Image Processing Reference
In-Depth Information
control (MAC) address for LANs, and may be up to  bytes in length. BACnet routers route packets
basedonthenetworknumbers.heyarerequiredtobeself-learning.Appropriateprotocolservices
for topology discovery are provided.
The BACnet network layer provides an unacknowledged connectionless datagram service. Besides
unicasts, broadcasts to any single network or to the entire internetwork are possible. Multicast sup-
port (including group assignment) is considered a local matter of the underlying network, and thus
typically not present. The application layer adds reliable transfers and optional segmentation. The
size of an application layer data unit (APDU) may range from  to  octets, depending on the
capabilitiesofthedevice.ConnectionsinvolvingmultipleAPDUsarenotpartoftheconcept.
The BACnet data model follows an object-oriented approach. Currently,  object types are
deined. hese include simple types representing binary, multistate, and analogue inputs and outputs
(physical data points) and values (virtual data points) as well as complex objects for scheduling, aver-
aging, trending, and event reporting. Besides mainly application domain agnostic objects, a “Loop
Object” representing PID controllers and objects for life safety systems are to be found as well. he
“Device Object” is mandatory for every device. It holds information on device characteristics such
as the maximum APDU length accepted and the other objects present.
As an example, Table . shows the “Binary Output” object and its properties. Some properties are
mandatory (R = read-only, W = read/write), support for others is optional (O). he properties “object-
identifier,” “object-name,” and “object-type” are mandatory for every object. The object-identifier
uniquely identifies an object within a given BACnet device; the object-identifier of the device object
is additionally required to be unique internetwork-wide. he “present-value” property is present in
every object representing a data point. The BACnet object model can be extended to include new
objects or properties in a compatible way.
The BACnet objects are manipulated via appropriate services, following a client-server model.
Currently,  services are defined, which are divided into five groups: Alarm and Event, Object Access
(property manipulation, supporting multiple priority levels), Remote Device Management, File
Access, and Virtual Terminal Services. he latter two are implemented less often today, Internet/Web
standards (HTTP/HTML, Telnet, FTP etc.) are preferred instead.
TABLE . BACnet “Binary Output” Object
Property Identifier
Property Datatype
Conformance Code
Object_Identifier
BACnetObjectIdentifier
R
Object_Name
CharacterString
R
Object_Type
BACnetObjectType
R
Present_Value
BACnetBinaryPV
W
Description
CharacterString
O
Device_Type
CharacterString
O
Status_Flags
BACnetStatusFlags
R
Event_State
BACnetEventState
R
Reliability
BACnetReliability
O
Out_Of_Service
BOOLEAN
R
Polarity
BACnetPolarity
R
Inactive_Text
CharacterString
O
Active_Text
CharacterString
O
Change_Of_State_Time
BACnetDateTime
O
Change_Of_State_Count
Unsigned
O
Time_Of_State_Count_Reset
BACnetDateTime
O
Elapsed_Active_Time
Unsigned
O
Time_Of_Active_Time_Reset
BACnetDateTime
O
Minimum_Off_Time
Unsigned
O
Minimum_On_Time
Unsigned
O
Priority_Array
BACnetPriorityArray
R
Relinquish_Default
BACnetBinaryPV
R
Time_Delay
Unsigned
O
Notification_Class
Unsigned
O
Feedback_Value
BACnetBinaryPV
O
Event_Enable
BACnetEventTransitionBits
O
Acked_Transitions
BACnetEventTransitionBits
O
Notify_Type
BACnetNotifyType
O
Event_Time_Stamps
BACnetARRAY[] of BACnetTimeStamp
O
Profile_Name
CharacterString
O
 
Search WWH ::




Custom Search