Choosing a Building Management System (BMS): What you should know about technical protocols

Abtar Singh, President, kWh360 & Singh360 Inc.

If you or a client is choosing a building management system (or BMS), it’s important to understand how it communicates information with digital devices such as controllers, meters, and input/output boards, and computers.

The details are important because some BMS use languages—or technical protocols—that lock you into using their vendor’s proprietary technology. Use of such protocols may force you and your client to pay higher prices for software and hardware available from only one vendor or its licensees.

This article describes common categories of BMS protocols. It recommends that you avoid proprietary protocols and favor more open ones.

A BMS communicates through protocols

To exchange data, digital devices must use a common data structure and a common channel or medium of communication.

The figure below shows a master BMS that communicates with devices that use microprocessors. They include a roof-top unit (or RTU), refrigeration controllers, energy meters, and other input/output boards within a building. The building controller also uses the Internet to share temperature, operating parameters, or energy data with remote users through enterprise servers or personal computers  


A BMS protocol defines the format and meaning of each data element, in much the same way a dictionary defines the spelling and meaning of words.

The data exchange often occurs through a physical wire such as a twisted-pair RS485 or an Ethernet CAT5 cable). It may also occur wirelessly over wi-fi network, through an internet protocol (or IP).

The phrase “BACNet over IP” means the BACNet protocol communicates through an IP network.

Some protocols are more open than others

Protocols fit in one of four categories, depending on their relative “openness:”

  1. Open. The protocol is readily available to everyone.
  2. Standard. All parties agree to a common data structure. The protocol may be an industry standard, such as BACnet and Modbus.
  3. Inter-operable. The protocol is vendor agnostic. A controller from one vendor can replace one from a different vendor.
  4. Proprietary. The data structure is restricted to the creator of the device.

Why you want a BMS with open protocols

A BMS with proprietary protocols locks the system owner into using a single BMS vendor. For example, you can’t remotely change the set points of a proprietary BMS unless you use the vendor’s software.

In contrast, with open and standard BMS protocols you can shop for alternative providers of digital devices and enterprise software.

This is why use of proprietary protocols is inconsistent with best practice. The lesson is clear:

In choosing a BMS, be sure its protocols are not proprietary.


How to know whether a BMS protocol is open

To determine whether a BMS protocol is open, ask the vendor two simple questions:

  1. Can your competitors exchange data with your BMS?
  2. Is the system’s protocol published in such a way that it’s easily accessible to everyone (including competitors)?

Best open protocols: BACNet, Modbus, and XML

For a master controller that exchanges data with devices and meters within a building, prefer the BACNet, Modbus or any other standard protocol. Otherwise, make sure it’s at least open enough so anyone with proper security access can read and write information.

For remote enterprise access (protocol B in the figure), organizations often use BACnet over IP.

The current trend is toward use of additional Internet technologies. Companies like Honeywell Tridium (Niagara framework), Resource Data Management (Data Manager), Echelon Corps (iLon), and many others have exchanged data through standard internet eXtensible Markup Language (or XML) with web services.

Even the ASHRAE BACNet committee has convened a working group to define use of XML with BACnet systems. The group is also working to define web services that will enable data exchange between building automation and control systems and various enterprise management systems.

In summary, use these criteria when you’re choosing devices and BMS:

  • For devices such as RTUs and refrigeration controllers, look for ones that use open protocols such as BACnet or Modbus.
  • Make sure these devices give you both “read” and “write” capabilities so you can change set points.
  • For easy enterprise access, choose a BMS with web services and XML capabilities.
  • Make sure the web services of the BMS allow both read and write capabilities.
  • Be sure the BMS supplier provides the XML dictionary and definitions of web services to anyone, including competitors.

About the Author

Dr. Abtar Singh is the founder & CEO of Singh360 Inc. (, a facility management consulting firm. He also founded a cloud and mobile based energy efficiency company kWh360 Inc. ( He specializes in facility management systems, design, engineering, performance monitoring, predictive analytics, and optimization of energy and maintenance systems. You can reach him at