The DataAccess model extends the variable model by defining VariableTypes. The DataItemType is the base type. ArrayItemType, BaseAnalogType and DiscreteItemType are specializations. See Figure 2. Each of these VariableTypes can be further extended to form domain or server specific DataItems.

Figure 2 – DataItem VariableType hierarchy

The StatusCode also contains an informational bit called SemanticsChanged.

Servers that implement Data Access shall set this Bit in notifications if certain Property values defined in this standard change. The corresponding Properties are specified individually for each VariableType.

Clients that use any of these Properties should re-read them before they process the data value.

This VariableType defines the general characteristics of a DataItem. All other DataItem Types derive from it. The DataItemType derives from the BaseDataVariableType and therefore shares the variable model as described in OPC 10000-3 and OPC 10000-5. It is formally defined in Table 1.

Table 1 – DataItemType definition

Attribute	Value
BrowseName	DataItemType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	BaseDataType
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the BaseDataVariableType defined in OPC 10000-5; i.e the Properties of that type are inherited.
HasSubtype	VariableType	BaseAnalogType	Defined in 5.3.2.2
HasSubtype	VariableType	DiscreteItemType	Defined in 5.3.3
HasSubtype	VariableType	ArrayItemType	Defined in 5.3.4
HasProperty	Variable	Definition	String	PropertyType	Optional
HasProperty	Variable	ValuePrecision	Double	PropertyType	Optional
Conformance Units
Data Access DataItems

Definition is a vendor-specific, human readable string that specifies how the value of this DataItem is calculated. Definition is non-localized and will often contain an equation that can be parsed by certain clients.

Example: Definition::= “(TempA – 25) + TempB”

ValuePrecision specifies the maximum precision that the Server can maintain for the item based on restrictions in the target environment.

ValuePrecision can be used for the following DataTypes:

• For Float, Double, and Decimal values it specifies the number of digits after the decimal place when it is a positive number. When it is a negative number, it specifies the number of insignificant digits to the left of the decimal place.

For example a ValuePrecision of -2 specifies that the precision of the Value is to the nearest 100. The ValuePrecision should always be a whole number and it shall always be interpreted as a whole number by rounding it to the nearest whole number.

• For DateTime values it shall always be a positive number which indicates the minimum time difference in nanoseconds. For example, a ValuePrecision of 20 000 000 defines a precision of 20 ms. The ValuePrecision should always be a whole number and it shall always be interpreted as a whole number by rounding it to the nearest whole number.
• ValuePrecision can also be used for other subtypes of Double (like Duration) and other Number subtypes that can be represented by a Double.

The ValuePrecision Property is an approximation that is intended to provide guidance to a Client. A Server is expected to silently round any value with more precision that it supports. This implies that a Client may encounter cases where the value read back from a Server differs from the value that it wrote to the Server. This difference shall be no more than the difference suggested by this Property.

The algorithm for rounding should follow the so-called “Banker’s rounding” (aka Round half to even), in which numbers which are equidistant from the two nearest integers are rounded to the nearest even integer. Thus, 0.5 rounds down to 0; 1.5 rounds up to 2.

Other decimal fractions round as you would expect: 0.4 to 0, 0.6 to 1, 1.4 to 1, 1.6 to 2, etc. Only x.5 numbers get the "special" treatment.

The VariableTypes in this subclause define the characteristics of AnalogItems. The types have identical semantics and Properties but with diverging ModellingRules for individual Properties.

The Properties are only described once - in 5.3.2.2. The descriptions apply to the Properties for the other VariableTypes as well.

This VariableType is the base type for analog items. All Properties are optional. Subtypes of this base type will mandate some of the Properties. The BaseAnalogType derives from the DataItemType. It is formally defined in Table 2.

Table 2 – BaseAnalogType definition

Attribute	Value
BrowseName	BaseAnalogType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Number
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DataItemType defined in 5.3.1; i.e. the Properties of that type are inherited.
HasSubtype	VariableType	AnalogItemType	Defined in 5.3.2.3
HasSubtype	VariableType	AnalogUnitType	Defined in 5.3.2.4
HasProperty	Variable	InstrumentRange	Range	PropertyType	Optional
HasProperty	Variable	EURange	Range	PropertyType	Optional
HasProperty	Variable	EngineeringUnits	EUInformation	PropertyType	Optional
Conformance Units
Data Access BaseAnalogType

The following paragraphs describe the Properties of this VariableType. If the analog item’s Value contains an array, the Properties shall apply to all elements in the array.

