Defined terms of OPC UA specifications, types and their components defined in OPC UA specifications and in this specification are highlighted with italic in this document.

BACnet related terms and names are always used together with BACnet.

In building automation systems, multiple entities may control and manipulate an object. There may be a need to arbitrate between a schedule program, manual operation by the facility manager, and from the service technician. For so-called commandable properties (like the present value of output object types), BACnet uses a 16-level prioritization schema where 1 is the highest priority (Manual Life Safety) and 16 is the lowest. The value in the highest priority slot takes precedence over all others. A default value is taken from the property “Relinquish_Default” in case all slots in the priority array are uninitialized (NULL).

Change-of-Value is used to inform client processes about changes of values spontaneously without the need for polling. For analog data a hysteresis (COV-increment) is provided as a special property to allow reducing the traffic and only notify relevant changes.

Note 1 to entry: The mapping to OPC UA concepts is described in Annex B.

The software that implements the mapping between BACnet and OPC UA defined in this specification.

A&EAlarms & Events

ANSIAmerican National Standards Institute

APIApplication Program Interface

ARCNETAttached Resource Computer NETwork

ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers

ASN.1Abstract Syntax Notation One (ISO 8824)

BACnetBuilding Automation and Control networks

COVChange of Value

DAData Access

HDAHistorical Data Access

HMI Human-Machine Interface

IECInternational Electrotechnical Commission

IPInternet Protocol - RFC 791

ISOInternational Organization for Standardization

LANLocal Area Network

MESManufacturing Execution System

NaN"Not a Number", a unique binary pattern representing an invalid number

(see ANSI/IEEE 754-1985)

NATNetwork Address Translation - RFC 2663

PICS Protocol Implementation Conformance Statement

UA Unified Architecture

UTCUniversal Time Coordinated

XMLExtensible Markup Language

Node definitions are specified using tables (See Table 1)

Table 1 – Type Definition Table

Attribute		Value
Attribute name		Attribute value. If it is an optional Attribute that is not set “--“ will be used.
References	NodeClass		BrowseName	DataType	TypeDefinition	Other
ReferenceType name	NodeClass of the TargetNode.		BrowseName of the target Node. If the Reference is to be instantiated by the server, then the value of the target Node’s BrowseName is “--“.	Attributes of the referenced Node, only applicable for Variables and Objects.		Referenced Other of the referenced Object.
Notes – Notes referencing footnotes of the table content.

Attributes are defined by providing the Attribute name and a value, or a description of the value.

References are defined by providing the ReferenceType name, the BrowseName of the TargetNode and its NodeClass.

• If the TargetNode is a component of the Node being defined in the table the Attributes of the composed Node are defined in the same row of the table. That implies that the referenced Node has a HasModelParent Reference with the Node defined in the Table as TargetNode (see OPC 10000-3 for the definition of ModelParents).
• The DataType is only specified for Variables; “[<number>]” indicates a single-dimensional array, for multi-dimensional arrays the expression is repeated for each dimension (e.g. [2][3] for a two-dimensional array). For all arrays the ArrayDimensions is set as identified by <number> values. If no <number> is set, the corresponding dimension is set to 0, indicating an unknown size. If no number is provided at all the ArrayDimensions can be omitted. If no brackets are provided, it identifies a scalar DataType and the ValueRank is set to the corresponding value (see OPC 10000-3). In addition, ArrayDimensions is set to null or is omitted. If it can be Any or ScalarOrOneDimension, the value is put into “{<value>}”, so either “{Any}” or “{ScalarOrOneDimension}” and the ValueRank is set to the corresponding value (see OPC 10000-3) and the ArrayDimensions is set to null or is omitted. In Table 2 examples are given.

Table 2 – Examples of DataTypes

Notation	DataType	ValueRank	ArrayDimensions	Description
Int32	Int32	-1	omitted or NULL	A scalar Int32
Int32[]	Int32	1	omitted or {0}	Single-dimensional array of Int32 with an unknown size
Int32[][]	Int32	2	omitted or {0,0}	Two-dimensional array of Int32 with unknown sizes for both dimensions
Int32[3][]	Int32	2	{3,0}	Two-dimensional array of Int32 with a size of 3 for the first dimension and an unknown size for the second dimension
Int32[5][3]	Int32	2	{5,3}	Two-dimensional array of Int32 with a size of 5 for the first dimension and a size of 3 for the second dimension
Int32{Any}	Int32	-2	omitted or NULL	An Int32 where it is unknown if it is scalar or array with any number of dimensions
Int32{ScalarOrOneDimension}	Int32	-3	omitted or NULL	An Int32 where it is either a single-dimensional array or a scalar

• The TypeDefinition is specified for Objects and Variables.
• The TypeDefinition column specifies a NodeId of a TypeDefinitionNode, i.e. the specified Node points with a HasTypeDefinition Reference to the corresponding TypeDefinitionNode. The symbolic name of the NodeId is used in the table.
• The Other of the referenced component is provided by specifying the symbolic name of the rule in the Other column. In the AddressSpace, the Node shall use a HasOther Reference to point to the corresponding Other Object.

