BACnet Devices, Objects and Properties

This is the second part of BACnet Protocol Stack Tutorial

BACnet Protocol Stack Tutorial Part 2

The BACnet protocol represents control equipment as a collection of "Devices" containing "Objects". Usually, the building control industry uses the term "Points" - they can be sensor inputs, control outputs or control values, with different characteristics according to the manufacturer. A BACnet Object can be used to represent a Point.

BACnet defines a standard set of "Object Types", each of which has a standard set of "Properties"; that describe an Object and its current status to other devices on a BACnet internetwork. It is through these properties that the Object may be controlled by other BACnet devices. Learn more

BACnet Services

The BACnet protocol also defines various services that BACnet devices can be asked to perform. Those services are divided into 5 categories:

• Object Access services, which are used to access and manipulate the properties of BACnet objects;
• Alarm and Event services, which are used to obtain alarms, events and change of value notifications;
• File Access services, which are used to access and manipulate files contained in BACnet devices;
• Remote Device Management services, which are used for device discovery, time synchronization, device communication control and reinitialization;
• Virtual Terminal Services, which are used to facilitate the bi-directional exchange of character-oriented data.

What is a BACnet Device 

A BACnet Device is the network-visible representation of control equipment connected to a BACnet network, which responds to BACnet service requests. A Device represents the control equipment by maintaining a collection of BACnet Objects. The Objects contain Properties that can be accessed and manipulated using BACnet's Read-Property and Write-Property service requests. The Objects within any Device are uniquely identified by an Object type and by an instance number (together these data make up an Object Identifier). Each Device must contain one Device Object to represent the BACnet capabilities of the Device. 

Here is an example of such a Device which has objects and properties that you can view with BACnet Explorer

Each Device on a BACnet internetwork must have a unique Device Object instance number and, consequently, a unique Device Object identifier. Devices can be contacted directly if their BACnet Address is known, or they can be discovered using the Who-Is and Who-Has broadcasts.

In applications built using the Cimetrics BACstac, a Device is represented in the BACstac Object Database by a Device handle. The BACstac internally maintains the Device information such as the BACnet Address of the Device and the list of Device’s Objects. The BACstac uses this information when it generates or responds to service requests.

What is a BACnet Object 

Clause 12 of the BACnet Standard, “MODELING CONTROL DEVICES AS A COLLECTION OF OBJECTS,” contains descriptions of the Properties required to be present in Objects that conform to a standard object type. An Object in a BACnet Device is uniquely identified within the current Device by an Object Identifier, which consists of an Object type and instance number. A Device must contain one Device Object, which has an instance number that is unique within the BACnet internetwork. All other objects in the Device do not have to be unique within the network.

Objects are represented in BACstac applications by an Object handle. The BACstac maintains information internally about an Object, such as the list of Properties that it contains. An Object’s handle can be obtained by searching through the list of Objects contained in a Device. An Object’s handle can also be obtained if its Object Identifier (type and instance number) is known. 

While this Standard prescribes the most widely applicable object types and their properties, implementers are free to create additional object types if desired.

There are currently 60 standard BACnet Object Types:

Here is an example of an Analog Input Object, as displayed in the Cimetrics BACnet Explorer:

What is a BACnet Property

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