What is BACnet | BACnet vs Modbus | BACnet MSTP

Worldwide manufacturers prefers BACnet as a data communication protocol for products with building automation and related markets. Let’s Start with very basics what is BACnet 

What is BACnet

BACnet also called as Building Automation Control Network is a data communication protocol for building automation and control networks.

BACnet was developed by American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) in 1987.

BACnet has been an ANSI standard and ISO standard since 1987 and 2003

BACnet is a standard data communication protocol that enables interoperability between different building systems and devices in building automation and control applications

The plug and play interchangeability is not enabled by BACnet and so it provides the means for many kinds of basic and complex inter operations to take place using standardized techniques which are proven flexible in over several years of practice in tens of millions of devices

BACnet does not replace the need for Direct Digital Control or control logic and does not attempt to standardize how devices are programmed.

Modbus vs BACnet

Here is the our best article about:

Modbus vs Bacnet (Difference between Modbus and Bacnet)


BACnet also uses Ethernet or IP-centric infrastructure and a simple, low cost twisted pair communication called Master Slave Token Passing protocols a network options

MSTP is based on EIA-485 and within the same unified standard it provides a sophisticated routing capability allows scaling of BACnet inter networks into large and efficient systems.

A BACnet device is often comprised of a microprocessor-based controller and software combination that is designed to understand and use the BACnet protocol.

MS/TP means Master-Slave/Token-Passing.

MS as the name itself says that any device on MS/TP bus has to be a master or a salve.

Master can initiate the communication once it has the token and the slave respond to the request, the slave cannot initiate communication or keep the token in a hold state.

TP indicates that entire MS/TP bus is controlled by a single token,

this single token is then passed from master to master in order of MS/TP MAC address and any device can respond to a communication request but only the device with the token can initiate the communication.

MS/TP (Master-Slave/Token-Passing) transmission mode which is unique to BACnet.

The MS/TP datalink sits on top of an EIA-485 physical layer and uses common electronic components to provide a low cost implementation.

MAC addresses are used to pass messages on MS/TP bus.

MS/TP is used for signaling up to 4000 feet.

MS/TP is the most popular type of BACnet LAN for application-specific and unitary controllers, and is also the lowest cost.

BACnet Gateway / Controller Specification

A BACnet device is typically a controller, gateway, or user interface

Some specification defines all aspects of the BACnet protocol are listed below :-

BACnet uses an object-oriented model for abstracting and representing different aspects of a control system. For example

  1. A physical Device (i.e. device objects)
  2. A temperature input (i.e. analog input)
  3. A relay control (i.e. binary output)

BACnet also allows the implementer to create their own non-standard objects that can be easy to communicate with other devices that choose to use them.

BACnet also has an extensive application services model that provides many types of useful services that implementer may elect to support in their devices. Services are grouped into five categories of functionality

a).Object access which performs write, read, create, delete;

b). Device management i.e.time synchronization, initialize, backup and restore database;

c). Alarm and Event i.e. alarms and changes of state;

d).File Transfer i.e. trend data, program transfer;

e). Virtual terminal i.e. human machine interface via prompts and menus.The objects that provides a services can be a server or a client or else can be both depending on system, Server is something which provides the service and Client is something that requests the service.

A property contains information about an object whereas the object may contain last amount of properties. The following properties that every object in the BACnet should follow :

  • object_identifier
  • object_name
  • object_type

The transport system uses different types of electronic messaging standards and methods to convey coded messages. Even though different transport methods are used, the coded message content remains the same.

This philosophy allows the designer or specifier to choose the most cost-effective transport method for a given application.

There are seven network types defined by BACnet standard, which behaves as the transport for its messages. The seven supported network types are:

  • BACnet/IP
  • BACnet MS/TP (Master-Slave/Token Passing)
  • BACnet ISO 8802-3 (Ethernet)
  • BACnet over ARCNET
  • BACnet Point-to-Point (EIA-232 and Telephone)
  • BACnet over LonTalk Foreign Frames
  • BACnet over ZigBee

Some of the BACnet controllers are listed below :-

  • Stryker Controller-Honeywell
  • AC/S1.2.1-ABB
  • TAC I/A Series-Schneider
  • TC-1000 Modular Series- Simens
  • PXC00-E.D-Simens

Some of BACnet Gateways are listed below :-

  • BACnet / CAN – Converter
  • BACnet Slave / CAN Converter :-HD67677
  • BACnet Master / CAN Converter :-HD67717
  • BACnet / CANopen Converter
  • BACnet Slave / CANopen Converter :-HD67678
  • BACnet Master / CANopen Converter :-HD67718 is a
  • BACnet / DeviceNet– Converter
  • BACnet Slave / DeviceNet Master Converter :-HD67680
  • BACnet Slave / DeviceNet Slave Converter :-HD67681

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