InstrumentRange defines the value range that can be returned by the instrument.

Example: InstrumentRange::= {-9999.9, 9999.9}

Although defined as optional, it is strongly recommended for Servers to support this Property. Without an InstrumentRange being provided, Clients will commonly assume the full range according to the DataType.

The InstrumentRange Property may also be used to restrict a Built-in DataType such as Byte or Int16) to a smaller range of values.

Examples:

UInt4: InstrumentRange::= {0, 15}

Int6: InstrumentRange::= {-32, 31}

The Range DataType is specified in 5.6.2.

EURange defines the value range likely to be obtained in normal operation. It is intended for such use as automatically scaling a bar graph display.

Sensor or instrument failure or deactivation can result in a returned item value which is actually outside of this range. Client software must be prepared to deal with this possibility. Similarly a Client may attempt to write a value that is outside of this range back to the server. The exact behaviour (accept, reject, clamp, etc.) in this case is Server-dependent. However, in general Servers shall be prepared to handle this.

Example: EURange::= {-200.0,1400.0}

See also 7.2 for a special monitoring filter (PercentDeadband) which is based on the engineering unit range.

NOTE If EURange is not provided on an instance, the PercentDeadband filter cannot be used for that instance (see clause 7.2).

EngineeringUnits specifies the units for the DataItem’s value (e.g., DEGC, hertz, seconds). The EUInformation type is specified in 5.6.3. The NonHierarchical References HasQuantity (see 6.5.2) and HasEngineeringUnitDetail (see 6.5.1) can be used to expose further information for the unit.

It is important to note that understanding the units of a measurement value is essential for a uniform system. In an open system in particular where Servers from different cultures might be used, it is essential to know what the units of measurement are. Based on such knowledge, values can be converted if necessary before being used. Therefore, although defined as optional, support of the EngineeringUnits Property is strongly advised.

OPC UA recommends using the “Codes for Units of Measurement” (see UN/CEFACT: UNECE Recommendation N° 20). The mapping to the EngineeringUnits Property is specified in 5.6.3.

Examples for unit mixup: In 1999, the Mars Climate Orbiter crashed into the surface of Mars. The main reason was a discrepancy over the units used. The navigation software expected data in newton second; the company who built the orbiter provided data in pound-force seconds. Another, less expensive, disappointment occurs when people used to British pints order a pint in the USA, only to be served what they consider a short measure.

The StatusCode SemanticsChanged bit shall be set if any of the EURange (could change the behaviour of a Subscription if a PercentDeadband filter is used) or EngineeringUnits (could create problems if the Client uses the value to perform calculations) Properties are changed (see clause 5.2 for additional information).

This VariableType requires the EURange Property. The AnalogItemType derives from the BaseAnalogType. It is formally defined in Table 3.

Table 3 – AnalogItemType definition

Attribute	Value
BrowseName	AnalogItemType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Number
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the BaseAnalogType defined in 5.3.2.2; i.e. the Properties of that type are inherited.
HasSubtype	VariableType	AnalogUnitRangeType	Defined in 5.3.2.5
HasProperty	Variable	EURange	Range	PropertyType	Mandatory
Conformance Units
Data Access AnalogItemType

This VariableType requires the EngineeringUnits Property. The AnalogUnitType derives from the BaseAnalogType. It is formally defined in Table 4.

Table 4 – AnalogUnitType definition

Attribute	Value
BrowseName	AnalogUnitType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Number
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the BaseAnalogType defined in 5.3.2.2; i.e. the Properties of that type are inherited.
HasProperty	Variable	EngineeringUnits	EUInformation	PropertyType	Mandatory
Conformance Units
Data Access AnalogUnitType

The AnalogUnitRangeType derives from the AnalogItemType and additionally requires the EngineeringUnits Property. It is formally defined in Table 5.

Table 5 – AnalogUnitRangeType definition

Attribute	Value
BrowseName	AnalogUnitRangeType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Number
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the AnalogItemType defined in 5.3.2.3; i.e. the Properties of that type are inherited.
HasProperty	Variable	EngineeringUnits	EUInformation	PropertyType	Mandatory
Conformance Units
Data Access AnalogUnitRangeType

This VariableType is an abstract type. That is, no instances of this type can exist. However, it might be used in a filter when browsing or querying. The DiscreteItemType derives from the DataItemType and therefore shares all of its characteristics. It is formally defined in Table 6.

