What is BACnet? Objects, BACnet IP and BBMD

What is BACnet? The building automation protocol explained

BACnet (Building Automation and Control Networks) is the widely used worldwide standard with which heating, ventilation, air conditioning, lighting and meters talk to the building management system. Instead of register addresses, BACnet describes every device with objects and properties that the management system recognises by itself.

Key facts

BACnet is standardised in ASHRAE 135 and ISO 16484-5 and independent of manufacturers.

Every device is a device object with a unique instance number. Measured values are, for example, analog input objects.

BACnet IP runs over UDP, on port 47808 by default. BACnet MS/TP runs over RS485.

A BBMD forwards BACnet broadcasts across subnet boundaries.

Device profiles such as B-ASC describe which functions a device masters at minimum.

On this page

How is BACnet structured?

BACnet thinks in objects. A device on the network is a device object with an instance number that may occur only once in the entire BACnet network. Below it are further objects: an analog input for a measured value such as active power, a binary input for a switching state, an analog value for a calculated value.

Every object has properties. The most important is Present Value, the current value. There are also Object Name, Units and Status Flags, among others. Because name and unit are stored in the device, the management system needs no register list to interpret a value correctly.

Object type	Meaning	Example
Device	the device itself	Meter, controller, gateway
Analog Input	measured value	Energy, power, voltage
Binary Input	state on or off	Signal contact, tariff input
Analog Value	calculated or set value	Total, setpoint

How do BACnet devices exchange data?

Through services. With Who-Is the management system asks which devices are on the network, and each device answers with I-Am. With ReadProperty it reads individual values, with ReadPropertyMultiple many at once.

Instead of polling constantly, the management system can also subscribe to changes with SubscribeCOV (COV stands for change of value). The device then reports by itself when a value changes by more than a set amount.

BACnet IP, MS/TP and BACnet/SC: what is the difference?

Variant	Transmission	Typical use
BACnet IP	Ethernet, UDP port 47808	Standard in new installations
BACnet MS/TP	RS485, master-slave/token-passing	Field devices, existing installations
BACnet/SC	Ethernet, TLS-encrypted via a hub	Installations with high IT security requirements

What does a BBMD do?

Who-Is and I-Am are broadcasts, and routers do not forward broadcasts into other subnets. If the management system and the devices are in different subnets, a BACnet Broadcast Management Device (BBMD) takes this over: it receives the broadcasts and sends them specifically to the BBMDs of the other subnets. Alternatively, a single device can register with a BBMD as a foreign device.

What do B-ASC and BTL mean?

BACnet describes typical device types with profiles. A B-ASC (BACnet Application Specific Controller) is a device with a fixed task, such as a meter or a gateway, that provides its values and answers requests. The BACnet Testing Laboratories (BTL) check whether a device meets the requirements of its profile.

How does meter data get into BACnet?

Most billing meters do not have BACnet but M-Bus or Modbus. A gateway translates: it reads the meters through their own interface and makes each meter available as a BACnet device of its own with analog input objects. To the management system it looks as if every meter had BACnet.

The EMU M-Center is certified as a B-ASC and supports BBMD. It takes on this task as M-Bus to BACnet IP, RS485 to BACnet IP and Modbus TCP to BACnet IP. How to approach this in planning is described in the article From M-Bus to BACnet IP.

Frequently asked questions

Which port does BACnet IP use?

UDP port 47808, BAC0 in hexadecimal. Further BACnet networks in the same building often use the following ports.

What is the difference between BACnet and Modbus?

Modbus transmits register values whose meaning is in the manual. BACnet transmits objects with name, unit and status that the management system recognises by itself. Modbus is widespread with meters and in industry, BACnet in building management.

Can BACnet run over RS485?

Yes, as BACnet MS/TP. It uses the same two-wire line as Modbus RTU, with an access method of its own (token passing). New installations mostly use BACnet IP.

What is a device instance number?

A number from 0 to 4,194,302 that uniquely identifies every BACnet device on the network. If an instance is assigned twice, the management system cannot find one of the devices.

Does every meter need its own IP address?

No. Behind a gateway, the meters share the gateway’s IP address. They are distinguished by a virtual BACnet network number and their own device instance.