XML DataType Mapping

The goal of the mapping is to provide a lossless, bi-directional mapping of built-in data types from OPC UA to XML and vice versa.

In some cases, it is possible to map an OPC UA DataType to an XML data type directly (see 5.2). In most cases, this is a lossless mapping, sometimes there is a loss of precision or information.
In other cases, there is an OPC UA DataType without a corresponding XML data type. For those cases, the XML Schema defined in OPC 10000-6, Annex D, provides appropriate XML data types that can be used. The mapping to those XML data types is defined in 5.3.
Finally, there are cases where there is an XML data type without a corresponding OPC UA DataType. For those cases, an OPC UA Information Model is defined (see 6). The mapping is defined in 5.4.

5.2 Direct Mapping of XML and OPC UA DataTypes

In Table 12, the direct mapping of XML data types and OPC UA DataTypes is defined. If there are any restrictions on a lossless, bidirectional mapping, this is described in the Comment column. If this column is empty, there are no restrictions.

Table 12 – Mapping of DataTypes

OPC UA DataType	XML DataType	Comment
String	string	Implementations might limit the maximum String size.
Boolean	boolean
Decimal	decimal	Implementations might limit the maximum Decimal size.
Float	float
Double	double
Duration	duration	In this mapping there might be a loss in precision (XML only requires down to minimum fractional second precision of milliseconds, OPC UA based Duration is a double (representing milliseconds).
DateTime	dateTime	Many implementations will not be able to represent the full range of DateTime values. To maximize interoperability, the following rules apply: When going from XML to an implementation: If the XML value is <= MinDate then the value is MinDate; If the XML value is >= MaxDate then the value is MaxDate; XML values without a TimezoneOffset are assumed to be in UTC; XML values with a TimezoneOffset are converted to UTC; Fractions of seconds not supported by the implementation are dropped. When going from an implementation to XML: If the value is MinDate the XML is ‘0001-01-01T00:00:00Z’; If the value is MaxDate the XML is ‘9999-12-31T23:59:59Z’; XML values are always in UTC; Fractions of seconds, if supported by the implementation, are included. Where: MinDate is the earliest date supported by an implementation; MaxDate is the latest date supported by an implementation; All implementations are required to support seconds. The value of MinDate is the default value and semantically equivalent to a Null value.
UtcTime	dateTimeTimestamp	See limitations of DateTime
ByteString	base64Binary	XML has two ByteString representations – hexBinary and base64Binary. The mapping of OPC UA ByteString to XML base64Binary is defined in OPC 10000-6. The mapping to XML hexBinary is defined in section 5.4. Implementations might limit the maximum ByteString size.
Int64	long
Int32	int
Int16	short
SByte	byte
UInt64	unsignedLong
UInt32	unsignedInt
UInt16	unsignedShort
Byte	unsignedByte
NormalizedString	normalizedString	Implementations might limit the maximum String size. In OPC UA, limitations on the string are only defined in the specification but not explicitly defined by the data structure, so implementations not following the specification may provide invalid values.
LocaleId	language	Both data types are based on strings defined in IETF RFC 5646 and should represent the same format, e.g., “en-US”. OPC UA allows an empty or null string, which is not allowed in XML. In OPC UA, limitations on the string are only defined in the specification but not explicitly defined by the data structure, so implementations not following the specification may provide invalid values. In XML, the schema definition only restricts the allowed values to a certain degree, so implementations not following the specification may provide invalid values. XML only restricts the allowed values to a certain degree in its schema definition, so potentially values could be invalid.
UriString	anyURI	Implementations might limit the maximum String size.

5.3 New XML data types for built-in OPC UA DataTypes

In Table 13, the mapping of built-in OPC UA DataTypes to XML data types is defined, where there is no corresponding built-in XML data type available. An XML Schema defined in OPC 10000-6, Annex D, provides the appropriate XML data types for this mapping.