Table 6 – DiscreteItemType definition

Attribute	Value
BrowseName	DiscreteItemType
IsAbstract	True
ValueRank	−2 (−2 = ‘Any’)
DataType	BaseDataType
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DataItemType defined in 5.2; i.e. the Properties of that type are inherited.
HasSubtype	VariableType	TwoStateDiscreteType	Defined in 5.3.3.2
HasSubtype	VariableType	MultiStateDiscreteType	Defined in 5.3.3.3
HasSubtype	VariableType	MultiStateValueDiscreteType	Defined in 5.3.3.4
Conformance Units
Data Access DiscreteItemType

This VariableType defines the general characteristics of a DiscreteItem that can have two states. The TwoStateDiscreteType derives from the DiscreteItemType. It is formally defined in Table 7.

Table 7 – TwoStateDiscreteType definition

Attribute	Value
BrowseName	TwoStateDiscreteType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Boolean
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DiscreteItemType defined in 5.3.3; i.e. the Properties of that type are inherited.
HasProperty	Variable	TrueState	LocalizedText	PropertyType	Mandatory
HasProperty	Variable	FalseState	LocalizedText	PropertyType	Mandatory
Conformance Units
Data Access TwoState

TrueState contains a string to be associated with this DataItem when it is TRUE. This is typically used for a contact when it is in the closed (non-zero) state.

for example: "RUN", "CLOSE", "ENABLE", "SAFE", etc.

FalseState contains a string to be associated with this DataItem when it is FALSE. This is typically used for a contact when it is in the open (zero) state.

for example: "STOP", "OPEN", "DISABLE", "UNSAFE", etc.

If the item contains an array, then the Properties will apply to all elements in the array.

The StatusCode SemanticsChanged bit shall be set if any of the FalseState or TrueState Properties are changed (see 5.2 for additional information).

This VariableType defines the general characteristics of a DiscreteItem that can have more than two states. The MultiStateDiscreteType derives from the DiscreteItemType. It is formally defined in Table 8.

Table 8 – MultiStateDiscreteType definition

Attribute	Value
BrowseName	MultiStateDiscreteType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	UInteger
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DiscreteItemType defined in 5.3.3; i.e. the Properties of that type are inherited.
HasProperty	Variable	EnumStrings	LocalizedText[]	PropertyType	Mandatory
Conformance Units
Data Access MultiState

EnumStrings is a string lookup table corresponding to sequential numeric values (0, 1, 2, etc.)

Example:

"OPEN"

"CLOSE"

"IN TRANSIT" etc.

Here the string "OPEN" corresponds to 0, "CLOSE" to 1 and "IN TRANSIT" to 2.

Clients should be prepared to handle item values outside of the range of the list; and robust servers should be prepared to handle writes of illegal values, by providing errorcode “Bad_OutOfRange”.

If the item contains an array then this lookup table shall apply to all elements in the array.

NOTE The EnumStrings property is also used for Enumeration DataTypes (for the specification of this DataType, see OPC 10000-3).

The StatusCode SemanticsChanged bit shall be set if the EnumStrings Property is changed (see 5.2 for additional information).

This VariableType defines the general characteristics of a DiscreteItem that can have more than two states and where the state values (the enumeration) do not consist of consecutive numeric values (may have gaps) or where the enumeration is not zero-based. The MultiStateValueDiscreteType derives from the DiscreteItemType. It is formally defined in Table 9.

Table 9 – MultiStateValueDiscreteType definition

Attribute	Value
BrowseName	MultiStateValueDiscreteType
IsAbstract	False
ValueRank	−2 (−2 = ‘Any’)
DataType	Number
References	NodeClass	BrowseName	DataType TypeDefinition	ModellingRule
Subtype of the DiscreteItemType defined in 5.3.3; i.e. the Properties of that type are inherited.
HasProperty	Variable	EnumValues	EnumValueType[]PropertyType	Mandatory
HasProperty	Variable	ValueAsText	LocalizedTextPropertyType	Mandatory
Conformance Units
Data Access MultiStateValueDiscrete

EnumValues is an array of EnumValueType. Each entry of the array represents one enumeration value with its integer notation, a human-readable representation, and help information. This represents enumerations with integers that are not zero-based or have gaps (e.g. 1, 2, 4, 8, 16). See OPC 10000-3 for the definition of this type. MultiStateValueDiscrete Variables expose the current integer notation in their Value Attribute. Clients will often read the EnumValues Property in advance and cache it to lookup a name or help whenever they receive the numeric representation.

