R.6.4
SEL-421 Relay Reference Manual Date Code 20111215
DNP3 Communications
Introduction to DNP3
DNP also supports static polling, simple polling of the present value of data
points within the remote. By combining event data, unsolicited polling, and
static polling, you can operate your system in one of the four access methods
shown in Table 6.3.
The access methods listed in Table 6.3 are in order of increasing
communication efficiency. With various tradeoffs, each method is less
demanding of communication bandwidth than the previous one. For example,
unsolicited report-by-exception consumes less communication bandwidth
because of the elimination of polling messages from the master required by
polled report-by-exception. You must also consider overall system size and
the volume of data communication expected in order to properly evaluate
which access method provides optimum performance for your application.
Binary Control
Operations
DNP masters use Object 12 control relay output block to perform DNP binary
control operations. The control relay output block has both a trip/close
selection and a code selection. The trip/close selection allows a single DNP
index to operate two related control points, such as trip and close or raise and
lower. Trip/close pair operation is not recommended for new DNP3 devices,
but is often included for interoperability with older DNP master
implementations.
The control relay output block code selection specifies either a latch or pulse
operation on the point. In many cases, DNP remotes have only a limited subset
of the possible combinations of the code field. Sometimes, DNP remotes
assign special operation characteristics to the latch and pulse selections.
Tab le 6.15 and Table 6.16 describe control point operation for the SEL-421.
Conformance Testing
In addition to the protocol specifications, the DNP User’s Group has approved
conformance testing requirements for Level 1 and Level 2 remote devices.
Some implementers perform their own conformance specification testing,
while some contract with independent companies to perform conformance
testing.
Conformance testing does not always guarantee that a master and remote will
be fully interoperable (work together properly for all implemented features).
Conformance testing does help to standardize the testing procedure and move
the DNP implementers toward a higher level of interoperability. The SEL-421
is certified as having passed DNP3 Level 2 Slave conformance tests by a third-
party organization, and the conformance certificate is on file at SEL.
Data Link Layer
Operation
DNP employs a three-layer version of the seven-layer OSI (Open Systems
Interconnect) model called the enhanced performance architecture. The layer
definition helps to categorize functions and duties of various software
components that make up the protocol. The middle layer, the Data Link Layer,
includes several functions for error checking and media access control.
Table 6.3 DNP Access Methods
Access Method Description
Polled static Master polls for present value (Class 0) data only.
Polled report-by-
exception
Master polls frequently for event data and occasionally for
Class 0 data.
Unsolicited report-
by-exception
Remote devices send unsolicited event data to the master, and the
master occasionally polls for Class 0 data.
Quiescent Master never polls and relies on unsolicited reports only.
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