ControlNetwork, or BACnet for short, is a data communication protocol specifically designed for the integrated control of building equipment. It is a unified standard protocol for data communication that standardizes the interconnection between automatic control systems of building equipment such as air conditioning, water supply and drainage, and power supply and distribution, making them more useful, open, and interoperable.
In January 1987, the Society of Heating, Refrigerating and Air-Conditioning Engineers (SHEEE) of the United States organized a working group called "SPCI35P," comprised of 20 volunteers from the building control industry worldwide, including universities, controller manufacturers, government agencies, and consulting firms. The group's goal was to develop a communication protocol for building energy management and control systems (EMCS). Over the course of more than eight years, 741 comments were received from 12 countries. After three rounds of public review, the "Building Automation Network Data Communication Protocol" (BACnet Data Communication Protocol) was officially adopted, becoming the ASHRAE 135.1995 standard and designated as a US national standard. In August 2000, the International Organization for Standardization (ISO) Technical Committee 205 (Building Environment Design Technical Committee) published the BACnet Data Communication Protocol as a formal "Committee Draft" for public review. After appropriate modifications, the draft became the official international standard ISO 16484.5, the only ISO standard in the field of building equipment control.
The ISO 16484 standard consists of seven parts:
(1) Building Automation and Control System (BACS)
(2) Hardware
(3) Functions
(4) Applications
(5) Communication Protocol (BACnet)
(6) Protocol testing
(7) Project implementation.
BACnet is a standard communication and data exchange protocol.
According to the original intent of the agreement, as long as manufacturers develop controllers or interfaces compatible with this standard, controllers from different manufacturers can eventually exchange data and achieve interoperability. In other words, this agreement establishes a rule under which various compatible systems conforming to this protocol can achieve interoperability without considering the manufacturer or relying on any dedicated chipset.
Openness and interoperability. Currently, more than 140 internationally renowned building equipment manufacturers have developed over 1,000 products that support the BACnet protocol, including manufacturers of building automation systems, power distribution monitoring systems, lighting control systems, security systems, fire protection equipment, refrigeration equipment, boiler equipment, elevator equipment, and more.
Technical Background of Building Automation Network Data Communication Protocol (BACnet)
Technical Features of BACnet
The BACnet data communication protocol describes the functions of a building automation network, the methods by which system components share data, the communication media used, the available functions, and all the rules governing information translation. The BACnet data communication protocol consists of the following parts: five local area network communication protocols, the protocols for communication between them, the representation of information data, and the functions of building automation equipment.
Since BACnet is an open computer control network, it must conform to the Open Systems Interconnection (OSI) reference model of the International Organization for Standardization (ISO). However, BACnet specifies data communication between controllers of electromechanical equipment within a building, implementing services and protocols for computer-controlled air conditioning, water supply and drainage, power distribution, and other building equipment systems. Therefore, the BACnet protocol is relatively simple. The BACnet protocol establishes a layered architecture with four layers, as shown in Figure 1. Figure 1 also shows the correspondence between the BACnet network layers and the OSI network layers.
The BACnet standard currently supports five types of physical layer/data link technologies, forming its protocol. These five technologies are: ISO 8802.3 Ethernet, ARC, Master/Slave/Token Passing (MS/TP), Point-to-Point (PTP) connection, and LonTalk protocol network. Building control systems consist of many distributed, independently functioning field controllers. The internal software data structures of Direct Digital Controllers (DDCs) from different manufacturers vary significantly. BACnet aims to enable communication between DDCs from different manufacturers via a network.
The BACnet data communication protocol employs object-oriented technology, defining a set of objects with attributes to represent the functions of building equipment. The values of these attributes describe the characteristics and functions of the objects. A BACnet object is a data structure representing the functional elements of a particular device. Objects are a set of data structures transmitted between devices; the attributes of an object are the information within that data structure. Devices can read information from and write information to objects (data structures); these are the operations performed on the objects.
Communication between devices in BACnet involves the device's application loading the corresponding object (data structure) into the device's Application Layer Protocol Data Unit (AFDU) and transmitting it to the corresponding device according to the protocol. The information carried in the object (data structure) is the object's attribute value. The application in the receiving device operates on these attributes to complete the information communication.
In building control systems, the function and task of a Direct Digital Controller (DDC) are defined by various standard "objects" within BACnet, which are collections of all data. BACnet uses these "objects" to transform the internal data structure of the DDC into a general, explicit, and abstract data structure to achieve data communication. BACnet defines 18 standard objects; different combinations of these objects enable different control functions of the DDC, thus describing the DDC's tasks.
In addition, BACnet defines 123 standard attributes. Attributes are essentially further descriptions of objects. Obtaining information from an "object" and issuing commands to it are both done through attributes. An object describes a set of data structures for building automation equipment; attributes are the information within those data structures; and service functions are the means to access and manage the information issued by these objects, command them to perform certain operations, or notify them of certain events. A BACnet service is a method a BACnet device can use to request information from other BACnet devices, command other devices to perform certain operations, or notify other devices of certain events. The BACnet data communication protocol defines 35 services and divides these 35 services into 5 categories.
Since not all devices in a building equipment control system need to support all BACnet functions, the BACnet protocol also defines 6 performance levels and 13 function groups. The higher the performance level, the richer the services offered by BACnet, but the greater the communication volume. Different types of devices can select different performance levels as needed, ensuring network response speed without affecting network rate. Function groups supplement performance levels; devices at lower performance levels can obtain relevant data from the system through function groups in network communication to achieve functions not available at their current performance level, thus enabling lower-performance-level devices to achieve high-performance-level functions through network communication.
The BACnet standard is designed for building systems such as HVAC, water supply and drainage, fire protection, and security. It provides various building equipment models, enabling interoperability and collaborative operation among different devices. BACnet defines a communication protocol between network workstations in different LAN environments. It utilizes Ethernet for communication interconnection, and its transmission performance is significantly improved compared to traditional control networks.