Only DataTypes that can be represented with EnumValues are allowed for Variables of MultiStateValueDiscreteType. These are Integers up to 64 Bits (signed and unsigned).

The numeric representation of the current enumeration value is provided via the Value Attribute of the MultiStateValueDiscrete Variable. The ValueAsText Property provides the localized text representation of the enumeration value. It can be used by Clients only interested in displaying the text to subscribe to the Property instead of the Value Attribute.

The StatusCode SemanticsChanged bit shall be set if the EnumValues Property value is changed (see clause 5.2 for additional information).

This abstract VariableType defines the general characteristics of an ArrayItem. Values are exposed in an array but the content of the array represents a single entity like an image. Other DataItems might contain arrays that represent for example several values of several temperature sensors of a boiler.

ArrayItemType or its subtype shall only be used when the Title and AxisScaleType Properties can be filled with reasonable values. If this is not the case DataItemType and subtypes like AnalogItemType, which also support arrays, shall be used. The ArrayItemType is formally defined in Table 10.

Table 10 – ArrayItemType definition

Attribute	Value
BrowseName	ArrayItemType
IsAbstract	True
ValueRank	0 (0 = OneOrMoreDimensions)
DataType	BaseDataType
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the DataItemType defined in 5.3.1; i.e. the Properties of that type are inherited.
HasSubtype	VariableType	YArrayItemType	Defined in 5.3.4.2
HasSubtype	VariableType	XYArrayItemType	Defined in 5.3.4.3
HasSubtype	VariableType	ImageItemType	Defined in 5.3.4.4
HasSubtype	VariableType	CubeItemType	Defined in 5.3.4.5
HasSubtype	VariableType	NDimensionArrayItemType	Defined in 5.3.4.6
HasProperty	Variable	InstrumentRange	Range	PropertyType	Optional
HasProperty	Variable	EURange	Range	PropertyType	Mandatory
HasProperty	Variable	EngineeringUnits	EUInformation	PropertyType	Mandatory
HasProperty	Variable	Title	LocalizedText	PropertyType	Mandatory
HasProperty	Variable	AxisScaleType	AxisScaleEnumeration	PropertyType	Mandatory
Conformance Units
Data Access ArrayItem2Type

InstrumentRange defines the range of the Value of the ArrayItem.

EURange defines the value range of the ArrayItem likely to be obtained in normal operation. It is intended for such use as automatically scaling a bar graph display.

EngineeringUnits holds the information about the engineering units of the Value of the ArrayItem.

For additional information about InstrumentRange, EURange, and EngineeringUnits see the description of BaseAnalogType in 5.3.2.2.

Title holds the user readable title of the Value of the ArrayItem.

AxisScaleType defines the scale to be used for the axis where the Value of the ArrayItem shall be displayed.

The StatusCode SemanticsChanged bit shall be set if any of the InstrumentRange, EURange, EngineeringUnits or Title Properties are changed (see 5.2 for additional information).

YArrayItemType represents a single-dimensional array of numerical values used to represent spectra or distributions where the x axis intervals are constant. YArrayItemType is formally defined in Table 11.

Table 11 – YArrayItemType definition

Attribute	Value
BrowseName	YArrayItemType
IsAbstract	False
ValueRank	1
DataType	BaseDataType
ArrayDimensions	{0} (0 = UnknownSize)
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the ArrayItemType defined in 5.3.4.1
HasProperty	Variable	XAxisDefinition	AxisInformation	PropertyType	Mandatory
Conformance Units
Data Access YArrayItemType

The Value of the YArrayItem contains the numerical values for the Y-Axis. Engineering Units and Range for the Value are defined by corresponding Properties inherited from the ArrayItemType.

The DataType of this VariableType is restricted to SByte, Int16, Int32, Int64, Float, Double, ComplexNumberType and DoubleComplexNumberType.

The XAxisDefinition Property holds the information about the Engineering Units and Range for the X-Axis.

The StatusCode SemanticsChanged bit shall be set if any of the following five Properties are changed: InstrumentRange, EURange, EngineeringUnits, Title or XAxisDefinition (see 5.2 for additional information).

Figure 3 shows an example of how Attributes and Properties may be used in a graphical interface.

Figure 3 – Graphical view of a YArrayItem

Table 12 describes the values of each element presented in Figure 3.

