BACnet

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BACnet is a communications protocol that is typically used by building management systems or process controllers to connect, monitor and control process values. BACnet can be used to connect to assets and allows multiple devices to initiate conversation and share data and information. BACnet can be used to control HVAC systems, BMS systems and other devices as well as allow them to to connect to graphical interfaces for the end user.  BACnet is both an international (ISO) and ANSI standard for interoperability between cooperating building automation devices.

The protocol defines a set of rules and standards, such as the structure of messages or network addressing, and more specifically, it defines what kind of information can be exchanged. 

BACnet provides live real time data for all zones connected to the BACnet network. For example, a facility manager can see if the boiler is running, or if an air-conditioner is cooling the building. This gives us incredible visibility into energy monitoring, as we can see exact energy usage in specific assets, which we can use to pinpoint inefficiency. 

ASHRAE developed the BACnet standard to eliminate this, and the standard continues to evolve as new technologies arise. Since then, Originally developed in 1987 under the auspices of the American Society of Heating Refrigerating and Airconditioning Engineers (ASHRAE), BACnet has been an ANSI standard since 1995 and an ISO standard since 2003. BACnet is a registered trademark of ASHRAE.

The term “interoperability” has a variety of meanings from simple information exchange, to deeper integration, to complete and complex interoperation between component devices and systems. While BACnet does not enable "plug and play" device interchangeability, 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.

BACnet offers a flexible range of networking options including the use of Ethernet or IP-centric infrastructure and a simple, low cost twisted pair communication called MS/TP that is based on EIA485. A sophisticated routing capability allows scaling of BACnet internetworks into large and efficient systems, all within the same unified standard. 

BACnet includes 54 standard objects that cover many common and generally useful applications. 

BACnet provides a collection of objects manufacturers can combine to build devices tailored to specific applications.

BACnet also has an extensive application services model that provides many types of useful services that implementers may elect to support in their devices. These services are grouped into the following logical areas: object access, alarm and event management, scheduling, trending, files, device and network management.

Building owners and system specifiers can also use BACnet as a tool for the specification of interoperable systems. BACnet does not replace the need for specifying what a user wants or needs. It simply provides some standardized tools to help enable the creation and specification of systems that can interoperate.

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, etc. 

Devices:

A BACnet device is often comprised of a microprocessor-based controller and 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. 

Device Interoperability 

BACnet divides the task of device interoperability into three distinct areas: Objects (information), Services(action requests), and Transport systems (internetworking, electronic messages). BACnet defines methods and requirements for implementation of each of these areas.

Objects 

All information within an interoperable BACnet device is modeled in terms of one or more information objects. 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.

Services: 

BACnet 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); and virtual terminal (human machine interface via prompts and menus). The service defines each request, and any parameters that need to be conveyed in the request and its reply.

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.

 

Architecture of BACnet.