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2. DNP3 Protocol Primer
PXM 4/6/8K DNP3 Ethernet Communications User Manual
MN150005EN January 2017 www.eaton.com
2.1.2 Layering
The master and the outstation shown in
Figure 1
each have
two software layers. The top layer is the DNP3 user layer.
In the master, it is the software that interacts with the data-
base and initiates the requests for the outstation’s data. In
the outstation, it is the software that fetches the requested
data from the outstation’s database for responding to mas-
ter requests. It is interesting to note that if no physical
separation of the master and outstation existed, eliminating
the DNP3 might be possible by connecting these two upper
layers together. However, since physical, or possibly logi-
cal separation of the master and outstation exists, DNP3
software is placed at a lower level. The DNP3 User’s code
uses the DNP3 software for transmission of requests or
responses to the matching DNP3 User’s code at the other
end. More will be said about data types and software lay-
ers later, but first we want to examine a few typical system
architectures where DNP3 is used.
Figure 2. Common System Architectures.
2.1.3 System Architecture
Figure 2
shows common system architectures in use
today. At the top is a simple one-on-one system having
one master station and one outstation. The physical con-
nection between the two is typically a dedicated or dial-up
telephone line.
The second type of system is known as a multi-drop design.
One master station communicates with multiple outstation
devices. Conversations are typically between the master
and one outstation at a time. The master requests data
from the first outstation, then moves onto the next outsta-
tion for its data, and continually interrogates each outsta-
tion in a round robin order. The communication media is
a multi-dropped telephone line, fiber optic cable, or radio.
Each outstation can hear messages from the master and
is only permitted to respond to messages addressed to it.
Outstations may or may not be able to hear each other.