Table 12 – YArrayItem item description

Attribute / Property	Item value
Description	Magnitude Response (dB)
axisScaleType	AxisScaleEnumeration.LINEAR
InstrumentRange.low	-90
InstrumentRange.high	5
EURange.low	-90
EURange.high	2
EngineeringUnits.namespaceUrl	http://www.opcfoundation.org/UA/units/un/cefact
EngineeringUnits.unitId	12878
EngineeringUnits.displayName	“en-us”, “dB”
EngineeringUnits.description	“en-us”, “decibel”
Title	Magnitude
XAxisDefinition.EngineeringUnits.namespaceUrl	http://www.opcfoundation.org/UA/units/un/cefact
XAxisDefinition.EngineeringUnits.unitId	4933722
XAxisDefinition.EngineeringUnits.displayName	“en-us”, “kHz”
XAxisDefinition.EngineeringUnits.description	“en-us”, “kilohertz”
XAxisDefinition.Range.low	0
XAxisDefinition.Range.high	25
XAxisDefinition.title	“en-us”, “Frequency”
XAxisDefinition.axisScaleType	AxisScaleEnumeration.LINEAR
XAxisDefinition.axisSteps	null

Interpretation notes:

• Not all elements of this table are used in the graphic.
• The X axis is displayed in reverse order, however, the XAxisDefinition.Range.low shall be lower than XAxisDefinition.Range.high. It is only a graphical representation that reverses the display order.
• There is a constant X axis

XYArrayItemType represents a vector of XVType values like a list of peaks, where XVType.x is the position of the peak and XVType.value is its intensity. XYArrayItemType is formally defined in Table 13.

Table 13 – XYArrayItemType definition

Attribute	Value
BrowseName	XYArrayItemType
IsAbstract	False
ValueRank	1
DataType	XVType
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the ArrayItemType defined in 5.3.4.1
HasProperty	Variable	XAxisDefinition	AxisInformation	PropertyType	Mandatory
Conformance Units
Data Access XYArrayItemType

The Value of the XYArrayItem contains an array of structures (XVType) where each structure specifies the position for the X-Axis (XVType.x) and the value itself (XVType.value), used for the Y-Axis. Engineering units and range for the Value are defined by corresponding Properties inherited from the ArrayItemType.

XAxisDefinition Property holds the information about the Engineering Units and Range for the X-Axis.

The axisSteps of XAxisDefinition shall be set to NULL because it is not used.

The StatusCode SemanticsChanged bit shall be set if any of the InstrumentRange, EURange, EngineeringUnits, Title or XAxisDefinition Properties are changed (see 5.2 for additional information).

ImageItemType defines the general characteristics of an ImageItem which represents a matrix of values like an image, where the pixel position is given by X which is the column and Y the row. The value is the pixel intensity.

ImageItemType is formally defined in Table 14.

Table 14 – ImageItemType definition

Attribute	Value
BrowseName	ImageItemType
IsAbstract	False
ValueRank	2 (2 = two dimensional array)
DataType	BaseDataType
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the ArrayItemType defined in 5.3.4.1
HasProperty	Variable	XAxisDefinition	AxisInformation	PropertyType	Mandatory
HasProperty	Variable	YAxisDefinition	AxisInformation	PropertyType	Mandatory
Conformance Units
Data Access ImageItemType

Engineering units and range for the Value are defined by corresponding Properties inherited from the ArrayItemType.

The DataType of this VariableType is restricted to SByte, Int16, Int32, Int64, Float, Double, ComplexNumberType and DoubleComplexNumberType.

The ArrayDimensions Attribute for Variables of this type or subtypes shall use the first entry in the array ([0]) to define the number of columns and the second entry ([1]) to define the number of rows, assuming the size of the matrix is not dynamic.

XAxisDefinition Property holds the information about the engineering units and range for the X-Axis.

YAxisDefinition Property holds the information about the engineering units and range for the Y-Axis.

The StatusCode.SemanticsChanged bit shall be set if any of the InstrumentRange, EURange, EngineeringUnits, Title, XAxisDefinition or YAxisDefinition Properties are changed.

CubeItemType represents a cube of values like a spatial particle distribution, where the particle position is given by X which is the column, Y the row and Z the depth. In the example of a spatial partical distribution, the value is the particle size. CubeItemType is formally defined in Table 15.

