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2. DNP3 Protocol Primer
PXM 4/6/8K DNP3 Ethernet Communications User Manual
MN150005EN January 2017 www.eaton.com
2.1.1 Master and Outstation Databases
Figure 1
shows the master-outstation relationship and gives
a simplistic view of the databases and software processes
involved. The master is on the left side of
Figure 1
, and the
outstation is on the right side.
A series of square blocks at the top of the outstation depict
data stored in its database and output devices. The various
data types are conceptually organized as arrays. An array of
binary input values represents states of physical or logical
Boolean devices. Values in the analog input array represent
input quantities that the outstation measured or computed.
An array of counters represents count values, such as kilo-
watt hours, that are ever increasing (until they reach a maxi-
mum and then roll over to zero and start counting again).
Control outputs are organized into an array representing
physical or logical on-off, raise-lower, and trip-close points.
Lastly, the array of analog outputs represents physical or
logical analog quantities such as those used for set points.
Figure 1. Master and Outstation Relationship.
The elements of the arrays are labeled 0 through N - 1
where N is the number of blocks shown for the respective
data type. In DNP3 terminology, the element numbers are
called the point indexes. Indexes are zero-based in DNP3,
that is, the lowest element is always identified as zero.
Indexes are zero-based in DNP3, that is, the lowest element
is always identified as zero.
Notice that the DNP3 master also has a similar database
for the input data types (binary, analog and counter.) The
master uses values in its database for the specific purposes
of displaying system states, closed-loop control, alarm noti-
fication, billing, and much, much more. An objective of the
master is to keep its database updated. It accomplishes
this by sending requests to the outstation asking it to return
the values in the outstation’s database. This is termed
polling. The outstation responds to the master’s request
by transmitting the contents of its database. Arrows are
drawn at the bottom of
Figure 1
showing the direction of
the requests (toward the outstation) and the direction of the
responses (toward the master). Later, we will discuss sys-
tems whereby the outstations transmit responses without
being asked.
