Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Fundamentals of distributed network protocol 129
Each object is denoted by a group number and a variation number. DNP3 represents the
group and variation numbers each with 8-bit fields. There are a number of general types
of objects, and these are organized into decade ranges. There are usually more than one
specific group type within each range. For each group, there are usually a number of
variations.
Table of Object Groups
Group Range
Object Group Description
0 - 9 Binary Input Objects
10 - 19 Binary Output Objects
20 - 29 Counter Objects
30 - 39 Analog Input Objects
40 - 49 Analog Output Objects
50 - 59 Time Objects
60 - 69 Class Objects
70 - 79 File Objects
80 - 89 Device Objects
90 - 99 Application Objects
100+ Alternate Numeric Objects
As can be seen from the table, the object groups are organized into ranges of similar
source types, such as binary input objects (0–9), or file objects (70–79). It may also be
noted that some data objects appear closely related to physical data monitoring and control
points, and others (range 50 on) appear less so. In the latter case, the data object is often
not representing physical things, for example file objects represent software, and class
objects represent an attribute that may be assigned to other objects.
In a few cases, there are objects defined that have no data attached to them at all. Whilst
these do not sound very useful, their function is performed merely by the transmission of
the object identifier. For example, the class object is used in a request message only to
identify the class attribute of objects that are being requested.
Within each object group there may be defined a number of variations. It is important to
note that each variation is normally just a different representation of the same underlying
data point. It may have more or less information than other variations, but the same point
is being represented by the object.
For the objects that are related to physical points, the type of data is classified into four
different types. These are shown in the following table. Each physical point will generally
be able to be represented by each of the four types, which are defined as different variations
for each object group.
The differences between these are illustrated further in the following paragraphs.
The static type is probably the simplest to understand. Static simply refers to the current
value of any point at the time the information is transmitted. This might be the current
value of a binary input or output, the most recently converted value of an analog input, the
value of a register, or the value of a counter. Static values are associated directly with
physical inputs and outputs, or with internal registers and counters. There is a one-to-one
relationship between these devices and the static points, so therefore there are as many
static points as there are physical devices, registers, or counters.
Event data is different to static data in that it is generated on the occurrence of a particular
event, such as a change in state of a binary point. If a physical device such as a binary
input has an event variation configured for it, then whenever it changes an event data
object will be generated for that input-change. In the case of analog inputs, events are