16.3
Date Code 20171006 Instruction Manual SEL-400 Series Relays
DNP3 Communication
Introduction to DNP3
Qualifier Codes and Ranges
DNP3 masters use qualifier codes and ranges to make requests for specific
objects by index. Qualifier codes specify the style of range, and the range speci-
fies the indices of the objects of interest. DNP3 masters use qualifier codes to
compose the shortest, most concise message possible when requesting points
from a DNP3 remote.
For example, the qualifier code 01 specifies that the request for points will
include a start address and a stop address. Each of these two addresses uses two
bytes. An example request using qualifier code 01 might have the four-hexadeci-
mal byte range field, 00h 04h 00h 10h, that specifies points in the range 4–16.
Access Methods
DNP3 has many features that help it obtain maximum possible message effi-
ciency. DNP3 Masters send requests with the least number of bytes using special
objects, variations, and qualifiers that reduce the message size. Other features
eliminate the continual exchange of static (unchanging) data values. These fea-
tures optimize use of bandwidth and maximize performance over a connection of
any speed.
DNP3 event data collection eliminates the need to use bandwidth to transmit val-
ues that have not changed. Event data are time-stamped records that show when
observed measurements changed. For binary points, the outstation device logs
changes from logical 1 to logical 0 and from logical 0 to logical 1. For analog
points, the remote device logs changes that exceed a dead band. DNP3 outstation
devices collect event data in a buffer that either the master can request or the
device can send to the master without a request message. Data sent from the out-
station to the master without a polling request are called unsolicited data.
DNP3 data fit into one of four event classes: 0, 1, 2, or 3. Class 0 is reserved for
reading the present value (static data). Classes 1, 2, and 3 are event data classes.
The meaning of Classes 1 to 3 is arbitrary and defined by the application at hand.
With remotes that contain great amounts of data or in large systems, the three
event classes provide a framework for prioritizing different types of data. For
example, you can poll once a minute for Class 1 data, once an hour for Class 2
data, and once a day for Class 3 data.
Class 0 polling is also known as static polling, or simple polling of the present
value of data points within the outstation. By combining event data polls, unsolic-
ited messaging, and static polling, you can operate your system in one of the four
access methods shown in Table 16.3.
The access methods listed in Table 16.3 are in order of increasing communica-
tions efficiency. With various trade-offs, each method is less demanding of com-
munications bandwidth than the previous one. For example, unsolicited report-
by-exception consumes less communications bandwidth because of the elimina-
tion of polling messages from the master required by polled report-by-exception.
You must also consider overall system size and the volume of data communica-
tion expected in order to properly evaluate which access method provides opti-
mum performance for your application.
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