Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

146 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Level 3 sub-set definition
Level 3 functions – additional to levels 1 and 2:
• Slave will process read requests for many specific objects and variations
• Supports a larger range of requests and function codes
• Enabling and disabling of unsolicited responses on a class-by-class basis
• Eg, will process the following:
– Class 1 (object 60, var 2, qualifier 6)
– Class 2 (object 60, var 3, qualifier 6)
– Class 3 (object 60, var 4, qualifier 6)
• Dynamic assignment of data objects to classes
Qualifier field quick reference
The meanings of the function qualifier field codes were presented in an earlier chapter,
showing the index size and qualifier sub-fields separately. The reader will recall that these
are structured as below:
Qualifier field
In the implementation tables the qualifier field values are simply shown as a two-digit
hexadecimal value, from which the I-size and Q-code values may be read directly.
The reader will soon notice that only certain combinations of these appear in the
implementation tables. It is convenient therefore to have a ‘quick-reference’ table for these
combinations. This is shown below.
This table shows that for the defined sub-sets, only a small number of methods of
referencing data are actually used. These are:
• Range-index mode, where the range field has the start and stop index values
• Non-ranged mode with no indexes, where the range field has the number of
data objects. In a response the data objects simply are appended