BACnet Overview

6 Minute Read

| Introduction

BACnet is a standard data communication protocol that enables interoperability between different building systems and devices in building automation and control applications. 

The 75F® system is integrated with an existing BMS using the standard BACnet specification. In addition to monitoring and controlling the site using the 75F® Facilisight portal, this integration allows the facility manager to: 

• Monitor sensor points such as temperature, humidity, discharge air temperature, CO2, VOC, illuminance, sound, and occupancy on per zone basis including trend data
• Send command from the BMS to set parameters such as desired temperature set points, system modes, etc. for the 75F® system
• Generate alarms per BACnet standard based on 75F® alerts

| What is BACnet 

BACnet stands for Building Automation and Control Network. BACnet is a data communication protocol for building automation and control networks. A data communication protocol is a set of rules governing the exchange of data over a computer network that covers everything from what kind of cable to use to how to form a particular request or command in a standard way. It was officially unveiled by ASHRAE in 1996 and is both an international (ISO) and ANSI standard for interoperability between cooperating building automation devices. BACnet is a registered trademark of ASHRAE. 

With its open architecture and ability to control and monitor any building automation process, BACnet meets the needs of users, integrators, and equipment vendors. Its popularity has steadily grown over the years until it became one of the leading, if not the leading, networking technology in building automation. BACnet’s success is the result of a series of critical design decisions: 

• Use an object-oriented approach to standardize the representation of processes and data within a device
• Provide standard services to access the data within a device 
• Provide more than one physical interface to accommodate small, medium, large, and huge systems 

BACnet is a standard data communication protocol that enables interoperability between different building systems and devices in building automation and control applications. The term interoperability means from simple information exchange to deeper integration to complete and complex interoperation between component devices and systems. BACnet provides the means for many kinds of basic and complex interoperations to take place using standardized techniques that have proven to be flexible and robust in over 15 years of practice in tens of millions of devices.  

The BACnet data communication protocol defines standard methods that manufacturers can implement to make components and systems that can be interoperable with other BACnet components and systems. BACnet includes, but is not limited to, HVAC applications. It is intended to apply to all types of automated building systems. There are interoperable products available in each of these categories: fire, security, lighting, HVAC, elevators, and so on.  

BACnet addresses the goal of interoperability by defining a generalized model of how automation devices work, a method for describing the information that they contain, and a method for describing protocols that one device can use to ask another device to perform some desired action. 

| Devices 

A BACnet device often comprises a microprocessor-based controller and a software combination that is designed to understand and use the BACnet protocol. A BACnet device is typically a controller, gateway, or user interface. Every BACnet device contains a device object that defines certain device information, including the device object identifier or instance number. A BACnet device object instance number must be field-configurable to be unique across the entire BACnet network where the device is installed. In addition to the device instance, each BACnet device contains a collection of information about the device and any input and output points that it monitors and controls.  

| Device Interoperability 

BACnet divides the task of device interoperability into three distinct areas:  

• Objects (information)
• Services(action requests)
• Transport systems (internetworking, electronic messages)

BACnet defines methods and requirements for the implementation of each of these areas. 

| Objects 

All information within an interoperable BACnet device is modeled in terms of one or more information objects. Each object represents some important component of the device or some collection of information that may be of interest to other BACnet devices. Objects may represent single pieces of information or a collection of multiple pieces of information such as a logical grouping. Objects represent either physical or virtual information such as analog and binary inputs and outputs, control algorithms, specific applications, and calculations.  

The BACnet standard defines 54 different standard object types. Each object is identified with an object identifier. An object identifier is a 32-bit binary number containing a code for the object type and the object instance number. In addition, every object, no matter its purpose or function, has a collection of properties that define the object. Each property includes at least a name and a value.  

A few of these common objects types are: 

| Properties 

 A BACnet property conveys information about a BACnet object. Objects have a collection of properties based on the function and purpose of the object. Each property contains:  

• Object_Identifier 
• Object_Name 
• Object_Type 

| Services 

BACnet Services are functions that can be requested and initiated by BACnet devices. These services are formal requests that one BACnet device sends to another BACnet device to ask it to do something. Services are grouped into five categories of functionality:

• Object Access (read, write, create, delete)
• Device Management (discover, time synchronization, initialize, backup, and restore database)
• Alarm and Event (alarms and changes of state)
• File Transfer (trend data, program transfer)
• Virtual Terminal (human-machine interface via prompts and menus)  

| Transport Systems 

The transport system uses different types of electronic messaging standards and methods to convey coded messages. Even though different transport methods are used, the coded message content remains the same. This philosophy allows the designer or specifier to choose the most cost-effective transport method for a given application.  

| Network Types 

The 2012 BACnet standard defines seven network types, which serve as the transport for BACnet messages. The seven supported network types are:  

• BACnet/IP
• BACnet MS/TP (Master-Slave/Token Passing)
• BACnet ISO 8802-3 (Ethernet)
• BACnet over ARCNET 
• BACnet Point-to-Point (EIA-232 and Telephone)
• BACnet over LonTalk Foreign Frames
• BACnet over ZigBee

The network types encompass the physical and datalink layers of the protocol. This combination of physical and datalink layers is often called the MAC (Medium Access Control) layer. A BACnet message itself is independent of the MAC layer used to transport the message. Therefore, in BACnet, messages to command or monitor information are the same, no matter which MAC layer is used for transport.