Any value for a Built-In type that is NULL shall be encoded as the JSON literal ‘null’ if the value is an element of an array. If the NULL value is for a field within a Structure or Union, the field shall not be encoded.
Note that JSON objects are unordered sets of name-value pairs. When a Built-In type is encoded as a JSON object, the order of fields specified is not preserved.
A Boolean value shall be encoded as the JSON literal ‘true’ or ‘false’.
Integer values other than Int64 and UInt64 shall be encoded as a JSON number.
Int64 and UInt64 values shall be formatted as a decimal number encoded as a JSON string
(See the XML encoding of 64-bit values described in 5.3.1.3).
Normal Float and Double values shall be encoded as a JSON number.
Special floating-point numbers such as positive infinity (INF), negative infinity (-INF) and not-a-number (NaN) shall be represented by the values “Infinity”, “-Infinity” and “NaN” encoded as a JSON string. See 5.2.2.3 for more information on the different types of special floating-point numbers.
String values shall be encoded as JSON strings.
Any characters which are not allowed in JSON strings are escaped using the rules defined in RFC 8259.
Strings with embedded nulls (‘\u0000’) are not guaranteed to be interoperable because not all DevelopmentPlatforms can handle Strings with embedded nulls. For this reason, embedded nulls are not recommended. Encoders may encode Strings with embedded nulls. Decoders shall read all bytes in String; however, decoders may truncate the String at the first embedded null before passing it on to the application.
DateTime values shall be formatted as specified by ISO 8601-1 and encoded as a JSON string.
DateTime values which exceed the minimum or maximum values supported on a platform shall be encoded as “0001-01-01T00:00:00Z” or “9999-12-31T23:59:59Z” respectively. During decoding, these values shall be converted to the minimum or maximum values supported on the platform.
ISO 8601-1 DateType values may specify an arbitrary number of decimal places representing fractions of seconds. Encoders shall support as many decimal places needed to represent the full range of the DateTime type on their DevelopmentPlatform. Decoders may truncate decimal places that exceed the range supported by the DateTime type on their DevelopmentPlatform.
DateTime values equal to “0001-01-01T00:00:00Z” are considered to be NULL values.
Guid values shall be formatted as described in 5.1.3 and encoded as a JSON string.
ByteString values shall be formatted as a Base64 text and encoded as a JSON string.
Any characters which are not allowed in JSON strings are escaped using the rules defined in RFC 8259.
XmlElement value shall be encoded as a String as described in 5.4.2.5.
NodeId values shall be encoded as a JSON object with the fields defined in Table 31.
The abstract NodeId structure is defined in OPC 10000-3 and has three fields Id, IdType and NamespaceIndex. The representation of these abstract fields are described in the table.
Table 31 – JSON Object Definition for a NodeId
|
Name |
Description |
|
IdType |
The IdentifierType encoded as a JSON number. Allowed values are: 0 - UInt32 identifier encoded as a JSON number. 1 - A String identifier encoded as a JSON string. 2 - A Guid identifier encoded as described in 5.4.2.7. 3 - A ByteString identifier encoded as described in 5.4.2.8. This field is omitted for UInt32 identifiers. |
|
Id |
The identifier. The value of the IdType field specifies the encoding of this field. |
|
Namespace |
For reversible encoding this field is a JSON number with the NamespaceIndex. The field is omitted if the NamespaceIndex is 0. For non-reversible encoding this field is the JSON string containing the NamespaceUri associated with the NamespaceIndex unless the NamespaceIndex is 0. If the NamespaceIndex is 0 the field is omitted. |
ExpandedNodeId values shall be encoded as a JSON object with the fields defined in Table 32.
The abstract ExpandedNodeId structure is defined in OPC 10000-4 and has five fields Identifier, IdentifierType, NamespaceIndex, NamespaceUri and ServerIndex. The representation of these abstract fields are described in the table.
Table 32 – JSON Object Definition for an ExpandedNodeId
|
Name |
Description |
|
IdType |
The IdentifierType encoded as a JSON number. Allowed values are: 0 - UInt32 Identifier encoded as a JSON number. 1 - A String Identifier encoded as a JSON string. 2 - A Guid Identifier encoded as described in 5.4.2.7. 3 - A ByteString Identifier encoded as described in 5.4.2.8. This field is omitted for UInt32 identifiers. |
|
Id |
The Identifier. The value of the IdType field specifies the encoding of this field. |
|
Namespace |
For reversible encoding this field is a JSON string with the NamespaceUri if the NamespaceUri is specified. Otherwise, it is a JSON number with the NamespaceIndex. The field is omitted if the NamespaceIndex is 0. For non-reversible encoding this field is the JSON string containing the NamespaceUri or the NamespaceUri associated with the NamespaceIndex unless the NamespaceIndex is 0 or 1. If the NamespaceIndex is 0 the field is omitted. |
|
ServerUri |
For reversible encoding this field is a JSON number with the ServerIndex. The field is omitted if the ServerIndex is 0. For non-reversible encoding this field is the JSON string containing the ServerUri associated with the ServerIndex unless the ServerIndex is 0. If the ServerIndex is 0 the field is omitted. |
StatusCode values shall be encoded as a JSON number for the reversible encoding. If the StatusCode is Good (0) it is only encoded if it is an element of a JSON array.
For the non-reversible form, StatusCode values shall be encoded as a JSON object with the fields defined in Table 33.
