: RS485/MODBUS is a popular communication method today, characterized by its simplicity and convenience. Many instruments now support RS485, and all instrument manufacturers support RS485/MODBUS. The reason is simple: RS485 conversion interfaces are much cheaper and more varied. At least in the low-end market, RS485/MODBUS will remain the primary networking method, and this will not change in the next two to three years.