BACnet (Building Automation and Control Network) is a communication protocol that is widely used in the building automation industry due to its manufacturer-neutral, high level of interoperability. It was developed in 1995 by the American patron ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers). Since 2003, the protocol has been documented in ISO 16484 Part 5, the international standard for building automation. When it comes to networking smart buildings, the BACnet communication protocol is the key element. Find out more about the BACnet protocol!
What is BACnet?
BACnet is a protocol in building automation that can be used to network intelligent devices in technical building equipment regardless of the manufacturer. It offers a high level of interoperability between systems and devices from different manufacturers. The high level of interoperability due to standardized, detailed rules and structures for communication exchange represents the key added value compared to other communication protocols in the building automation sector. The other communication standards are not sufficiently standardized to achieve comparable interoperability. The BACnet communication protocol is therefore particularly suitable for setting up a manufacturer-independent integral overall system in a building.
BACnet certification
To ensure a high degree of interoperability, BACnet-compatible devices undergo a complex certification process. This involves checking whether the device complies with the standardized rules for communication in accordance with its assigned BACnet device profile. BACnet profiles contain different requirements that the corresponding devices must fulfill. BACnet devices that have successfully passed certification are marked with a BTL logo. The main device profiles include (in descending order of performance)
Management operating devices
- Management and operating station with a wider range of features than B-OWS (B-AWS -Advanced Workstation)
- Management and operating station (B-OWS – Operator Workstation)
- On-site operating device (B-OD – Operator Display)
Automation equipment/field devices
- Freely programmable automation station (B-BC – Building Controller)
- Automation station with a configurable application (B-AAC – Advanced Application Controller)
- Automation station with a fixed application (B-ASC – Application Specific Controller)
- Switching or actuating device (B-SA – Smart Actuator)
- Sensor (B-SS – Smart Sensor)
AMEV certification
BACnet-compatible devices have an extended seal of quality if they have been tested and certified according to an AMEV device profile in addition to being certified according to a BACnet device profile. Based on the BACnet device profiles, the Working Group for Mechanical and Electrical Engineering of State and Municipal Administrations (AMEV) has developed its own profiles, the requirements of which are even more closely aligned with the challenges of practical use. Classic AMEV profiles are the AS-A and AS-B profiles.
Revisions
The scope and structure of these communication rules are regularly revised, optimized and expanded in so-called revisions. In order to keep a BACnet-compatible device up to date, regular recertifications must therefore be carried out in accordance with the current revision. A list of all certified BACnet devices can be viewed on the website of the International BACnet Organization – you can find our BACnet-certified devices here. Regular updates of revisions are created by various localized working groups worldwide and ultimately published by the international BACnet interest group. An important update that was recently published was BACnet Secure Connect (BACnet/SC). With BACnet/SC, BACnet communication networks can now be encrypted.
How is BACnet structured?
According to the OSI layer model, the BACnet protocol has a high degree of flexibility in terms of communication transmission paths. BACnet supports transmission via Ethernet or via RS 485, allowing flexible use of BACnet at both IP level and serial level. Significantly larger data volumes can be exchanged when data is transmitted via Ethernet. Furthermore, the classic networking options from the IT world offer great flexibility for linking devices with each other via BACnet IP. It does not matter whether the devices are located within a building or whether they are networked across building boundaries. Traditionally, IP networks are set up in a star configuration and can be flexibly interconnected and expanded using routers. In addition, only networking via Ethernet offers the option of encryption using BACnet/SC.
Serial communication via BACnet
At serial level, BACnet uses the token-passing method as a special feature for managing the communication participants. This is where BACnet differs greatly from other serial protocols in building automation. All network participants are able to actively communicate information into the network. Permission for active communication is granted by transferring a token to the device. All other devices in the network can then only “listen” to the device with the token. This type of communication management in a serial network has significant advantages. The data load on the transmission path is minimized and optimally utilized, as each device in the network is able to actively and independently communicate messages and information into the network. Unnecessary communication overhead caused by cyclical “polling” of all network participants for new information is thus avoided. Furthermore, important value changes or fault messages are immediately visible in the network, as the network participants can actively communicate important and urgent messages to the network as soon as they occur. In this way, important information is also transmitted and visible on the serial transmission path almost in real time.
Server and client communication
The BACnet standard makes a fundamental distinction between server and client communication. If a BACnet-compatible device is configured as a BACnet server, it provides data for another BACnet-compatible device, the BACnet client. If a BACnet-compatible device is configured as a BACnet client, it receives data from a device configured as a BACnet server.
Communication between different automation stations is based on the object types standardized in the BACnet protocol with their specific properties. These object types define the structure with which the communication content between two automation stations is described.
