POWERLINK Objects and their attributes – OPC UA for POWERLINK

5.2.1 POWERLINK Objects and their attributes

One of the very significant differences between POWERLINK and OPC UA is that OPC UA provides metadata to each object directly through the Server, whereas POWERLINK can transport metadata only through XDD files or specification documents. Table 9 and Table 10 show examples for object definitions in the POWERLINK Communication Profile EPSG DS 301.

Table 9 – Example for the description of Objects in POWERLINK specifications

Index	1006h	Object Type	VAR
Name	NMT_CycleLen_U32
Data Type	UNSIGNED32	Category	M
Value Range	refer below	Access	rws, valid on reset
Default Range	-	PDO Mapping	No

Table 10 – Example for the description of SubObjects in POWERLINK specifications

Sub-Index	09h
Name	Prescaler_U16
Data Type	UNSIGNED16	Category	MN: M, CN: O
Value Range	0, 1-1000	Access	rws, valid on reset
Default Range	2	PDO Mapping	No

Figure 14 shows the same information in the XDD format defined in EPSG DS311.

Table 11 lists the attributes that are specified for POWERLINK Objects, and how they are mapped to OPC UA mechanisms.

Table 11 – Mapping of attributes

POWERLINK Attribute	Description
Index	Index and Sub-Index are provided by the Information Model as Properties of the Objects as defined in 5.2.2, 5.2.3 and 5.2.4.
SubIndex
Name	The name of the POWERLINK Object shall be used as the BrowseName and the DisplayName of the OPC UA Node
Object Type	The relevant object types of POWERLINK are VAR, ARRAY and RECORD. The VariableTypes PowerlinkArrayType, PowerlinkRecordType and PowerlinkVariableType are used to represent such objects from the POWERLINK Object Dictionary and extend the BaseDataVariableType by POWERLINK specific information about the object. POWERLINK Objects of the type ARRAY shall be modelled as PowerlinkArrayType (5.2.2) POWERLINK Objects of the type RECORD shall be modelled as PowerlinkRecordType (5.2.3) POWERLINK Objects of the type VAR shall be modelled as PowerlinkVariableType (5.2.4)
Data Type	The mapping of primitive datatypes is defined in Table 22. In certain cases the Information Model makes an exception and uses a Structure DataType to improve the usability. Examples for such exceptions: PowerlinkErrorEntryDataType (7.5.1), encoded as DOMAIN in POWERLINK PowerlinkPDOMappingEntryDataType (7.5.3), encoded as UINT64 in POWERLINK Also for usability reasons some variables are modelled as Enumeration. Examples for such cases: NMT_CurrNMTState_U8 (PowerlinkNMTStateEnumeratioin) NMT_ResetCmd_U8 (PowerlinkNMTResetCmdEnumeration)
Value Range	The Value Range of the POWERLINK Object can be provided by the optional Property Range of the PowerlinkVariableType (5.2.4).
Category	POWERLINK defines the 3 categories Mandatory (M), Optional (O) and Conditional (Cond). OPC UA defines the ModellingRules Mandatory and Optional. Since OPC UA does provide an ModellingRule which can be mapped to Conditional of POWERLINK, the mapping is the following: Category M becomes the ModellingRule Mandatory Category O becomes the ModellingRule Optional Category Cond becomes the ModellingRule Optional and requires a textual description at the objects definition about the condition that makes the object mandatory.
Access	Access and PDO Mapping are provided by the Property PowerlinkAttributes defined for the PowerlinkVariableType (5.2.4).
PDO Mapping
Default Value	The default value of the object is provided by the optional Property DefaultValue of the PowerlinkVariableType (5.2.4).