What is meant by Modbus protocol?

Modbus Protocol is a serial communication protocol, which was published by Modicon company (now Schneider Electric) in 1979 for the use of programmable logic controller (PLC) communication. Modbus has become the industry standard (De facto) of communication protocols in the industrial field and is now a common connection method between industrial electronic devices.

What is Modbus Protocol?

Modbus Protocol Features

The main reasons why Modbus is more widely used than other communication protocols are:

  • Published publicly and without copyright requirements
  • Easy to deploy and maintain
  • For suppliers, there are not many restrictions on modifying mobile local bits or bytes

Modbus allows multiple (approximately 240) devices to be connected on the same network to communicate. For example, a device that measures temperature and humidity and sends the results to a computer. In the data acquisition and monitoring control system (SCADA), Modbus is usually used to connect the monitoring computer and the remote terminal control system (RTU).

How does a Modbus work?

Modbus is a master/slave communication mode with a single master station. There can only be one master station on the Modbus network, and the master station has no address on the Modbus network. The address range of the slave station is 0-247, where 0 is the broadcast address. The actual address range of the slave station is 1-247. Modbus communication standard protocol can be transmitted through various transmission methods, such as RS232C, RS485, optical fiber, radio, etc.

Modbus has two serial transmission modes, ASCII and RTU. They define different ways of how data is packaged and decoded. Devices that support Modbus protocol generally support the RTU format. Both communication parties must support one of the above modes at the same time.

The above is the official definition, let’s use ordinary words to explain:

This is actually the communication protocol, and any protocol is similar. There are only two communication formats and communication specifications.

If you want, you can also define a communication specification yourself. Use PLC or VB language to process according to this specification defined by yourself. If the reliability exceeds Modbus, then your communication specification is the most popular!

The difference and connection between RS 485 and MODBUS

I often see RS485 and MODBUS written together, and Sino-Inst will talk to you about the difference and connection between RS485 and MODBUS.

RS485 is a physical interface, which is simply hardware.

MODBUS is an international standard communication protocol used to exchange data between devices of different manufacturers (usually for industrial purposes); the so-called protocol can also be understood as the “language” someone said above, or simply software.

Under normal circumstances, two devices transmit data through the MODBUS protocol: RS232C was first used as the hardware interface, (that is, the serial communication port (serial port) on an ordinary computer). There is also RS422, and there is also commonly used RS485. This interface The transmission distance is long, and it is often used in general industrial scenes.

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

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

And MODBUS TCP is to conform to the development trend of the world today. Anything can be connected by Ethernet or Internet to transmit data.

The hardware interface of MODBUS TCP mode is the Ethernet (Ethernet) port, which is the network port generally used on our computer.

We categorize industrial networks into three categories: RS485 network, HART network and fieldbus network.

HART network

HART is a transitional bus standard proposed by Emerson. It mainly superimposes a digital signal on top of a 4-20 mA current signal.

The physical layer uses BELL202 frequency shift keying technology. In order to realize the functions of some smart meters. But this agreement is not a truly open standard, and you have to join his foundation to get the agreement. Part of the cost of joining the foundation.

The technology is mainly monopolized by several large foreign companies, and in the past two years, some domestic companies have started to do it again. But it has not yet reached the level of foreign companies.

Nowadays, a large part of smart meters are equipped with HART round cards and all have HART communication function.

But from the domestic point of view, this part of the function has not been really used, at most it is only used to set the parameters of the handheld communicator. The HART intelligent instrument has not played its due function, and it has not been connected to the Internet for equipment monitoring.

In the long run, due to the low communication rate of HART, the networking is difficult and other reasons. The procurement volume of HART instruments will experience a downward trend.

However, because HART instruments have a history of more than ten years, the number of instruments installed is very large. For some system integrators, there is still a lot of room for use.

Fieldbus network

Fieldbus technology is one of the hotspots of technology development in the field of automation today. Known as the 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 the instruments installed in the control site and the control equipment installed in the control room. The key symbol is the ability to support bidirectional, multi-node, bus-style all-digital communication.

In recent years, fieldbus technology has become a hot spot in the development of automation and instrumentation in the world. The appearance of the fieldbus network is a revolutionary change in the traditional control system structure. It is the automatic control system that is moving in the direction of intelligence, digitization, informationization, networking, and decentralization. Form a new type of network integrated fully distributed control system-Fieldbus Control System FCS (Fieldbus Control System).

