Building Automation Control Network Data Communication Protocol BACnet

Abstract: This paper summarizes the current status of communication protocols and fieldbus in intelligent buildings, and introduces BACnet, a new data communication protocol emerging in this field.

Keywords: BACnet intelligent building building automation object-oriented

With the rapid development of computer, communication, control and graphic display technology, namely 4C technology, and the global construction of information highways, intelligent buildings, the combination of digitalization, networking and informationization began to enter people's field of vision. However, today's various control functions in intelligent buildings have become more powerful and complex, resulting in the use of equipment produced by different manufacturers in the same building, but each manufacturer basically develops its own proprietary communication protocol, so all kinds of Such communication protocols and devices bring a lot of inconvenience to the system integration and management use of intelligent buildings, and users are subject to manufacturers, which increases the cost, the use and maintenance costs. Therefore, the development of an open, unified communication protocol standard and the formation of a plug and play environment has become an urgent problem to be solved.

At present, in the field of intelligent building, fieldbus and communication protocols mainly include: (1) bus protocols originally applied in the field of industrial control, such as representative Profibus, Lonworks bus, CAN bus, etc.; (2) specifically for intelligent buildings Bus and communication protocols, such as BACnet and CEBus in the US, EIB in Europe, etc. This article gives a detailed introduction to BACnet.

Figure 1 BACnet architecture hierarchy

1 BACnet Protocol Overview

Building automation control network data communication protocol BACnet (A Data

The Communication Project for Building Automation and Control Network was officially adopted by the Standards Project Committee 135P organized by the American Society of Heating, Refrigerating and Air-Conditioning Engineers in June 1995. The standard number is ANSI/ASHRAE Standard l35-1995. In December of the same year, it officially became the US national standard and was recognized by the European Standards Committee as a draft EU standard. In January 2000, representatives of 15 countries (China, France, Japan, the United Kingdom, the United States, etc.) of the ISO organization TC205 Committee unanimously passed a resolution to make extensive comments on BACnet as a “committee draft”, and appropriately revised it as “International Standardization Draft ", and finally become an international standard.

The general building automation equipment is functionally divided into two parts: one part specifically handles the control function of the equipment; the other part specializes in the data communication function of the equipment. BACnet is to establish a unified data communication standard, so that devices can interoperate. The BACnet protocol only specifies the rules for communication between devices and does not cover implementation details.

The BACnet protocol model is: (1) all network devices are peer-to-peer except for the MS/TP protocol; (2) each device is an "object" entity, each Objects are described by their "attributes" and provide methods for identifying and accessing devices in the network; devices communicate with each other by reading/writing the attributes of certain device objects and using the "services" provided by the protocol; (3) The Sophistication of a device, its ability to implement a service request or to understand the type of object type, is reflected by the device's Conformance Class.

1.1 BACnet architecture

BACnet is an open network protocol for intelligent buildings. It follows the OSI model architecture. The BACnet architecture hierarchy is shown in Figure 1. The BACnet protocol currently supports five combined types of data link/physical layer specifications in terms of hardware/software implementation, data transfer rate, system compatibility, and network applications. The Master/Slave Pass (MS/TP) protocol is a data link specification specifically designed for building automation equipment. BACnet supports twisted pair, coaxial cable and fiber optic cable on physical media. In terms of topology, star and bus topologies are supported.

BACnet does not strictly define the network topology, as shown in Figure 2. The segment is a segment interval formed by connecting multiple physical network segments through a repeater (R); the network is formed by connecting multiple network segments through a bridge (B), and each network forms a MAC. Address field; BACnet/Internet network is an Internet formed by interconnecting multiple networks using different LAN technologies with routers (RTs).

In the BACnet topology, there is only one logical path between devices, which does not require the optimal routing algorithm of the WAN. Secondly, BACnet has a single local address space, so BACnet has developed a simplified network layer protocol with reference to the OSI model, providing no confirmation to the application layer. Connectionless data unit delivery service. Each BACnet device is uniquely identified by a network number and a MAC address.

The network layer implements the connection of two or more heterogeneous BACnet LANs (different number chain layers) through the "router", and performs automatic configuration of the "router", routing table maintenance, and congestion control through protocol packets. The connection between the BACnet router and each network is called a "port." The routing table contains the following items of the port: (1) the MAC address and network number of the network to which the port is connected; and (2) the list of network numbers that the port can reach the network and the connection status with these networks. In Figure 2, "1/2 RT" is a semi-router, which forms a complete BACnet router by PTP connection, that is, the BACnet Internet network shields the WAN technology from the application layer.

The BACnet application layer, the BACnet application entity, serves the upper layer application through an API (application programming interface) and communicates with the peer application layer entity. An application entity consists of two parts: a subscriber unit and an application service unit (ASE). ASE is a set of application services for specific content. The subscriber unit supports the local API, saves the transaction context information, generates the request ID, the application service response corresponding to the record ID, the counter required to maintain the timeout retransmission mechanism, and maps the device behavior requirements to the object.

