The Modbus communication protocol is one of the communication standards in building automation. Compared to other classic communication protocols such as BACnet, KNX, DALI, M-Bus or SMI, Modbus is easy and inexpensive to implement in devices of all kinds. Find out more about Modbus now!
What is Modbus?
Modbus is a communication protocol that plays a central role in building automation today. The protocol is one of the oldest and most widespread protocols in building automation. A basic distinction can be made between Modbus RTU, the serial data transmission method, and Modbus TCP, data transmission via IP. Modbus RTU was developed back in 1979 and the standard over IP was added in 2007.
About the Modbus organization the Modbus communication standard is further developed and promoted in individual market segments. The Modbus organization consists of a group of independent users and suppliers of automation devices. The Modbus communication protocol is characterized by the fact that it is easy and inexpensive to implement in automation devices. For this reason, Modbus-enabled devices are cheaper to purchase than comparable devices that support a different communication standard. This has led to the development of a wide range of Modbus-capable devices.
Use the advantages of Modbus to implement efficient and future-proof building automation in your non-residential building! Contact us, we will be happy to advise you.
What types of Modbus are there?
Modbus RTU (Remote Terminal Unit):
Modbus RTU is a widely used variant of the Modbus protocol for serial serial communication. It uses a Binary data transmission via RS-232 or RS-485 Connections. RS-485 enables the communication with several devices in a network. The RTU format includes data fields for addressing devices, sending commands and receiving data. Due to its efficiency and widespread support Modbus RTU is particularly popular in industrial automation systems where real-time communication and reliability are required.
Modbus ASCII:
Modbus ASCII is an older older variant of the Modbus protocolwhich is also used for serial communication. In contrast to Modbus RTU, however, ASCII uses a text-based format for data transmission. This makes it easier to interpret and debugbut also slower than RTUas it requires more bandwidth. Modbus ASCII is used less frequently nowadays and is often found in applications that require a require slower serial communication or special ASCII-based legacy systems must support.
Modbus TCP/IP:
Modbus TCP/IP is a modern modern extension of the Modbus protocol for communication via Ethernet networks. It uses the TCP protocol for the transmission of data packets over IP networks, including LANs, WANs and even the Internet. This variant enables a easier integration into IT networks and offers greater reach and flexibility than the serial variants. Modbus TCP/IP is used in modern industrial automation and control systems widely used where a fast and reliable communication via Ethernet is required.
How does Modbus work?
Data transmission with Modbus is generally possible via the serial path or via the Internet Protocol (IP). The transmission method via the serial standards RS232 and RS485 is called Modbus Remote Terminal Unit (RTU). The possible baud rates range from 1,200 kBit/s to 115,200 kBit/s. In building automation, the use of the RS485 serial standard and the baud rate of 19,200 kbit/s has proven to be particularly practical. If the RS485 serial standard is used, it must also be ensured that bus terminating resistors (120 Ohm) are used at the beginning and end of the communication line. Otherwise, interference will occur during communication. The transmission method via the IP standard Ethernet is called Modbus Transmission Control Protocol (TCP). The data transmission rates here are 10/100/1000 Mbit/s. Wireless transmission is also possible.
How is Modbus structured?
Depending on the transmission method, different topologies must be taken into account. A connection via the serial standard RS 232 represents a point-to-point connection. In the case of a connection via the serial standard RS 485, communication takes place in a line structure. In this case, a network line consists of a “master” and at least one “slave”. The master manages the network by communicating continuous read and write requests (polling) in the network and to all participating devices. Individual device addresses are used so that each device and its tasks can be clearly identified in a network. The valid range here is from 1 to 247.
If communication takes place via Modbus TCP, all standard IP network topologies can be used. Communication with Modbus TCP is managed in the same way as communication via Modbus RTU. However, the device that manages the communication in a network is referred to differently as the client. All other devices in the network are referred to as servers.
A key difference is that, unlike a Modbus RTU network, there can be several clients (masters) in a Modbus TCP network. The unique addressing of devices in a Modbus TCP network takes place via the IP address.
How is Modbus wired?
The cabling varies depending on the Modbus protocol used. The two most common variants are Modbus RTU and Modbus TCP, each of which places different demands on the cabling.
Modbus RTU cabling
Modbus RTU is based on serial communication and is often transmitted via RS-485although RS-232 and RS-422 can also be used. With the RS-485 cabling is recommended, twisted pair cables to minimize disruptions. The Main signal lines connected in parallel. To avoid reflections, terminating resistors of typically 120 ohms should be fitted at both ends of the RS-485 bus. This type of cabling follows a Bus topology, in which all devices are connected in a so-called daisy chain configuration are connected in parallel.
Modbus TCP cabling
In contrast to Modbus RTU, Modbus TCP uses Ethernet as the physical medium, which simplifies the cabling considerably. Standard CAT5e or CAT6 Ethernet cables are used here. The devices can be connected either via an Ethernet switch or directly to each other. The network follows a star topology in which all devices are connected centrally via a switch or router. The maximum cable length between two active network devices is 100 meters.
Modbus data types
The Modbus communication protocol defines the following data types for data transmission. The data sheet of a Modbus-enabled device defines which registers (data points) correspond to which data type and can therefore only be read/written or read and written.
| Discrete Input | Input bit | read only |
| Coil | Output bit | readable and writable |
| Input register | Input register (16 bit) | read only |
| Holding register | Output register (16 bit) | readable and writable |
Modbus function codes
Based on the data types, function codes standardize the exchange of information between two Modbus devices. The range of functions supported by a Modbus device depends on the manufacturer and can also be found in the data sheet. Central function codes are
| FC 01 | Read coils | Reading one or more bit outputs |
| FC 02 | Read Discrete Inputs | Reading one or more bit inputs |
| FC 03 | Read Holding Register | Reading output registers |
| FC 04 | Read Input Register | Reading input registers |
| FC 05 | Write Single Coil | Writing a bit output |
| FC 06 | Write Single Register | Write one register output |
| FC 15 | Write Multiple Coils | Writing multiple bit values |
| FC 16 | Write Multiple Registers | Write multiple registers |
| FC 23 | Read/Write Multiple Registers | Write and read multiple registers |
Modbus in building automation and areas of application
The Modbus communication protocol is mainly used in building automation for networking devices from the field level with the automation level. Fast and simple networking of a large number of devices. Modbus-capable devices are also very inexpensive to purchase. These price advantages are particularly important when a large number of devices are networked together.
Another area of application for Modbus is the integration of heating, ventilation or cooling systems into a central building automation system. The manufacturers of such systems often supply the heating, ventilation or cooling system together with a control and regulation system tailored to the system, including automation equipment. The heating, ventilation or cooling system thus represents a self-sufficient automation system. Since Modbus as a communication protocol is easy and inexpensive to implement, many of the autonomously operating heating, ventilation or cooling systems communicate via Modbus.
Advantages of Modbus
- Simple and cost-effective implementation of the communication protocol for manufacturers
- Inexpensive Modbus-capable devices available
- Standardization of communication and data exchange via data types and function codes
- Communication serial or via IP
- RS 485: Bus terminating resistors are necessary
What is the difference between Modbus and RS485?
Modbus and RS485 are two different concepts used in industrial communication, but they are not directly comparable. are not directly comparableas they are based on different levels of communication technology. operate at different levels of communication technology.
RS485, also known as EIA-485, is a standard for the physical physical layer of serial communication. He describes the electrical properties and signal transmission for serial connections. RS485 is particularly useful in industrial environments as it enables communication via long distances and the connection of several devices to a single bus.
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