BACnet: The Building Automation Protocol for 35 Years
BACnet (ISO 16484-5) has been the dominant communication protocol in Building Management Systems (BMS) since 1987. Originally developed by ASHRAE to enable interoperability between HVAC equipment from different manufacturers, it has established itself as a global standard in the commercial, healthcare, industrial, and data center sectors.
Its object-oriented model is its strength: each device (AHU controller, heating controller, energy meter, etc.) exposes a list of standardized objects with predefined properties. A BACnet supervisor can automatically discover all devices on a network and read their objects without any prior configuration—a major advantage over Modbus.
BACnet Network Architecture
A typical BACnet network is organized into two layers:
Supervisor Layer (BACnet/IP Backbone)
- Building Management System (BMS) controllers connected via Ethernet
- BACnet/IP communication, UDP port 47808
- BACnet routers to interconnect subnets
Field Layer (BACnet MS/TP)
- Zone controllers (fan coils, VAVs, local air handling units) on RS485
- BACnet MS/TP communication, linear bus topology
- BACnet MS/TP → BACnet/IP gateway to connect the two layers
Cloud (Eziwan)
▲
│ MQTT TLS / 4G
▼
Eziwan Gateway ──── BACnet/IP (Ethernet) ──┬── JACE Controller
├── Siemens PXC Controller
└── Compteur BACnet
──── BACnet MS/TP (RS485) ──┬── Fan-coil Distech
└── VAV Schneider
BACnet Objects: The Standard Nomenclature
BACnet defines 54 standardized object types. The most common ones in building automation are:
| Object Type | Abbreviation | Description | Examples |
|---|---|---|---|
| Analog Input | AI | Read-only analog measurement | Temperature, humidity, CO2 |
| Analog Output | AO | Analog output (control) | AHU setpoint, valve opening |
| Analog Value | AV | Calculated value or parameter | Comfort setpoint |
| Binary Input | BI | Read-only logical state | On/off, fault, alarm |
| Binary Output | BO | Logical output | On/off control |
| Binary Value | BV | Internal binary value | Summer/winter mode |
| Multi-State Value | MSV | Multi-state mode | HVAC mode: off/vent/heat/cool |
| Trend Log | TL | Device-integrated history | 30-day temperature archive |
Cloud-Based Building Management System Monitoring: Use Cases
The BACnet → cloud connection enables remote monitoring of the entire building management system (BMS) infrastructure:
Commercial Buildings (Offices, Shopping Centers)
- HVAC monitoring: zone temperatures, setpoints, AHU status
- Monitoring of electricity, heating, and cooling consumption by zone
- Comfort indicators: CO2, temperature, humidity by space
Hospitals and Laboratories
- Cleanroom monitoring (positive pressure, humidity, temperature)
- Monitoring of pharmaceutical cold rooms
- Traceability of environmental conditions for compliance
Data Centers
- Real-time PUE (Power Usage Effectiveness)
- Cooling monitoring (CRAC units, cooling towers)
- Server temperature and relative humidity alerts
Multi-site operations (real estate companies, hotel chains, retail chains)
- Unified view of all buildings on a single dashboard
- Centralized alerts routed to the appropriate site manager
- Benchmarking between comparable sites
Automatic Discovery of BACnet Devices
A major advantage of BACnet over Modbus is automatic discovery (auto-discovery). The Eziwan gateway sends a Who-Is message as a broadcast on the BACnet network. Each device responds with its I-Am (Device Instance Identifier). The gateway then queries the object list (ReadProperty Object List) of each device and automatically generates the measurement points to be monitored.
This automatic detection eliminates the need to manually enter register addresses—a tedious and error-prone process in large buildings with hundreds of controllers.
Integration with Existing Building Management System (BMS) Controllers
For sites equipped with a building management system (BMS) controller (JACE Niagara, Schneider EcoStruxure, Siemens Desigo CC), the cloud gateway can be integrated in two ways:
Option 1 — Direct BACnet Reading (Supervisor Bypass) The Eziwan gateway reads BACnet field devices directly, in parallel with the existing supervisor. The supervisor continues to operate for local control. The gateway adds the cloud layer without modifying the existing system.
Option 2 — Reading from the supervisor (MQTT export) If the supervisor supports MQTT Publish (Niagara 4.x with MQTT module, Ignition, etc.), the gateway subscribes to the supervisor’s MQTT data. This is the simplest approach if the supervisor is already properly configured.