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We often see RS485 and MODBUS mentioned together. What are their differences and connections?
RS485 is a physical interface, simply put, it is hardware.
MODBUS is an internationally standardized communication protocol used for data exchange between devices from different manufacturers (generally for industrial purposes);
The so-called protocol can also be understood as the “language” mentioned by someone above, simply put, it is software.
Generally, two devices transmit data via the MODBUS protocol:
Initially, RS232C was used as the hardware interface (which is the serial communication port on a regular computer);
There are also RS422 used,
and commonly used RS485, which has a longer transmission distance and is more widely used in general industrial sites.
The MODBUS protocol is divided into three modes: MODBUS RTU, MODBUS ASCII, and the later developed MODBUS TCP:
Among them, the first two (MODBUS RTU, MODBUS ASCII) use serial communication ports (RS232, RS422, RS485) as their physical hardware interfaces.
MODBUS TCP, on the other hand, is designed to align with the current global trend of connecting everything via Ethernet or the Internet for data transmission. Thus, the MODBUS TCP mode uses Ethernet ports as its hardware interface, which is the network port commonly used on our computers.
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 the 4-20 mA current signal. 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; one must join its foundation to obtain the protocol, and joining the foundation requires a fee. The technology is mainly monopolized by a few large foreign companies, and in recent years some domestic companies have been working on it, but they have not yet reached the level of foreign companies. Currently, a significant portion of intelligent instruments are equipped with HART cards, all of which have HART communication capabilities. However, domestically, there has not been a real utilization of this functionality; at most, it is used for parameter settings via handheld devices, without fully leveraging the capabilities of HART intelligent instruments for networked device monitoring. In the long term, due to the low communication speed and networking difficulties of HART, the procurement volume of HART instruments will likely trend downward. However, due to the fact that HART instruments have been around for over a decade, they are still widely installed, leaving significant room for system integrators to utilize.
Fieldbus networks: Fieldbus technology is one of the hot topics in the current automation field, hailed as the local area network of automation. Its emergence marks the beginning of a new era in automation control technology. Fieldbus connects instruments located in the control field with control devices set up in the control room through a digital, serial, multi-station communication network. Its key feature is the ability to support bidirectional, multi-node, bus-type all-digital communication. In recent years, fieldbus technology has become a hotspot for automation and instrumentation development internationally. Its emergence has brought revolutionary changes to the structure of traditional control systems, advancing 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 of fieldbus currently coexist in parallel, each with its own survival domain, and a truly unified standard has not yet been formed. The key issue is that it is unclear when a unified standard will be developed, and the technology is not yet mature. Additionally, the variety of fieldbus instruments available for selection is still relatively small, and prices are high. From the end-user’s perspective, many are still in a wait-and-see state, hoping to consider it after the technology matures, resulting in limited implementation.
RS485 network: RS485/MODBUS is currently a popular networking method characterized by its simple implementation and convenience. Moreover, there are many instruments that support RS485, especially in the oil industry where RS485/MODBUS is almost ubiquitous. Currently, instrument manufacturers are also increasingly supporting RS485/MODBUS for a simple reason: it is very difficult and expensive to buy a conversion port for the original HART instruments, while RS485 conversion interfaces are much cheaper and 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.
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