Table 15 – CubeItemType definition

Attribute	Value
BrowseName	CubeItemType
IsAbstract	False
ValueRank	3 (3 = three dimensional array)
DataType	BaseDataType
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the ArrayItemType defined in 5.3.4.1
HasProperty	Variable	XAxisDefinition	AxisInformation	PropertyType	Mandatory
HasProperty	Variable	YAxisDefinition	AxisInformation	PropertyType	Mandatory
HasProperty	Variable	ZAxisDefinition	AxisInformation	PropertyType	Mandatory
Conformance Units
Data Access CubeItemType

Engineering units and range for the Value are defined by corresponding Properties inherited from the ArrayItemType.

The DataType of this VariableType is restricted to SByte, Int16, Int32, Int64, Float, Double, ComplexNumberType and DoubleComplexNumberType.

The ArrayDimensions Attribute for Variables of this type or subtypes should use the first entry in the array ([0]) to define the number of columns, the second entry ([1]) to define the number of rows, and the third entry ([2]) define the number of steps in the Z axis, assuming the size of the matrix is not dynamic.

XAxisDefinition Property holds the information about the engineering units and range for the X-Axis.

YAxisDefinition Property holds the information about the engineering units and range for the Y-Axis.

ZAxisDefinition Property holds the information about the engineering units and range for the Z-Axis.

The StatusCode SemanticsChanged bit shall be set if any of the InstrumentRange, EURange, EngineeringUnits, Title, XAxisDefinition, YAxisDefinition or ZAxisDefinition Properties are changed (see 5.2 for additional information).

This VariableType defines a generic multi-dimensional ArrayItem.

This approach minimizes the number of types however it may be proved more difficult to utilize for control system interactions.

NDimensionArrayItemType is formally defined in Table 16.

Table 16 – NDimensionArrayItemType definition

Attribute	Value
BrowseName	NDimensionArrayItemType
IsAbstract	False
ValueRank	0 (0 = OneOrMoreDimensions)
DataType	BaseDataType
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the ArrayItemType defined in 5.3.4.1
HasProperty	Variable	AxisDefinition	AxisInformation []	PropertyType	Mandatory
Conformance Units
Data Access NDimensionArrayItemType

The DataType of this VariableType is restricted to SByte, Int16, Int32, Int64, Float, Double, ComplexNumberType and DoubleComplexNumberType.

AxisDefinition Property holds the information about the EngineeringUnits and Range for all axis.

The StatusCode SemanticsChanged bit shall be set if any of the InstrumentRange, EURange, EngineeringUnits, Title or AxisDefinition Properties are changed (see 5.2 for additional information).

DataItems are always defined as data components of other Nodes in the AddressSpace. They are never defined by themselves. A simple example of a container for DataItems would be a “Folder Object” but it can be an Object of any other type.

Figure 4 illustrates the basic AddressSpace model of a DataItem, in this case an AnalogItem.

Figure 4 – Representation of DataItems in the AddressSpace

Each DataItem is represented by a DataVariable with a specific set of Attributes. The TypeDefinition reference indicates the type of the DataItem (in this case the AnalogItemType). Additional characteristics of DataItems are defined using Properties. The VariableTypes in 5.2 specify which properties may exist. These Properties have been found to be useful for a wide range of Data Access clients. Servers that want to disclose similar information should use the OPC-defined Property rather than one that is vendor-specific.

The above figure shows only a subset of Attributes and Properties. Other Attributes that are defined for Variables in OPC 10000-3 (e.g., Description) may also be available.

This subclause lists the Attributes of Variables that have particular importance for Data Access. They are specified in detail in OPC 10000-3. The following Attributes are particularly important for Data Access:

• Value
• DataType
• AccessLevel
• MinimumSamplingInterval

Value is the most recent value of the Variable that the Server has. Its data type is defined by the DataType Attribute. The AccessLevel Attribute defines the Server’s basic ability to access current data and MinimumSamplingInterval defines how current the data is.

When a client requests the Value Attribute for reading or monitoring, the Server will always return a StatusCode (the quality and the Server’s ability to access/provide the value) and, optionally, a ServerTimestamp and/or a SourceTimestamp – based on the Client’s request. See OPC 10000-4 for details on StatusCode and the meaning of the two timestamps. Specific status codes for Data Access are defined in 7.3.

Following is a description of the DataTypes defined in this specification.

DataTypes like String, Boolean, Double or LocalizedText are defined in OPC 10000-3. Their representation is specified in OPC 10000-5.

This structure defines the Range for a value. Its elements are defined in Table 17.

Table 17 – Range DataType structure

Name	Type	Description
Range	structure
low	Double	Lowest value in the range.
high	Double	Highest value in the range.