If the NodeId of a DataType is provided, the symbolic name of the Node representing the DataType shall be used.

Nodes of all other NodeClasses cannot be defined in the same table; therefore only the used ReferenceType, their NodeClass and their BrowseName are specified. A reference to another of this document points to their definition.

If no components are provided, the DataType, TypeDefinition and Other columns may be omitted and only a Comment column is introduced to point to the Node definition.

Components of Nodes can be complex, i.e. containing components by themselves. The TypeDefinition, NodeClass, DataType and Other can be derived from the type definitions, and the symbolic name can be created as defined in 3.5.2.1. Therefore those containing components are not explicitly specified; they are implicitly specified by the type definitions.

The NodeIds of all Nodes described in this document are only symbolic names. Annex A defines the actual NodeIds.

The symbolic name of each Node defined in this document is its BrowseName, or, when it is part of another Node, the BrowseName of the other Node, a “.”, and the BrowseName of itself. In this case “part of” means that the whole has a 0:HasProperty or HasComponent Reference to its part. Since all Nodes not being part of another Node have a unique name in this document, the symbolic name is unique.

The namespace for this specification is defined in Annex A. The NamespaceIndex for all NodeIds defined in this specification is server specific and depends on the position of the namespace URI in the server namespace table.

Note: This specification does not only define concrete Nodes, but also requires that some Nodes have to be generated, for example one for each device type available in the frame application. The NodeIds of those Nodes are server-specific, including the Namespace. But the NamespaceIndex of those Nodes cannot be the NamespaceIndex used for the Nodes defined by this specification, because they are not defined by this specification but generated by the Server.

The text part of the BrowseNames for all Nodes defined in this specification is specified in the tables defining the Nodes. The NamespaceIndex for all BrowseNames defined in this specification is server specific and depends on the position of the namespace URI defined in this specification in the server namespace table.

If the BrowseName is not defined by this specification, a namespace index prefix like ‘0:EngineeringUnits’ is added to the BrowseName. This is typically necessary if a Property of another specification is overwritten or used in the OPC UA types defined in this specification. Table 260 provides a list of namespaces used in this specification.

For all Nodes specified in this specification, the Attributes named in Table 3 shall be set as specified in the table.

Table 3 – Common Node Attributes

Attribute	Value
DisplayName	The DisplayName is a LocalizedText. Each server shall provide the DisplayName identical to the BrowseName of the Node for the LocaleId “en”. Whether the server provides translated names for other LocaleIds is vendor specific.
Description	Optionally a vendor specific description is provided
NodeClass	Shall reflect the NodeClass of the Node
NodeId	The NodeId is described by BrowseNames as defined in 3.5.2.1 and defined in Annex A.
WriteMask	Optionally the WriteMask Attribute can be provided. If the WriteMask Attribute is provided, it shall set all Attributes to not writeable that are not said to be vendor-specific. For example, the Description Attribute may be set to writeable since a Server may provide a server-specific description for the Node. The NodeId shall not be writeable, because it is defined for each Node in this specification.
UserWriteMask	Optionally the UserWriteMask Attribute can be provided. The same rules as for the WriteMask Attribute apply.

For all Objects specified in this specification, the Attributes named in Table 4 shall be set as specified in the table.

Table 4 – Common Object Attributes

Attribute	Value
EventNotifier	Whether the Node can be used to subscribe to Events or not is vendor specific

For all Variables specified in this specification, the Attributes named in Table 5 shall be set as specified in the table.

Table 5 – Common Variable Attributes

Attribute	Value
MinimumSamplingInterval	Optionally, a vendor-specific minimum sampling interval is provided
AccessLevel	The access level for Variables used for type definitions is vendor-specific, for all other Variables defined in this part, the access level shall allow a current read; other settings are vendor specific.
UserAccessLevel	The value for the UserAccessLevel Attribute is vendor-specific. It is assumed that all Variables can be accessed by at least one user.
Value	For Variables used as InstanceDeclarations, the value is vendor-specific; otherwise it shall represent the value described in the text.
ArrayDimensions	If the ValueRank does not identify an array of a specific dimension (i.e. ValueRank <= 0) the ArrayDimensions can either be set to null or the Attribute is missing. This behaviour is vendor-specific. If the ValueRank specifies an array of a specific dimension (i.e. ValueRank > 0) then the ArrayDimensions Attribute shall be specified in the table defining the Variable.

For all VariableTypes specified in this specification, the Attributes named in Table 6 shall be set as specified in the table.

Table 6 – Common VariableType Attributes

Attributes	Value
Value	Optionally a vendor-specific default value can be provided
ArrayDimensions	If the ValueRank does not identify an array of a specific dimension (i.e. ValueRank <= 0) the ArrayDimensions can either be set to null or the Attribute is missing. This behaviour is vendor-specific. If the ValueRank specifies an array of a specific dimension (i.e. ValueRank > 0) then the ArrayDimensions Attribute shall be specified in the table defining the VariableType.