R.6.5
Date Code 20111215 Reference Manual SEL-421 Relay
DNP3 Communications
DNP3 in the SEL-421
A feature called data link confirmation is a mechanism that provides positive
confirmation of message receipt by the receiving DNP device. While this
feature helps you recognize a failed device or failed communications link
quickly, it also adds significant overhead to the DNP conversation. Consider
for your individual application whether you require this link integrity function
at the expense of overall system speed and performance.
The DNP technical bulletin (DNP Confirmation and Retry Guidelines 9804-
002) on confirmation processes recommends against using data link
confirmations because these processes can add to traffic in situations where
communications are marginal. The increased traffic will reduce connection
throughput further, possibly preventing the system from operating properly.
Network Medium
Contention
When more than one device requires access to a single network medium, you
must provide a mechanism to resolve the resulting network medium
contention. For example, unsolicited reporting results in network medium
contention if you do not design your network as a star topology of point-to-
point connections or use carrier detection on a multidrop network.
To avoid collisions among devices trying to send messages, DNP3 includes a
collision avoidance feature. Before sending a message, a DNP3 device listens
for a carrier signal to verify that no other node is transmitting data. The device
transmits if there is no carrier or waits for a random time before rechecking for
a carrier signal. However, if two nodes both detect a lack of carrier at the same
instant, these two nodes could begin simultaneous transmission of data and
cause a data collision. If your network allows for spontaneous data
transmission including unsolicited event data transmissions, you also must use
application confirmation to provide a retry mechanism for messages lost due
to data collisions.
DNP3 in the SEL-421
The SEL-421 is a DNP3 Level 2 remote (slave) device. Additional
implementation documentation describing DNP in the relay is in DNP3
Documentation on page R.6.12
.
Data Access
You can use any of the data access methods listed in Table 6.4. Table 6.4 also
lists the SEL-421 DNP3 settings. You must configure the DNP master for the
data access method you select.
Table 6.4 DNP Access Methods (Sheet 1 of 2)
Access Method Master Polling SEL-421 Settings
Polled static Class 0 Set ECLASSB, ECLASSC,
ECLASSA, ECLASSV to Off,
UNSOL to No.
Polled report-by-
exception
Class 0 occasionally, Class
1, 2, 3 frequently
Set ECLASSB, ECLSSC,
ECLASSA, ECLASSV to the
desired event class, UNSOL to No.
NOTE: Because unsolicited
messaging only operates properly in
some situations, for maximum
performance and minimum risk of
configuration problems, use the
polled report-by-exception access
method. Configure the master to
perform at least 10 event polls for
every integrity poll.
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