BMS
- What you should know about technical protocols
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 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) 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.
Put in short, 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.
