L-DALI User Manual
133
LOYTEC
Version 5.2
LOYTEC electronics GmbH
6.6 BACnet Technology
6.6.1 BACnet Data Points
Data points in the BACnet technology are known as BACnet objects. They have a specific
type (e.g. analog input or binary output) and a set of properties, which describe the data
point more closely. The actual value is stored in the “Present_Value”.
On the device, there exist two classes of BACnet data points:
BACnet server objects
(SO): These BACnet objects configured by the Configurator
software to be allocated
locally
on the device. These objects can be accessed by the
BACnet building control system or operating workstations. They support COV
subscriptions to deliver value changes in an event-driven way. Local server objects can
be created as AI, AO, AV, BI, BO, BV, MI, MO, MV, Accumulator, Pulse_Converter,
Loop objects.
BACnet client mappings
(CM): For certain applications, it is necessary that the device
acts as a BACnet client. This functionality can be configured by activating a
client
mapping
. Client mappings can be of the type
Poll
,
COV
,
Write
,
Auto
or
Value
. This
specifies how the BACnet client accesses other BACnet objects on the BACnet
network. The
Auto
method determines the best way (poll, COV, or write) to talk with
other server objects.
Poll
is used for objects that need to read data from other BACnet
objects in a periodic manner.
COV
is used to subscribe for COV at other BACnet
objects in order to get updates in an event-driven fashion.
Write
is used to send updates
to other BACnet objects.
Value
refers to a combined read and write client mapping.
When writing a value to this client mapping, the value is written to the remote BACnet
object. As soon as the Present_Value of the remote BACnet object is updated, the value
is transferred back.
The direction of BACnet server objects deserves a closer look. The direction specified for
data points in the Configurator software always refers to the network view of the
communication. The definition of input and output objects in BACnet, however, refers to
the process view, which is opposite to the network. Therefore, a BACnet analog input (AI)
object is modeled as an analog output data point. The direction of client mappings naturally
refers to the network communication. Therefore, a write client mapping is represented as an
analog output data point.
In BACnet commandable objects can be written with values at a certain priority. The value
with the highest priority is in effect. When revoking a written value, the NULL value is
written. This takes back the value. When all written values are withdrawn, the
Relinquish_Default value is in effect.
For BACnet server objects the write priority defines which priority is written to the
commandable server object. It is possible to create additional
priority write
data points,
which can be configured to write at other priority levels. This may be necessary, if two parts
of an application are required to write with two different priorities. To know, which priority
slots are used in a commandable object, additional
priority read
data points can be added.
They reflect the value on a given priority slot.
The default value feature of a data point is mapped to the Relinquish_Default property for
commandable objects. For BACnet objects, which are not commandable, the Present_Value
is initialized with the specified default value.
6.6.2 BACnet Alarming
BACnet alarming on the device is based on the
intrinsic reporting
mechanism. Currently,
algorithmic reporting is not supported. Alarm conditions can only be applied to data points,
which map to BACnet server objects. If defined, the intrinsic reporting properties of the
