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BACnet Building Automation and Control Network

Marvel Premium BACnet Controller

BACnet (Building Automation and Control Network), is the communications protocol that defines communication services used between building control systems and building automation end-devices. The protocol displays how data is represented and moved between BACnet nodes on the network and the services used to move it. For example, by sharing sensors and data, BACnet gives our applications similar abilities to sight, hearing, taste, smell and touch to help us make better decisions.

In June of 1987, a group of HVAC and Building Automation professionals met in Nashville to develop this standard, open protocol specifically for the building automation industry. Furthermore, in June of 1995, ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers) also adopted this same standard. Therefore, BACnet became an alternative to the proprietary and closed protocols offered by large vendors that were in existence at that time.

How BACnet works

The BACnet protocol uses an Object-oriented approach (Object-oriented programming allows for simplified programming. Its benefits include reusability, refactoring, extensibility, maintenance and efficiency). As a result, this approach standardizes the representation of processes and data. Secondly, BACnet provides the ability to control and monitor any building automation process, to meet the needs of users, integrators, and equipment vendors. As a result, the BACnet protocol uses mobile and cloud-hosted devices, head-end computers, general-purpose direct digital controllers, and application-specific or unitary controllers with equal effect.

In a typical HVAC application BACnet would handle lighting, heating/cooling with a programmable thermostat, smoke detection and some form of security/safety applications. In these applications, motion detectors can be used to more efficiently measure human presence and activity and with a timer to make sure lights are off after hours. To further simplify things, with BACnet, devices from various manufacturers work in conjunction with each other with little or no additional integration. This leads to cost savings over proprietary systems, but also tremendous savings in resources, installation costs, maintenance costs, and energy savings.

Today, the majority of users prefer to use Ethernet. For example, the Control and Information Protocol (CIP) used in industrial application, BACnet uses objects to represent data on a network. These objects are defined by the BACnet specification and have both required and optional data. BACnet is an unconnected, peer network where any device can send service requests to any other device. Unlike connected protocols where devices have ongoing data transfers, communication in BACnet is unscheduled without any time critical operations. With ease of use in mind, BACnet is a certifiable standard, though certification is not required.  

What are the 2 Protocols?

There are two distinct BACnet protocols being BACnet IP and BACnet MSTP.

BACnet IP:

The BACnet/IP allows users to transfer data to and from devices over Ethernet using BACnet/IP Protocol. BACnet/IP communication is implemented by defining a new protocol layer called the “BACnet Virtual Link Layer” or BVLL. There are many advantages to this approach. First it is extensible to other, future transport mechanisms such as IPv6, ATM, Sonet among others. Secondly, the concept of defining an extensible mechanism for peer-to-peer management of BACnet messages means that other manipulations. This includes such things as encryption/decryption and compression/decompression that can be performed outside of the process of generating BACnet APDU/NPDUs, i.e., without altering the existing standard. Routing between BACnet/IP and non-BACnet/IP networks is specified, including the case where IP and non-IP BACnet devices reside on the same LAN.


The MS stands for Master – Slave although in practice there are not many slaves out there and the TP stands for Token Passing. This method of BACnet the most common use to connect field devices to controllers and routers, to control applications. Up to 128 devices can be installed on a single network in the physical layer using RS485 with a max physical length of 4000 feet, and speeds up to 115k baud. Nevertheless, if more length is needed, it can be increased by the use of repeaters. You can compare to Ethernet, where the spec allows a max of 100 meters (330ft) on a single, unrepeated segment. Common baud rates are 19200, 38400 and 76800 and all devices must operate at the same baud rate. More and more devices can auto sense the baud rate and configure themselves correctly.

However, a disadvantage of the token system is that any one device gets a limited use of the bandwidth. Thus, a device may need to keep an internal queue of application layer messages it wants to send waiting to use the token. There are some vendor systems which fill their queue and then drop subsequent messages without notifying the user of the problem. Finally, limited access, combined with the overhead, makes it easy to use up all the bandwidth on the network. This happens if there are many devices with many objects and many properties of interest.

Key Points

  • Developed by: ASHRAE
  • Use: Communication across devices
  • Markets: Industrial, Transportation, Energy Management, Building Automation, Regulatory and health and safety
  • Examples: Boiler Control, Tank Level Measurements
  • Proprietary: no
  • Transmission Modes: Ethernet, IP, MS/TP, Zigbee
  • Standards: ANSI/ASHRAE Standard 185 ;ISO-16484-5; ISO-16484-6
  • Costs: Low; No charge for usage or licensing fees
  • Network Interfaces: Existing LANs and LANs infrastructure
  • Testing: BACnet Testing Labs
  • Advantages:
    • Scalability between cost, performance and system size
    • Endorsement and adoption by nearly every major vendor in North America and many other countries
    • Robust internet working including multiple LAN types and dial-up
    • Unrestricted growth and the ability to add new innovations and new features anytime •
  • Disadvantages:
    • Limited the number of field devices that can connect to a master station except Ethernet TCP/IP
    • MT/TP-Wire Length
    • Ethernet-Infrastructure
    • New standard has security standard but not implemented in all devices