ASHRAE Journal - November 2013 - B11

The BACnet committee developed the WriteGroup service
to change properties in objects in multiple devices at the same
time. To do this, the WriteGroup is broadcast in a way that allows many devices to hear the same request simultaneously.
For this to make sense and not cause chaos, the WriteGroup
service includes a group number parameter that restricts the
effect to only those receiving devices that are members of that
group. The WriteGroup service further restricts the targets for
writing to those Channel objects within those devices that are
associated with the speciﬁed channel number(s).
In Figure 3, the writing device is writing a value to Group 27,
Channel 11. Devices 1 and 2 receive the write request. Device
2's Channel object is Group 14, Channel 11 so it does not execute the write, whereas Device 1 does execute the write.
Facilities management personnel beneﬁt from a BACnetbased lighting control system as they can operate this system and
other BACnet-based systems, such as HVAC, through a common
front end. At the device level, a BACnet-based VAV box in a
room can read the state of the occupancy sensors provided by
the BACnet-based lighting system. The VAV box can reduce the
air ﬂow to the space when the space is considered to be unoccupied to achieve part load occupancy savings needed by high
performance buildings. Separate HVAC occupancy sensors are
not needed as the sensor data can be shared through BACnet.

Writing Device

WriteGroup Group 27, Channel 11

Received by Both Devices...

Channel Object Group 27,
Channel 11

Object
1

Object
2

Object
n

Device 1

Channel Object Group 14,
Channel 11

Object
1

Object
2

Object
n

Device 2

Only Executed by Device 1

Figure 3: Control Groups limit WriteGroup effect to specific
Channel objects across many devices. (Source: Figure 12-11 from

SGPC 135-2012).

New Device Profiles
BACnet has had device proﬁles for some time. These device
proﬁles set the minimum BIBB support requirements and are
useful in specifying project requirements.
The major change in device proﬁles was to separate the original BACnet Operator Work Station (B-OWS) into three device
proﬁles called the B-AWS (Advanced Workstation), B-OWS
(Operator Workstation) and the B-OD (Operator Display). This
was done because the BACnet community wanted different devices that met speciﬁc BIBBs rather than having to support all
operator-related BIBBs in one device.
The B-AWS supports all the B-OWS BIBBs, plus the following:
* Event Log BIBB,
* Advanced View Modify BIBBs for data sharing, scheduling, trending and scheduling that allows writing to other object
properties not supported in the B-OWS, and
* Automatic Network Mapping, Device Mapping, Object
Create and Delete and other device management BIBBs.
The B-OD is a device that is an LCD screen usually attached
to BAS panel in a mechanical room. The B-OD supports the
ability to read and write data so the user does not have to carry
a laptop around and connect to a BAS panel.

Conclusion
Since its release in 1995, ASHRAE BACnet Standard 135 has
undergone substantial enhancements. The standard had 538 pages in 1995. The 2012 version has 1,039 pages. The standard was
republished with updates every four years. Now, the standard is
on a two-year publication cycle. More new BACnet objects and
services will be in the 2014 version of the standard. Stay tuned!
N ovem ber 2013

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B11

ASHRAE Journal - November 2013

Table of Contents for the Digital Edition of ASHRAE Journal - November 2013