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Bacnet Object Oriented Model

BACnet’s Object-Oriented World: Devices, Objects, and Properties

Imagine walking into a modern apartment building. Each apartment has a unique number, contains different rooms (bedroom, kitchen, living room), and is furnished with various items (sofa, fridge, thermostat). Now, think of a smart building as that apartment complex. The building itself is the network, each device is an apartment, objects are the rooms inside, and properties are the furniture that defines each room’s state and function. This is exactly how BACnet’s object-oriented model works.

In this article, we’ll explore BACnet’s object-oriented approach, using the apartment building analogy to make complex concepts easy to understand. We’ll cover the Device Object as the main identifier, introduce key object types, and explain how properties describe their state and configuration.

Object-Oriented Building Automation: A New Way to Think About Systems

Before BACnet, building automation used linear, device-centric communication. It was like having an apartment directory that only listed apartment numbers with no details about what was inside. BACnet’s object-oriented model changed this by providing a structured way to describe exactly what each device can do, like a detailed apartment listing that includes room types, square footage, and furniture.

What Does “Object-Oriented” Mean for Buildings?

In simple terms, object-oriented programming (and BACnet’s object-oriented model) organizes data and functionality into “objects” that represent real-world things. For BACnet, these objects represent physical devices, sensors, actuators, and logical functions in a building.

Apartment Building Analogy:

  • Furniture = BACnet Properties (temperature value, light status, schedule time)

The Device Object: The Apartment’s Front Door

Every BACnet device has one Device Object—its primary identifier. This is like the apartment’s front door with its number, resident information, and building rules posted outside.

Key Properties of the Device Object

Just as an apartment has basic information, the Device Object has essential properties:

| Property | Apartment Analogy | BACnet Purpose |

|————–|———————–|———————|

| Object Identifier | Apartment Number (e.g., 203) | Unique ID for the device |

| Object Name | Resident Name (e.g., “Smith Family”) | Human-readable device name |

| Vendor Name | Building Management Company | Manufacturer of the device |

| Model Name | Apartment Type (e.g., “2-Bedroom Deluxe”) | Model number of the device |

| Firmware Revision | Apartment Renovation Date | Software version of the device |

| Protocol Version | Building Code Compliance | BACnet protocol version supported |

| Status Flags | Apartment Occupancy Status | Device operational state (online/offline) |

Key BACnet Object Types: The Rooms of Your Building

Just as apartments have different rooms for different purposes, BACnet devices have different object types for different functions. Let’s explore the most common ones:

1. Analog Input (AI) Object: The Thermostat Room

What It Is: Measures continuous physical values (temperature, humidity, pressure)

Apartment Analogy: A thermostat in the living room that measures temperature

Key Properties:

  • Status Flags: Sensor health (e.g., normal, fault, out of range)

Real-World Example: An HVAC sensor measuring outdoor temperature

2. Binary Output (BO) Object: The Light Switch Room

What It Is: Controls on/off devices (lights, relays, motors)

Apartment Analogy: A light switch in the bedroom that turns lights on or off

Key Properties:

  • Reliability: Device health (e.g., reliable, failed)

Real-World Example: A relay that controls a rooftop exhaust fan

3. Schedule (SCH) Object: The Cleaning Schedule Room

What It Is: Defines timed events (when to turn devices on/off, adjust setpoints)

Apartment Analogy: A building’s cleaning schedule that specifies when common areas are cleaned

Key Properties:

  • Holiday Schedule: Special dates (e.g., “Christmas Day: Setpoint 65°F”)

Real-World Example: A schedule that turns office lights on at 8:00 AM and off at 6:00 PM

4. Analog Output (AO) Object: The Dimmer Switch Room

What It Is: Controls variable outputs (valve position, damper opening, light dimming)

Apartment Analogy: A dimmer switch that adjusts light brightness

Key Properties:

  • Write Priority: Who can adjust it (e.g., HVAC system, building manager)

Real-World Example: A damper actuator that controls air flow in a VAV (Variable Air Volume) system

5. Multi-State Value (MSV) Object: The Smart Lock Room

What It Is: Represents multiple discrete states (more than just on/off)

Apartment Analogy: A smart lock with states like “Locked”, “Unlocked”, “Key Card Access”, “Emergency Override”

Key Properties:

  • Description: What it controls (e.g., “Main Entrance Lock”)

Real-World Example: An access control system for a building entrance

How BACnet Objects Work Together: A Day in the Life of a Smart Building

Let’s see how these objects interact in a typical office building:

1. 6:00 AM: The Schedule Object activates, sending a command to the Analog Output Object controlling HVAC setpoints: “Raise temperature to 72°F”

2. 6:30 AM: The Analog Input Object for outdoor temperature reports 45°F to the HVAC system

3. 7:55 AM: The Binary Output Object for the front door lock receives a command from the access control system: “Unlock”

4. 8:00 AM: The Schedule Object commands the Binary Output Objects for office lights: “Turn ON”

5. 12:00 PM: The Multi-State Value Object for the conference room indicates “In Use” based on occupancy sensor input

6. 6:00 PM: The Schedule Object commands lights off and HVAC setpoints to 65°F

7. 10:00 PM: The Binary Output Object for the front door lock receives: “Lock”

This coordination happens because all these objects share the same BACnet object-oriented language—like residents in an apartment building all speaking the same language, making communication seamless.

Benefits of BACnet’s Object-Oriented Design

BACnet’s object-oriented model offers several key advantages:

1. Interoperability: “Speak the Same Language”

  • Like all apartments in a building following the same layout conventions, making it easy for maintenance to work on any apartment

2. Flexibility: “Rearrange the Furniture”

  • Like renovating an apartment by changing furniture instead of tearing down walls

3. Scalability: “Add More Apartments”

  • Like expanding an apartment building with new floors while maintaining the same management system

4. Clarity: “Know What’s in Each Room”

  • Like an apartment inventory list that shows exactly what’s in each room

5. Maintainability: “Easy to Fix”

  • Like knowing exactly which room in an apartment needs repair

Beyond the Basics: Advanced BACnet Objects

BACnet supports over 50 object types to handle complex building functions, including:

  • Calendar: Defines special dates for schedules

Each object type follows the same basic principles—properties that describe its state and functionality—making them easy to understand once you grasp the fundamentals.

Conclusion: The Apartment Building of the Future

BACnet’s object-oriented model transforms smart buildings into well-organized apartment complexes where every device (apartment) has a clear identity, contains specific objects (rooms), and is configured with properties (furniture). This structured approach enables seamless communication between different systems, regardless of who made them.

Just as a well-designed apartment building is efficient, comfortable, and easy to maintain, BACnet’s object-oriented design creates buildings that are energy-efficient, comfortable, and easy to manage. It’s the secret ingredient that makes true smart building integration possible.

So the next time you walk into a modern office, hotel, or hospital, remember the invisible BACnet objects working behind the scenes—like the apartments, rooms, and furniture that make up a well-run building—keeping everything running smoothly and efficiently.

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