NOTE For some DataTypes, e.g. Int64, UInt64, or Decimal, there may be a loss in precision in the representation of the range with a Double.

If a limit is not known a NaN shall be used.

Its representation in the AddressSpace is defined in Table 18

Table 18 – Range definition

Attribute		Value
BrowseName		Range
IsAbstract		False
References	NodeClass		BrowseName	DataType	TypeDefinition	Other
Subtype of Structure defined in OPC 10000-5.
Conformance Units
Base Info Range DataType

EUInformation contains information about the EngineeringUnits.

The intention of the OPC UA standard is not to define a set of units but a way to expose units based on existing systems. Since there is not a single worldwide set of units used in all industries, the EUInformation structure includes a separate field (the namespaceUri) to identify the system on which the exposed unit is based.

The default OPC UA mapping is based on UN/CEFACT as defined in clause 5.6.3.4, because it can be programmatically interpreted by generic OPC UA Clients. However, the EUInformation structure has been defined such that other standards bodies can incorporate their engineering unit definitions into OPC UA. If Servers use such an approach then they shall identify this standards body by using a proper URI in EUInformation.namespaceUri.

Servers can enhance EUInformation by providing the Quantity and Unit model (see 0) and referencing from EUInformation instances to the appropriate instances for quantity and unit. See Figure 5 for an example.

Figure 5 – Enhanced EUInformation example

The EUInformation elements are defined in Table 19.

Table 19 – EUInformation DataType structure

Name	Type	Description
EUInformation	structure
namespaceUri	String	Identifies the organization (company, standards organization) that defines the EUInformation.
unitId	Int32	Identifier for programmatic lookup. −1 is used if a unitId is not available.
displayName	LocalizedText	The displayName of the engineering unit is typically the abbreviation of the engineering unit, for example "h" for hour or "m/s" for meter per second.
description	LocalizedText	Contains the full name of the engineering unit such as "hour" or "meter per second".

Its representation in the AddressSpace is defined in Table 20.

Table 20 – EUInformation definition

Attribute		Value
BrowseName		EUInformation
IsAbstract		False
References	NodeClass		BrowseName	DataType	TypeDefinition	Other
Subtype of Structure defined in OPC 10000-5.
Conformance Units
Base Info EUInformation

This clause specifies how to apply the “Codes for Units of Measurement” published by the “United Nations Centre for Trade Facilitation and Electronic Business” (see https://www.unece.org/cefact/codesfortrade/codes_index.html). This recommendation establishes a single list of code elements to represent units of the International System of Units (SI Units) like units of measure for length, mass (weight), volume and other quantities and in addition covers administration, commerce, transport, science, technology, industry etc. It provides a fixed code that can be used for automated evaluation.

Table 21 contains a small excerpt of the relevant columns in the UNECE recommendation:

Table 21 – Examples from the UNECE Recommendation

Excerpt from Recommendation N°. 20, Annex 1
Common Code	Name	Symbol
C81	radian	rad
C25	milliradian	mrad
MMT	millimetre	mm
HMT	hectometre	hm
KMT	kilometre	km
KMQ	kilogram per cubic metre	kg/m3
FAH	degree Fahrenheit	°F

The UNECE recommendation in several cases defines multiple instances of the same unit (same name and symbol) for different quantities. Therefore, the relevant information for EUInformation.unitId, EUInformation.displayName, and EUInformation.description has been extracted by eliminating duplicates. This extract is available here:

http://www.opcfoundation.org/UA/EngineeringUnits/UNECE/UNECE_to_OPCUA.csv

This mapping has been generated as follows:

• The namespaceUri shall be http://www.opcfoundation.org/UA/units/un/cefact
• The Common Code (represented as an alphanumeric variable length of up to 3 characters) has been converted into a 32 Bit Integer and is used for the unitId. The following pseudo code specifies the conversion algorithm:

Int32 unitId = 0; Int32 c; for (i=0; i<=3;i++) {

c = CommonCode[i]; if (c == 0) break; // end of Common Code unitId = unitId << 8; unitId = unitId | c;}

• The Symbol field shall be used as invariant locale for displayName. Servers may configure multiple additional locales for each displayName. However, if none of the LocaleIds specified by the Client for the Session matches these additional locales, the Server shall return the invariant locale.
• The Name field shall be used as invariant locale for description. Servers may configure multiple additional locales for each description. However, if none of the LocaleIds specified by the Client for the Session matches these additional locales, the Server shall return the invariant locale.

This structure defines float IEEE 32 bits complex value. Its elements are defined in Table 22.

