Who-Is, I-Am, and ReadPropertyReadProperty, COV, WriteProperty)Analog Input with Present Value, which the BMS can read or subscribe to using COVIn today’s world of smart buildings, the ability for systems to communicate intelligently and reliably is a foundational requirement. From HVAC and lighting to access control and energy metering, building systems must work together in real time — and often across devices from different vendors.
This is where BACnet (Building Automation and Control Network) stands out.
Unlike traditional protocols such as Modbus, which rely on fixed registers and manual decoding, BACnet is built around two game-changing features:
For system integrators, this means fewer errors, faster deployments, and better interoperability — especially when compared to protocols that require digging through technical manuals just to understand what a data point represents. Understanding how BACnet works is key to unlocking scalable, interoperable, and future-ready building automation systems.
At the heart of BACnet’s efficiency is its self-describing structure — a sharp contrast with legacy protocols like Modbus. In BACnet, every device exposes its capabilities using a standard ontology: a consistent set of object types (e.g., Analog Input, Binary Output, Schedule, Device) and well-defined properties (Present Value, Units, Status Flags, etc.).
This means integrators and software platforms no longer need to guess what a value represents or manually map register addresses to real-world functions. The structure is clear, universal, and standardized.
Even more powerful is BACnet’s discovery mechanism. Using services like Who-Is, I-Am, ReadProperty, and ReadPropertyMultiple, tools such as Niagara Framework, YABE, or SCADA Engine can automatically:
This autonomous discovery makes BACnet a massive time-saver and significantly reduces errors during configuration and commissioning — especially in large or complex installations.
Thanks to this design, BACnet isn’t just a protocol — it’s a framework that enables plug-and-play interoperability across diverse building systems.
BACnet is built on a modular, peer-to-peer architecture designed specifically for building automation. Unlike master-slave models used in older protocols, BACnet allows any device — whether it’s a sensor, controller, or server — to initiate communication with any other device on the network.
This flexibility is one of the reasons BACnet has become the default protocol for Building Management Systems (BMS) around the world.
Key architectural principles:
BACnet’s architecture enables scalability from small single-building installations to vast multi-campus or multi-site systems, while maintaining a consistent structure that simplifies integration and expansion.
This makes BACnet particularly well-suited for organizations and operators investing in long-term, future-proof building automation strategies.
“BACnet provides a strong foundation for building automation, but when paired with LoRaWAN®, it unlocks entirely new capabilities — enabling long-range, low-power communication with wireless sensors across large facilities, even beyond the reach of traditional wiring.“
To manage the complexity of device communication across different types of networks and infrastructures, BACnet follows a layered architecture — similar in concept to the OSI model — that separates concerns like services, routing, and data transport.
Each layer serves a specific role in ensuring that devices can communicate effectively, regardless of the underlying medium.
This is where the actual BACnet services reside. Devices use these services to read or write data, discover other devices, subscribe to value changes, and more.
Common BACnet services include:
The network layer handles:
This layer is critical when integrating multiple buildings or crossing network boundaries.
BACnet supports several data link options, giving it great flexibility:
BACnet is designed to work over multiple transport layers — but in practice, two dominate most real-world installations: BACnet/IP and BACnet MS/TP. Understanding their differences is essential for selecting the right architecture depending on the building’s infrastructure, scale, and performance needs.
BACnet/IP runs over standard Ethernet or Wi-Fi using the UDP/IP protocol stack. It’s the most popular choice for modern buildings because it:
Use cases:
✔ Smart campuses
✔ High-performance commercial buildings
✔ Systems requiring integration with cloud or IoT platforms
MS/TP (Master-Slave/Token-Passing) is a serial communication protocol over RS-485. It’s still very common in field-level device networks due to its simplicity and low cost.
Key characteristics:
Use cases:
✔ Legacy building systems
✔ Economical controller-to-sensor wiring
✔ Buildings without Ethernet infrastructure
In most new or renovated installations, BACnet/IP is the recommended path forward, especially when preparing for IoT integrations. However, MS/TP remains highly relevant in cost-sensitive or retrofit scenarios — and hybrid architectures combining both are common.
At the core of BACnet’s power and flexibility is its object-oriented model. Instead of handling raw values or registers (as in Modbus), BACnet structures every function of a device into well-defined objects, each with a consistent set of properties.
This approach is what makes device discovery and interoperability seamless — every BACnet-compatible device presents its capabilities using the same universal structure.
Each object represents a logical function of a device. Some of the most frequently used include:
| Object Type | Typical Use Case |
|---|---|
| Analog Input | Temperature sensors, CO₂ levels, humidity |
| Analog Output | Fan speeds, valve positions |
| Binary Input | Contact closures, motion detectors |
| Binary Output | Relays, lighting control, digital actuators |
| Device | Metadata about the device itself |
| Schedule | Time-based automation of values |
| Trend Log | Historical logging of property values |
BACnet defines dozens of object types, and manufacturers can also add proprietary objects if needed.
