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the gap between Building Automation and Industry processes with BACnet
and OPC UA
Both markets increasingly require data exchange from building automation to industrial automation processes and the ability to connect machines to building management systems.
Marketing and Training Building Automation,
Big things with
It started as a side meeting at the light+building 2012 trade show in Frankfurt, Germany. A group of experts from the BACnet community and the OPC Foundation met for a first informal get-together. Very soon it became clear to all that both markets increasingly require data exchange from building automation to industrial automation processes and the ability to connect machines to building management systems. After a few teleconferences half a year later Matthias Damm and Frank Schubert were elected as leaders of the newly founded working group “BACnet OPC UA Information Model” and the group started on the specification work.
Figure 1: Foundation of the working group at the University of Dresden, Fall 2012 (from left to right)
Klaus Wächter, Siemens AG, member of the BACnet Interest
Group executive board
Matthias Damm, Ascolab
Frank Schubert, Beckhoff Automation
Stefan Hoppe, Vice President OPC Foundation
The driving forces -
The basic idea
In several face-to-face meetings, the group discussed potential use cases and decided to focus on connecting building automation systems to enterprise systems first.
The most commonly expected use cases include:
Data from building
automation systems (where the source is BACnet)
brought to enterprise systems
(where the destination is OPC UA)
General overview about communication
In every technical communication model it’s essential to clearly specify three different elements; the network media, the services to access the data and the data content itself.
Network media: The BACnet OPC UA Information Model does not mandate using specific network media. While BACnet supports a total of 8 different physical network layers including IPv4, IPv6 and serial networks using EIA-232, EIA-485 or wireless connections using Zigbee, OPC UA uses TCP/IP communication. A gateway application implementing this profile may choose any of the supported BACnet layers, even though it is expected that the majority of implementations will likely use IP.
Services: The BACnet OPC UA Information Model does not mandate how a gateway application implementing the BACnet OPC UA Information Model will gather the data from BACnet. An application may even fall back to polling the data points if retrieving the data using COV (change of value) is not possible at all or temporarily not available due to device limitations or a higher number of clients.
Data: The main
focus of the BACnet OPC UA Information Model is actually
the data mapping and representation of BACnet objects into OPC UA data
structures. Since both use an object-oriented data model, specifying
the mapping procedure was not complicated at all.
Data representation in BACnet and OPC UA
Both OPC UA and BACnet specify objects to model
Unlike in other standards or more simple communication protocols not
only the values are provided. Additional information like the name of
an object, a description text, the engineering units, alarm limits,
operating hours or change-of-state count, etc. is provided. The BACnet
OPC UA Information Model describes a gateway interface between the two
Scope of the mapping BACnet ➞ OPC UA
BACnet defines a set of object types where all common properties are repeated for each type. Type hierarchies and inheritance are not used in BACnet.
Since OPC UA supports type hierarchies, inheritance and aggregation, these concepts are used to avoid duplicated definitions in the OPC UA representation of BACnet. The example of a BACnet analog input object type is used in this overview chapter to describe how the different concepts are used in this mapping specification.
The BACnet OPC UA Information Model overview
Figure 3 below shows the main ObjectTypes of the OPC UA for
information model and their relationships. This schematic overview is
not intended to be complete.
The boxes in this diagram show the ObjectTypes used in this specification as well as some elements from other specifications. The upper grey box shows the OPC UA core ObjectType from which the OPC UA for BACnet information model ObjectTypes are derived and some VariableTypes used in the BACnet ObjectTypes.
The grey column on the left of the second level shows the main ObjectTypes that this specification introduces. They represent corresponding BACnet object types. A type hierarchy is used whenever identical components are used in different BACnet object types.
The middle grey column in the second level shows the ObjectTypes used
for grouping BACnet features. These groupings are used in the OPC UA
for BACnet information model ObjectTypes
shown in the left grey column.
Example mapping of an Analog Input value
Figure 4below provides an example of how to map a BACnet Analog Input object type to an OPC UA ObjectType:
The left grey column shows the list of BACnet properties of the BACnet Analog Input object type.
The middle grey column shows an instance of an OPC UA BACnetAnalogInput ObjectType.
The right grey column shows the OPC UA ObjectTypes used to represent a BACnet Analog Input object type.
Most of the BACnet properties are mapped to OPC UA
Properties using the
BACnet property name as OPC UA BrowseName.
They are either Properties
of the Object directly or Properties
of the EventReporting Object.
In BACnet, physical devices are represented using the Device Object Type. Beyond the representation of property data like the device name, vendor or model name and APDU parameters, the group decided to include device management services like Device Communication Control (e.g. to silencing a device), Reinitialize Device (reboot), Time Synchronization or Object Creation and Deletion (see fig. 5).
Mapping events and alarms
Events and alarms are conditions triggering a message to
recipient or a group to be notified. In BACnet these messages are
provided through notification class objects. A mapping gateway will
likely act as a recipient for all alarms and events, transporting these
messages to OPC UA systems at all times, 24/7 (see fig. 6).
Both technologies and standards, BACnet and OPC UA,
together. The BACnet OPC UA Information Model provides a standardized
and proven architecture to bridge the gap between different
applications in industry and building automation. Both groups, i.e. the
OPC Foundation and BACnet Interest Group Europe will continue their
Outlook: what’s next?
After receiving very positive feedback from companies
the BACnet OPC UA Information Model, the working group started a
60-days public review in June 2016. Public reviews are commonly used in
the development of the BACnet standard in order to allow feedback from
parties not being able to attend the face-to-face meetings. The
comments received within the public review period will be incorporated
into the document and if no substantial issues are recognized the
BACnet OPC UA Information Model will be released as Version 1 in fall
In a later stage the group may establish further meetings to specify connecting industry processes from OPC UA to building automation management systems to allow for other use cases where BACnet is the leading system and OPC UA provides the data to be connected.
Where to get the specification
BACnet OPC UA Information Model is provided free of charge and
license-free. The specification document is available from the BACnet
Interest Group Europe website.
It is neither planned to include this profile into the
BACnet-standard, nor into the OPC UA specifications, though.
It will be available as a separate document providing a recommended architecture to interconnect these two protocols.
Contact the working group
Feedback and support is always welcome, feel free to
Matthias Damm: Matthias.firstname.lastname@example.org
Frank Schubert: email@example.com
The working group received great support from the OPC
BACnet Interest Group Europe and the ASHRAE (American Society of
Heating, Refrigerating and Air-Condition Engineers), not to forget the
companies which sent their experts as volunteers to the working group
meetings. Thanks to all supporting parties and individuals!
About the Author
Frank Schubert, Marketing and Training Building Automation at Beckhoff Automation GmbH & Co. KG in Verl, Germany, serves in numerous working groups in the BACnet community, including ASHRAE SSPC135 (aka “the BACnet committee”) and the advisory board of the BACnet Interest Group Europe. He was one of the early adopters of BACnet in Europe starting in 1997 and conducts training seminars for the German Engineers Association (VDI) and BACnet Academy since the year 2000.
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