A 16-bit index is used to address all entries within the POWERLINK Object Dictionary. In the case of a simple variable, the index references the value of this variable directly. In the case of records and arrays, however, the index addresses the whole data structure.
To allow individual elements of structures of data to be accessed via the network a Sub-Index is defined. For single POWERLINK Object Dictionary entries such as an UNSIGNED8, BOOLEAN, INTEGER32 etc. the value for the Sub-Index is always zero. For complex POWERLINK Object Dictionary entries such as arrays or records with multiple data fields the Sub-Index references fields within a data-structure pointed to by the main index. The fields accessed by the Sub-Index can be of differing data types.