Each object contains properties, which provide readable or writable data about that object. For example, an Analog Input might have:
These properties are standardized across all vendors, so any BACnet-compatible platform knows how to interpret them without needing custom drivers or decoding maps.
With Modbus, a device might expose a value at Register 40025 — but the integrator has to check the manual to know what it means, what units it’s in, and how to scale it.
With BACnet, this same value would be exposed as an Analog Input with a Present Value property labeled “Temperature”, in °C — and discovered automatically by the BMS.
This semantic clarity is what makes BACnet ideal for scalable, vendor-independent building automation.
To truly understand how BACnet works, let’s walk through a real-world example: a BACnet-enabled temperature sensor connected to a building automation system (BMS).
This simple use case highlights the protocol’s power — from object modeling to real-time communication and automatic discovery.
The temperature sensor includes an Analog Input object. This object has several properties, including:
Because these object types and property names are standardized, the BMS doesn’t need custom code to interpret them.
Using BACnet’s Who-Is / I-Am services, the BMS controller sends a broadcast request to find devices on the network. The sensor replies with its Device ID, object list, and capabilities.
Then, using ReadProperty or ReadPropertyMultiple, the BMS can access the full list of available objects and properties — no manual mapping required.
When COV is enabled, the sensor will automatically push updates to the BMS only when the temperature changes — reducing network traffic and latency.
Once the BMS has the temperature data, it can:
All of this happens without custom parsing, scaling, or configuration — a clear demonstration of BACnet’s plug-and-play interoperability.
One of BACnet’s strengths as an open protocol is the wide range of tools and utilities available for developers, integrators, and facility managers. Whether you’re testing a new device, troubleshooting a communication issue, or analyzing network performance, these tools can greatly simplify the process.
Widely used in commercial building automation, Niagara offers native BACnet integration and:
It’s an enterprise-grade platform ideal for large-scale BMS deployments.
YABE is a popular free and open-source Windows tool that allows you to:
It’s widely used by engineers for its simplicity and effectiveness, especially in small to mid-sized projects.
Wireshark, the well-known packet analyzer, includes a built-in BACnet protocol dissector. With it, you can:
It’s an essential tool for deep debugging, especially on complex or segmented networks.
| Tool | Use Case |
|---|---|
| CAS BACnet Explorer | Professional-grade diagnostic and test tool |
| BACpypes (Python) | Build and simulate BACnet devices in Python |
| BACnet Stack (C/C++) | Open-source implementation for custom devices |
| BACnetSim | Simulate complex BACnet device networks |
| Visual Test Shell (VTS) | Validate protocol conformance (used in testing) |
Whether you’re a developer writing your own BACnet stack, or an integrator configuring a commercial building, these tools make BACnet systems more transparent, debuggable, and interoperable by design.
BACnet’s architecture — built on discovery, standard object modeling, and media independence — has made it the backbone of modern building automation. From HVAC systems and lighting to energy meters and access control, BACnet enables diverse devices to communicate seamlessly and reliably, no matter the manufacturer.
Its self-describing structure and auto-discovery capabilities make it far more efficient than older protocols like Modbus. Integrators spend less time reading manuals and mapping registers, and more time building scalable, high-performing systems.
As the building industry moves toward IP convergence, cloud-based control, and IoT-native deployments, BACnet continues to evolve. Combined with technologies like LoRaWAN®, supported by Actility, BACnet extends beyond the building — connecting remote sensors and edge devices in a unified, intelligent platform.
In a world where smart buildings are becoming the norm, understanding how BACnet works is more than technical know-how — it’s a strategic advantage.
Unlike Modbus, which uses register-based communication and often requires manual decoding, BACnet uses standardized object types and properties, allowing for automatic device discovery and seamless integration.
Device discovery allows systems like Niagara or BACnet explorers to automatically identify devices, read their object structures, and understand what services they support — with no manual mapping required.
A BACnet object represents a specific function within a device (e.g., temperature input, binary switch). Each object has defined properties such as present value, units, and status flags.
These are actions that BACnet devices use to communicate. ReadProperty retrieves a value from another device, while COV (Change of Value) allows devices to push updates automatically when values change.
Use BACnet/IP for modern, high-performance networks with Ethernet or Wi-Fi. Use MS/TP when you need a cost-effective solution for field-level devices on RS-485 cabling.
About Actility
Actility, one of the co-inventors of LoRaWAN® technology and a founding member of the LoRa Alliance, is the leader in industrial-grade low-power wide-area network (LPWAN) connectivity and IoT tracking solutions. Actility’s ThingPark™ platform, which supports multi-radio connectivity (LoRaWAN®, NB-IoT, LTE-M), powers the majority of public networks and numerous private and enterprise networks worldwide. Through its subsidiary Abeeway, Actility offers patented ultra-low power, multi-radio trackers and comprehensive indoor and outdoor geolocation services. Additionally, the ThingPark Market boast the largest catalog of LoRaWAN® devices, gateways, and solutions available.
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