5.2 Modeling concepts ToC Previous Next

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.

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Figure 14 – Example for XDD format

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).

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