The BACnet application layer provides both typed and unverified services. BACnet defines four service primitives: Request, Indication, Response, and Verification, which are passed through Application Layer Protocol Data Units (APDUs). Since BACnet is built on a connectionless communication mode, the OSI model provides a transport layer partial simplification function for end-to-end services, which is also implemented by the application layer: reliable end-to-end transmission and error checking; message segmentation and Flow control; message reorganization and sequence control.

1.2 BACnet objects, services and functional groups

BACnet uses object-oriented technology to provide a standard for building automation equipment. In BACnet, an object is a set of data structures transmitted between network devices. The network device implements interoperability by reading and modifying the object data structure encapsulated in the application layer APDU. BACnet currently defines 18 objects. As shown in Table 1, each object must have three attributes: Object_Identifier, Object_Name, and Object_Type. The object identifier is used to uniquely identify the object; the BACnet device can establish a connection with the device containing the related object by broadcasting the object name of an object contained in the object. The BACnet protocol requires each device to contain a "device object" that allows the network to obtain all of the device's information by reading its attributes.

Table 1 BACnet objects

In BACnet, the method of an object is called a service, and the object and its attributes provide an abstract description of the "network-visible information" of a building automation device, and the service provides commands and methods for how to access and manipulate the information. BACnet devices implement services by passing service requests and service response messages over the network. BACnet defines 35 types of services and divides them into six categories: (1) Alarm and Event Services contain changes in the status of eight service processing environments, providing a preset request value change notification for BACnet devices. , requesting alarm or event status summary, sending alarm or event notification, receiving alarm notification confirmation, etc.; (2) File Access Service (File Access Services) contains two kinds of services, providing methods for reading and writing files, including upload/download control The ability of programs and databases; (3) Object Access Services (Object Access Services) contains 9 kinds of services, providing methods for reading, modifying and writing attribute values ​​and adding and deleting objects; (4) Remote Device Management Services Includes 11 services that provide tools for maintenance and fault detection of BACnet equipment, (5) Virtual Terminal Services (Virtual Terminal Services) consists of three kinds of services, providing a character-oriented data bidirectional exchange mechanism, enabling other building automation devices with proprietary features to become a BACnet virtual terminal and enabling BACnet networks. Refactoring it; (6) Network Security Services includes two services, providing peer entity authentication, data source authentication, operator authentication, and data encryption.

The BACnet functional group specifies the combination of objects and services required to implement a specific control function. BACnet has defined 13 functional groups, including clock function group, event response function group, file function group, virtual terminal function group, device communication function group, and so on.

1.3 BACnet device level and device level description

In an actual building automation system, it is not necessary or possible for all devices to support and include all of the above objects and services. Therefore, BACnet defines six conformance categories (device level). The consistency category has a rating number of 1 to 6, and the lowest level is category 1. Each category specifies a minimum subset of services to be implemented by the device and includes all services at a low level.

In order to help users and engineers determine the interoperability between different BACnet devices, the manufacturer is required to provide a standard format file for each device to identify the content of the implemented BACnet standard in the device, that is, the file must include instructions for the device to comply with the BACnet level. This file is the PICS (Protocol Implementation Conformance Statement), which includes: (1) basic information identifying the manufacturer and describing the device; (2) the device conforms to the BACnet level; (3) the functional group supported by the device; and (4) the device Supported standards-based or proprietary services, the ability of a device to initiate or respond to service requests; (5) standard or proprietary object types supported by the device and their attribute descriptions; (6) data link technologies supported by the device; (7) Segmentation requests and responses supported by the device.

2 BACnet Internet Extension

Currently, the BACnet standard uses two technologies to interconnect with the Internet. The first type of technology annex H is called "tunnel" technology, and its equipment is called packet encapsulation / disassembly equipment, referred to as PAD. It acts like a gateway/router, which is reflected in Figure 2 where two and a half routers connect to the WAN to form a complete BACnet router. The second technology is called BACnet/IP in Annex J. The device directly encapsulates IP frames/packets for transmission on the BACnet network and the Internet.

The PAD encapsulates the BACnet message data in an IP protocol packet and decapsulates it in the destination BACnet network. Therefore, each BACnet network connected to the Internet must be configured with a PAD gateway/router. It can be a separate device or part of a building control device function.

ASHRAE officially released Annex J in January 1999 and became the US national standard. It standardizes the technology for supporting TCP/IP devices to form a BACnet network. It is called a BACnet/IP network, or B/IP for short. It is a collection of one or more IP subnets, and has a separate BACnet network number. The BACnet/IP network packet is an IP packet at the network layer and a UDP datagram at the transport layer, thereby realizing the convergence with the TCP/IP protocol of the Internet.

Open, compatible, flexible, widely supported communications protocols or fieldbus specifically for smart buildings are bound to become a development direction in the field of intelligent buildings. The BACnet protocol is such a pioneering technology that enables devices from different vendors to be interconnected, interchangeable and interoperable, creating a seamlessly linked building automation system that fully satisfies the owners, users and integrators. The demand also provides a variety of Internet interconnection and Internet access solutions, which facilitates the integration of systems within the intelligent building, making it easy for intelligent buildings to take information highways.