Table 33 – JSON Object Definition for a StatusCode
|
Name |
Description |
|
Code |
The numeric code encoded as a JSON number. The Code is omitted if the numeric code is 0 (Good). |
|
Symbol |
The string literal associated with the numeric code encoded as JSON string. e.g. 0x80AB0000 has the associated literal “Bad_InvalidArgument”. The Symbol is omitted if the numeric code is 0 (Good). If the string literal is not known to the encoder the field is omitted. |
For the non-reversible encoding, a StatusCode of Good (0) it is only encoded if it is an element JSON array.
DiagnosticInfo values shall be encoded as a JSON object with the fields shown in Table 34.
Table 34 – JSON Object Definition for a DiagnosticInfo
|
Name |
Data Type |
Description |
|
SymbolicId |
Int32 |
A symbolic name for the status code. The default value is -1. |
|
NamespaceUri |
Int32 |
A namespace that qualifies the symbolic id. The default value is -1. |
|
Locale |
Int32 |
The locale used for the localized text. The default value is -1. |
|
LocalizedText |
Int32 |
A human readable summary of the status code. The default value is -1. |
|
AdditionalInfo |
String |
Detailed application specific diagnostic information. The default value is null. |
|
InnerStatusCode |
StatusCode |
A status code provided by an underlying system. The default value is Good. |
|
InnerDiagnosticInfo |
DiagnosticInfo |
Diagnostic info associated with the inner status code. The default value is null. |
Each field is encoded using the rules defined for the built-in type specified in the Data Type column. Fields should be omitted if the value is null or the default value for the field. If a field is not present the value shall be interpreted as the default value.
The SymbolicId, NamespaceUri, Locale and LocalizedText fields are encoded as JSON numbers which reference the StringTable contained in the ResponseHeader.
QualifiedName values shall be encoded as a JSON object with the fields shown in Table 35.
The abstract QualifiedName structure is defined in OPC 10000-3 and has two fields Name and NamespaceIndex. The NamespaceIndex is represented by the Uri field in the JSON object.
Table 35 – JSON Object Definition for a QualifiedName
|
Name |
Description |
|
Name |
The Name component of the QualifiedName. |
|
Uri |
For reversible encoding this field is a JSON number with the NamespaceIndex. The field is omitted if the NamespaceIndex is 0. For non-reversible encoding this field is the JSON string containing the NamespaceUri associated with the NamespaceIndex unless the NamespaceIndex is 0. If the NamespaceIndex is 0 the field is omitted. |
LocalizedText values shall be encoded as a JSON object with the fields shown in Table 36.
The abstract LocalizedText structure is defined in OPC 10000-3 and has two fields Text and Locale.
Table 36 – JSON Object Definition for a LocalizedText
|
Name |
Description |
|
Locale |
The Locale portion of LocalizedText values shall be encoded as a JSON string |
|
Text |
The Text portion of LocalizedText values shall be encoded as a JSON string. |
For the non-reversible form, LocalizedText value shall be encoded as a JSON string containing the Text component.
ExtensionObject values shall be encoded as a JSON object with the fields shown in Table 37.
Table 37 – JSON Object Definition for an ExtensionObject
|
Name |
Description |
|
TypeId |
The NodeId of a DataTypeEncoding Node or a DataType Node formatted using the rules in 5.4.2.10. |
|
Encoding |
The format of the Body field encoded as a JSON number. This value is 0 if the body is Structure encoded as a JSON object (see 5.4.6). This value is 1 if the body is a ByteString value encoded as a JSON string (see 5.4.2.8). This value is 2 if the body is a XmlElement value encoded as a JSON string (see 5.4.2.9). This field is omitted if the value is 0. |
|
Body |
Body of the ExtensionObject. The type of this field is specified by the Encoding field. If the Body is empty, the ExtensionObject is NULL and is omitted or encoded as a JSON null. |
For the non-reversible form, ExtensionObject values shall be encoded as a JSON value containing only the value of the Body field. The TypeId and Encoding fields are dropped.
Variant values shall be encoded as a JSON object with the fields shown in Table 38.
Table 38 – JSON Object Definition for a Variant
|
Name |
Description |
|
Type |
The Built-in type for the value contained in the Body (see Table 1) encoded as JSON number. If type is 0 (NULL) the Variant contains a NULL value and the containing JSON object shall be omitted or replaced by the JSON literal ‘null’ (when an element of a JSON array). |
|
Body |
If the value is a scalar, it is encoded using the rules for type specified for the Type. If the value is a one-dimensional array it is encoded as JSON array (see 5.4.5). Multi-dimensional arrays are encoded as a one-dimensional JSON array which is reconstructed using the value of the Dimensions field (see 5.2.2.16). |
|
Dimensions |
The dimensions of the array encoded as a JSON array of JSON numbers. The Dimensions are omitted for scalar and one-dimensional array values. |
For the non-reversible form, Variant values shall be encoded as a JSON value containing only the value of the Body field. The Type and Dimensions fields are dropped. Multi-dimensional arrays are encoded as a multi-dimensional JSON array as described in 5.4.5.
DataValue values shall be encoded as a JSON object with the fields shown in Table 39.
Table 39 – JSON Object Definition for a DataValue
|
Name |
Data Type |
Description |
|
Value |
Variant |
The value. |
|
Status |
StatusCode |
The status associated with the value. |
|
SourceTimestamp |
DateTime |
The source timestamp associated with the value. |
|
SourcePicoseconds |
UInt16 |
The number of 10 Picosecond intervals for the SourceTimestamp. |
|
ServerTimestamp |
DateTime |
The Server timestamp associated with the value. |
|
ServerPicoseconds |
UInt16 |
The number of 10 Picosecond intervals for the ServerTimestamp. |
If a field has a null or DefaultValue it is omitted. Each field is encoded using the rules defined for the built-in type specified in the Data Type column.