However, various standards of current fieldbus exist in parallel and have their own areas of survival. There has not yet been a truly unified standard. The key is that we cannot see when a unified standard will be formed, and the technology is not mature enough.

In addition, the types of fieldbus instruments are still relatively small, and the room for selection is small, and the price is relatively high. From the perspective of end users, most of them are still on the sidelines. I want to wait until the technology is mature and consider it, but now there are few implementations.

RS485 network

RS485/MODBUS is a popular way of network layout. Its characteristic is simple and convenient to implement. And now there are many instruments that support RS485, especially in the oil industry.

RS485/MODBUS is simply dominating the world. Current instrument vendors have also switched to support RS485/MODBUS. The reason is simple, like the original HART instrument, it is very difficult and expensive to buy a conversion port. The RS485 conversion interface is much cheaper and has a wide variety.

At least in the low-end market, RS485/MODBUS will still be the most important networking method, and it will not change in the past two to three years.

Features of 4-20mA current loop and RS485 communication (Modbus RTU protocol) data acquisition

4-20mA current loop data acquisition characteristics

Use pressure, temperature transmitter and current acquisition module (RTU) or acquisition card on site for data acquisition and then communicate and transmit with the industrial computer. The data acquisition system using 4-20mA current loop instrument has the following characteristics:

  1. The construction is complicated, data cables need to be buried in advance, the number of cables is large, and the construction period is long
  2. The computer terminal needs a current acquisition card or a data acquisition module
  3. Complicated replacement and maintenance operations, troublesome expansion
  4. 4-20mA current loop data acquisition accuracy is low, and the error comes from 4-20mA output instrument and acquisition card (or data acquisition module)

RS485 communication (Modbus protocol) data acquisition characteristics

Use smart meters, transmitters, actuators, etc. with RS485 communication to directly communicate with the serial port of the industrial computer. The use of RS485 communication digital sensor meter network system has the following characteristics:

  1. The construction complexity is reduced, data cables need to be buried in advance, the number of cables is small, and the construction period is reduced
  2. The computer terminal only needs an isolated RS485/RS232 converter
  3. Easy to replace and maintain, and easy to expand
  4. High acquisition accuracy, eliminating the error of RTU, the system error only comes from the transmitter itself
  5. RS485 communication has good transmission stability and less network maintenance
  6. The cost of use is greatly reduced|
  7. The maximum communication distance of RS485 is 1200 meters

RS485 communication transmission of electromagnetic flowmeter Modbus protocol

The RS485 communication data transmission interface of the electromagnetic flowmeter is half-duplex. The standard rate is greater than 250kHz. The communication direction conversion time is 3.5us. Usually 16-way load can be connected. It can also be expanded to 32 channels. Standard shielded twisted pair cable is 1000m. Use master-slave multi-machine communication. When multiple electromagnetic flowmeters are interconnected. It can save signal lines and facilitate high-speed transmission.

RS-485 communication has good anti-noise interference. Long transmission distance and multi-station capability. Will not affect the synchronization of on-site and remote data due to PLC failure.

However, the electromagnetic flowmeter must have a 485 interface and support the MODBUS RTU protocol.

In order to maintain stability, it is better to use electromagnetic flowmeters of the same manufacturer and the same brand when communicating with multiple flowmeters.

RS485 is a kind of industrial data bus. The electromagnetic flowmeter has RS485 serial communication so that digital communication replaces the transmission of PLC analog signals and ordinary switch signals. The PLC simulation calculation is reduced, and the accurate data collection and transmission are realized.

RS485 precession vortex flowmeter

Precession vortex flowmeter is a new type of gas flowmeter. It can be used for the measurement of natural gas, propane, air, nitrogen and other gases.

The converter can output frequency pulse, 4 ~ 20mA analog signal, and has RS485 interface, can be directly connected to the computer network, transmission distance up to 1.2km;

Precession Vortex Gas Flowmeter, like vortex flowmeter, is a velocity flowmeter. The cost performance of the intelligent precession vortex flowmeter is higher than vortex flowmeter. The intelligent precession vortex flowmeter can only measure gas. Precession Vortex Gas Flowmeter can be made into an integrated temperature and pressure compensation. And can measure various gases. Such as compressed air, oxygen, biogas, natural gas, coal gas, etc.

Read more about Everything You Need To Know About Electronic Flow Meters