What Is a BACnet Object? Properties and Examples Explained
What Is a BACnet Object?
BACnet organizes information inside a device using objects. A BACnet object is a standardized representation of a function, value, or collection of information within a BACnet device.
Objects can represent physical inputs and outputs, software values, schedules, trends, devices, and other functions used in a building automation system.
A single physical device can contain many BACnet objects. For example, a BACnet thermostat may expose separate objects for room temperature, temperature setpoint, occupancy status, fan command, and other information.
BACnet Objects and Properties
Each BACnet object contains properties that describe the object and provide access to its data. Properties can contain identifying information, measured values, status information, configuration settings, units, and other information associated with the object.
Three properties are present in every BACnet object: Object Identifier, Object Name, and Object Type. Additional required and optional properties depend on the specific object type.
Some properties can only be read, while others may also be writable. Whether a property can be written depends on the property, the object type, and the device implementation.
Example of a BACnet Object
Consider a room temperature sensor represented as a BACnet Analog Input object. The object could contain properties such as:
| Property | Example Value |
| Object Identifier | Analog Input, Instance 1 |
| Object Name | Space Temperature |
| Object Type | Analog Input |
| Present Value | 71 |
| Units | Degrees Fahrenheit |
| Status Flags | Normal |
The properties available on an actual Analog Input object depend on the BACnet standard requirements and the capabilities implemented by the device manufacturer.
BACnet Object Identifiers
Every BACnet object has an Object Identifier. The identifier combines the object's type with an instance number so that the object can be uniquely identified within a BACnet device.
For example, a device could contain several Analog Input objects: Analog Input 1, Analog Input 2, Analog Input 3, and so on. Each represents a separate BACnet object even though they share the same object type.
BACnet clients use these identifiers when reading properties, writing properties where permitted, and otherwise interacting with objects in a BACnet device.
Common BACnet Object Types
BACnet defines many standardized object types. Some of the object types commonly encountered in building automation systems include:
- Analog Input - Represents an analog measured value such as temperature.
- Analog Output - Represents an analog command or output value.
- Analog Value - Represents an analog value that is not necessarily tied directly to a physical input or output.
- Binary Input - Represents a two-state input such as normal/fault or off/on.
- Binary Output - Represents a two-state command or output.
- Binary Value - Represents a two-state value used within a device or control system.
- Multi-state Input - Represents an input with more than two possible states.
- Multi-state Output - Represents a command with multiple possible states.
- Multi-state Value - Represents a value with multiple defined states.
- Schedule - Represents scheduled operation based on time and date.
- Calendar - Represents dates used by scheduling and control functions.
- Trend Log - Stores historical samples of BACnet data.
- Notification Class - Defines how event notifications are distributed.
- Device - Represents the BACnet device itself.
This is not a complete list. BACnet has added additional standardized object types as the protocol has evolved.
The BACnet Device Object
Every BACnet device includes a Device Object. The Device Object represents the BACnet device itself rather than an individual sensor, output, or control point.
It contains information used to identify the device and describe its BACnet capabilities. Other objects within the device represent the actual data, control points, schedules, logs, and functions made available through BACnet.
Why BACnet Objects Matter
The standardized object and property model is one of the reasons BACnet can support interoperability between equipment from different manufacturers.
A BACnet client does not need to understand the manufacturer's internal memory layout or proprietary data structure. Instead, it can interact with standardized BACnet objects and properties.
For example, a building management system can discover an Analog Input object and read its Present Value property using standard BACnet services, regardless of how the manufacturer stores that value internally.
Viewing BACnet Objects and Properties
BACnet diagnostic software can be used to discover devices and browse the objects and properties they expose.
The CAS BACnet Explorer can discover BACnet devices, browse their object lists, examine property values, monitor data, and test BACnet communication.
BACnet Objects in Software Development
Developers implementing BACnet in a product must determine which BACnet object types their device needs to expose and which properties and BACnet services are required for the application.
Chipkin's CAS BACnet Stack supports a wide range of BACnet object types for developers adding BACnet communication to embedded devices and software applications.
The BACnet Stack page contains the current list of object types supported by the CAS BACnet Stack.
Related BACnet Resources
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