Table 13 – New XML DataType

OPC UA DataType	XML DataType	Comment
DataValue	DataValue	XML data type defined in OPC 10000-6, Annex D
DiagnosticInfo	DiagnosticInfo	XML data type defined in OPC 10000-6, Annex D
ExpandedNodeId	ExpandedNodeId	XML data type defined in OPC 10000-6, Annex D Contains ServerIndex and potentially NamespaceIndex (both Server-specific)
Guid	Guid	XML data type defined in OPC 10000-6, Annex D Does not check specific GUID-format in XML
LocalizedText	LocalizedText	XML data type defined in OPC 10000-6, Annex D
NodeId	NodeId	XML data type defined in OPC 10000-6, Annex D Contains NamespaceIndex (Server-specific)
QualifiedName	QualifiedName	XML data type defined in OPC 10000-6, Annex D Contains NamespaceIndex (Server-specific)
StatusCode	StatusCode	XML data type defined in OPC 10000-6, Annex D
XmlElement	XmlElement	XML data type defined in OPC 10000-6, Annex D

5.4 New OPC UA DataTypes for built-in XML data types

In Table 14, the mapping of built-in XML data types to OPC UA DataTypes is defined, where there is no corresponding built-in OPC UA DataType available. Those OPC UA DataTypes are defined in 6. In some cases, the range is smaller than possible in the XML data type.

Table 14 – New OPC UA DataTypes

OPC UA DataType	XML DataType	Comment
XmlQName	QName	Contains XML document specific prefix Implementations might limit the maximum String size.
Array of XmlQName	NOTATION	Contains XML document specific prefixes Implementations might limit the maximum String size.
XmlToken	token	Implementations might limit the maximum String size.
XmlNmToken	NMTOKEN	Implementations might limit the maximum String size.
Array of XmlNmToken	NMTOKENS	Implementations might limit the maximum array length.
XmlName	Name	Implementations might limit the maximum String size.
XmlNcName	NCName	Implementations might limit the maximum String size.
XmlId	ID	Implementations might limit the maximum String size.
XmlIdRef	IDREF	Implementations might limit the maximum String size.
Array of XmlIdRef	IDREFS	Implementations might limit the maximum array length.
XmlEntity	ENTITY	Implementations might limit the maximum String size.
Array of XmlEntity	ENTITIES	Implementations might limit the maximum array length.
XmlInteger	integer	XmlInteger is a subtype of Int64 with a limited max. and min. value, whereas the size of integer is unlimited.
XmlNonPositiveInteger	nonPositiveInteger	The min. of XmlNonPositiveInteger is limited to min. of Int64, whereas the min. of nonPositiveInteger is unlimited.
XmlNegativeInteger	negativeInteger	The min. of XmlNegativeInteger is limited to min. of Int64, whereas the min. of negativeInteger is unlimited.
XmlNonNegativeInteger	nonNegativeInteger	The max. of XmlNonNegativeInteger is limited to max. of UInt64, whereas the max. of nonNegativeInteger is unlimited.
XmlPositiveInteger	positiveInteger	The max. of XmlPositiveInteger is limited to max. of UInt64, whereas the max. of positiveInteger is unlimited.
XmlHexBinary	hexBinary	Implementations might limit the maximum ByteString size.
XmlYearMonthDuration	yearMonthDuration	Implementations might limit the maximum String size.
XmlGYearMonth	gYearMonth	Implementations might limit the maximum String size.
XmlDayTimeDuration	dayTimeDuration	Implementations might limit the maximum String size.
XmlGYear	gYear	Implementations might limit the maximum String size.
XmlGMonth	gMonth	Implementations might limit the maximum String size.
XmlGDay	gDay	Implementations might limit the maximum String size.
XmlGMonthDay	gMonthDay	Implementations might limit the maximum String size.
XmlTime	time	Implementations might limit the maximum String size.
XmlDate	date	Implementations might limit the maximum String size.

5.5 Handling of Errors

If the concrete mapping of a data type does not specify more specific error handling rules, the following shall apply:

Return an error when the type conversion fails because of limitations in the destination range. In an OPC UA context, the StatusCode Bad_OutOfRange should be returned (e.g. in the Value of a Variable).
Return an error when an incorrect syntax is encountered. In an OPC UA context, the StatusCode Bad_SyntaxError should be returned (e.g. in the Value of a Variable).”