The main object types include
- Device (Device DEV)
- Analog input (Analog Input AI)
- Analog output (Analog Output AO)
- Analog value (Analog Value AV)
- Binary input (Binary Input BI)
- Binary output (Binary Output BO)
- Binary value (Binary Value BV)
- Multistate input (Multistate Input MI)
- Multistate output Multistate Output MO)
- Multistate value (Multistate Value MV)
- File (File FIL)
- Notification class (Notification Class NC)
- Schedule (Schedule SCHED)
- Calendar (Calendar CAL)
Communication services:
In addition to the object types, communication services(BIBBs = BACnet Interoperability Building Blocks) play a central role in describing the way information is exchanged. There are currently around 45 different servicesdefined in the BACnet standard, which are divided into 5 main categories:
- Alarm and event services
- File access services
- Object access services
- Device management services
- Virtual terminal services
With their help, communication in the network can be controlled and structured in the same quality regardless of the manufacturer. The communication services and object types that must be supported by a BACnet-capable device are defined via certifiable BACnet device profiles.
BACnet in building automation and areas of application
As an interoperable communication standard in building automation, BACnet is ideal for implementing the cross-discipline integration of HVAC, lighting control, shading, security and fire alarm technology. Intelligent automation stations play a central role here, creating a uniform overall system by networking with each other, with devices from the field level and with higher-level management operating devices (MBE). When networking using the BACnet data transfer protocol, the automation stations are often configured as a BACnet client and as a BACnet server. In this way, an automation station can optimally perform the role of the central, networking element. It integrates information from sub-networks at field device level via the client function and passes this data on to higher-level networks at management level after internal processing.
Communication via IP (BACnet IP) is chosen for communication between automation stations and to the management level, as it enables a significantly higher data transmission speed and more flexible network topologies compared to serial communication paths.
The integration of information from sub-networks often takes place via serial communication paths. BACnet MS/TP is often used for this purpose. The cabling then takes place via a 2-wire bus that can be laid quickly. This is a cost-effective solution that is particularly suitable for networking large numbers of field devices.
In a building in which the BACnet communication protocol is used throughout, structured data is available for all devices installed in the technical building equipment. This is an indispensable basis for analyzing the operating efficiency of a building.
The use of the BACnet protocol is therefore a key element in achieving the German government’s legal requirements for reducing CO2 emissions from existing buildings.
Advantages of BACnet
- Standardized data exchange for high interoperability in building automation
- Standardized communication at management, automation and field level
- Cross-trade integration of devices from different building control systems, for example ventilation systems, heating systems, elevator monitoring, access control, energy supply
- technology and manufacturer-independent
- Supports various communication transmission paths, such as BACnet/IP or MS/TP
- Tested by independent test laboratories
- Manufacturer-independent high quality thanks to certifiable device profiles
- Regular expansion and maintenance of standards
Frequently asked questions
You have questions, we have answers. Here we present the most frequently asked questions about the BACnet communication protocol.
How does BACnet communicate?
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Communication can take place via Ethernet or RS 485 can be used. Both BACnet MS/TP via RS485 and BACnet/IP via Ethernet can be used.
Which cable is needed for BACnet?
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Various cable types are used to connect BACnet-capable devices. different cable types are useddepending on the specific application and network requirements. Some of the most common cable types that can be used for BACnet networks are listed below:
- Twisted PairBACnet can often be connected via twisted pair cables such as Cat5e or Cat6 via Ethernet or RS-485 interfaces. This is a common choice for BACnet/IP networks.
- RS-485 cableFor BACnet MS/TP, a 2- or 3-wire RS-485 cable is often used. The wires are twisted to improve signal integrity.
- Ethernet cable: BACnet/IP can be transmitted via conventional Ethernet cables such as Cat5e or Cat6. This is particularly common in modern BACnet installations.
What is MS/TP?
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MS/TP stands for “Master-Slave/Token Passing” and is a specific implementation of the BACnet protocol.
Master-Slave/Token Passing refers to the way how devices in an MS/TP BACnet network communicate with each other. In an MS/TP network, there is a master that owns and determines the token (a type of authorization), Which device is allowed to “speak. The other devices in the network are so-called slaves and can only send data if they receive the token from the master.
Differences between BACnet vs Modbus
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A major advantage of Modbus is that it is publicly accessible and free of charge is. It is easy to implement and maintain as it is a simple protocol.
BACnet is very scalable in terms of performance and system size.
However, Modbus is not securewhich is a significant disadvantage. In addition, the number of data types is limited and large binary objects are not supported.
The new BACnet standard has a security standardwhich, however, is not implemented in all devices.
From a network perspective, BACnet works with Ethernet, IP and MS/TP, while Modbus works with the simpler protocols ASCII, RTU and TCP/IP.
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