Table 22 – ComplexNumberType DataType structure

Name	Type	Description
ComplexNumberType	structure
real	Float	Value real part
imaginary	Float	Value imaginary part

Its representation in the AddressSpace is defined in Table 23

Table 23 – ComplexNumberType definition

Attribute		Value
BrowseName		ComplexNumberType
IsAbstract		False
References	NodeClass		BrowseName	DataType	TypeDefinition	Other
Subtype of Structure defined in OPC 10000-5.
Conformance Units
Data Access Complex Number

This structure defines double IEEE 64 bits complex value. Its elements are defined in Table 24.

Table 24 – DoubleComplexNumberType DataType structure

Name	Type	Description
DoubleComplexNumberType	structure
real	Double	Value real part
imaginary	Double	Value imaginary part

Its representation in the AddressSpace is defined in Table 25.

Table 25 – DoubleComplexNumberType definition

Attribute		Value
BrowseName		DoubleComplexNumberType
IsAbstract		False
References	NodeClass		BrowseName	DataType	TypeDefinition	Other
Subtype of Structure defined in OPC 10000-5.
Conformance Units
Data Access DoubleComplex Number

This structure defines the information for auxiliary axis for ArrayItemType Variables.

There are three typical uses of this structure:

1. The step between points is constant and can be predicted using the range information and the number of points. In this case, axisSteps can be set to NULL.
2. The step between points is not constant, but remains the same for a long period of time (from acquisition to acquisition for example). In this case, axisSteps contains the value of each step on the axis.
3. The step between points is not constant and changes at every update. In this case, a type like XYArrayType shall be used and axisSteps is set to NULL.

Its elements are defined in Table 26.

Table 26 – AxisInformation DataType structure

Name	Type	Description
AxisInformation	structure
engineeringUnits	EUInformation	Holds the information about the engineering units for a given axis.
eURange	Range	Limits of the range of the axis
title	LocalizedText	User readable axis title, useful when the units are %, the Title may be “Particle size distribution”
axisScaleType	AxisScaleEnumeration	LINEAR, LOG, LN, defined by AxisSteps
axisSteps	Double[]	Specific value of each axis steps, may be set to “Null” if not used

Its representation in the AddressSpace is defined in Table 27.

Table 27 – AxisInformation definition

Attribute		Value
BrowseName		AxisInformation
IsAbstract		False
References	NodeClass		BrowseName	DataType	TypeDefinition	Other
Subtype of Structure defined in OPC 10000-5.
Conformance Units
Data Access AxisInformationType

When the steps in the axis are constant, axisSteps may be set to “Null” and in this case, the Range limits are used to compute the steps. The number of steps in the axis comes from the parent ArrayItem.ArrayDimensions.

This enumeration identifies on which type of axis the data shall be displayed. Its values are defined in Table 28.

Table 28 – AxisScaleEnumeration values

Name	Value	Description
LINEAR	0	Linear scale
LOG	1	Log base 10 scale
LN	2	Log base e scale

Its representation in the AddressSpace is defined in Table 29.

Table 29 – AxisScaleEnumeration definition

Attribute	Value
BrowseName	AxisScaleEnumeration
IsAbstract	False
References	NodeClass	BrowseName	DataType	TypeDefinition	ModellingRule
Subtype of the Enumeration type defined in OPC 10000-5
HasProperty	Variable	EnumStrings	LocalizedText[]	PropertyType
Conformance Units
Data Access ArrayItem2Type

This structure defines a physical value relative to a X axis and it is used as the DataType of the Value of XYArrayItemType. For details see 5.3.4.3.

Many devices can produce values that can perfectly be represented with a float IEEE 32 bits but, they can position them on the X axis with an accuracy that requires double IEEE 64 bits. For example, the peak value in an absorbance spectrum where the amplitude of the peak can be represented by a float IEEE 32 bits, but its frequency position required 10 digits which implies the use of a double IEEE 64 bits.

Its elements are defined in Table 30.

Table 30 – XVType DataType structure

Name	Type	Description
XVType	structure
x	Double	Position on the X axis of this value
value	Float	The value itself

Its representation in the AddressSpace is defined in Table 31.

Table 31 – XVType definition

Attribute		Value
BrowseName		XVType
IsAbstract		False
References	NodeClass		BrowseName	DataType	TypeDefinition	Other
Subtype of Structure defined in OPC 10000-5.
Conformance Units
Data Access XYArrayItemType