What It Is
BACnet represents application data as typed objects with defined properties, behaviors, and identifiers. That object model is a major reason BACnet integrations are more expressive than register-only protocols such as Modbus.
Every object is identified by an Object Identifier that combines type and instance, such as analogInput:1 or binaryOutput:12.
Common Object Families
| Object Type | Typical Use | Notes |
|---|---|---|
| Analog Input, Analog Output, Analog Value | Temperatures, setpoints, pressures, calculated values | Core analog object family used on almost every job |
| Binary Input, Binary Output, Binary Value | Alarm states, relay commands, enable flags | Used for discrete status and command behavior |
| Multi-State Input, Multi-State Output, Multi-State Value | Modes, stages, enumerated states | Useful where a point is more than simply on or off |
| Device | Identity and capabilities of the device itself | Every BACnet device has one Device object |
| Schedule, Calendar, Trend Log, Notification Class | Scheduling, trending, and event handling | Often important at the supervisory level |
Key Properties
| Property | Why It Matters |
|---|---|
| Object Identifier | Defines the object’s type and instance |
| Object Name | Human-readable label seen in tools and BMS front ends |
| Present Value | The current value of the point |
| Status Flags | Shows alarm, fault, override, and out-of-service state |
| Units | Adds engineering meaning to analog values |
| Description | Useful for commissioning and handoff clarity |
Why Object Modeling Matters
BACnet projects often fail because the transport is healthy while the object model is wrong. A BMS may expect a writable Analog Output and receive a read-only Analog Input. A schedule may need a Schedule object, not just another analog setpoint. A trend consumer may need history support that the exposed object set does not provide.
In practice, interoperability depends on choosing the right object type, assigning sensible names and instances, and confirming the expected properties and services are actually supported.