R.6.2
SEL-421 Relay Reference Manual Date Code 20111215
DNP3 Communications
Introduction to DNP3
Each level is a proper superset of the next lower-numbered level. A higher
subset level device can act as a master to a lower subset level device. For
example, a typical SCADA master is a Level 3 device and can poll a Level 2
or Level 1 device by using only the data types and functions that the lower-
level device uses. A lower-level device can also poll a higher-level device. For
example, a Level 1 device can poll a Level 3 device, but the Level 1 device can
only access the features and data available in Level 1.
In addition to the Basic 4 and the Subset Definitions, the protocol is further
refined by conformance requirements and a series of technical bulletins. The
technical bulletins supplement the specifications with discussion and
examples of specific features of DNP.
Data Handling
Objects
DNP3 uses a system of data references called objects, which the Basic 4
standard object library defines. Each subset level specification requires a
minimum implementation of object types and also recommends several
optional object types. Object types are commonly referred to as objects. DNP
objects are specifications for the type of data the object carries. An object can
include a single value or more complex data. Some objects serve as shorthand
references for collections of data or even all data within the DNP device.
If there can be more than one instance of a type of object, then each instance
of the object includes an index that makes it unique. For example, each binary
status point (Object 1) has an index. If there are 16 binary status points, these
points are Object 1, Index 0 through Object 1, Index 15.
Each object also includes multiple versions called variations. For example,
Object 1 has three variations: 0, 1, and 2. Variation 0 is a shorthand reference
used to request all Object 1 data from a DNP device. Variation 1 is used to
specify binary input values only and variation 2 is used to specify binary input
values with status information.
Each DNP3 device has both a list of objects and a map of object indices. The
list of objects defines the available objects, variations, and qualifier codes. The
map defines the indices for objects that have multiple instances and defines
what data or control points correspond with each index.
A master initiates all DNP message exchanges except unsolicited data. DNP
terminology describes all points from the perspective of the master. Binary
points for control that move from the master to the remote are called Binary
Outputs, while binary status points within the remote are called Binary Inputs.
Table 6.1 DNP3 Implementation Levels
Level Description Equipment Types
1 Simple: limited communication requirements Meters, simple IEDs
2 Moderately complex: monitoring and metering devices
and multifunction devices that contain more data
Protective relays,
RTUs
3 Sophisticated: devices with great amounts of data or
complex communication requirements
Large RTUs, SCADA
masters
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