D.4
SEL-751 Relay Instruction Manual Date Code 20170927
DNP3 Communications
Introduction to DNP3
unsolicited report-by-exception consumes less communications bandwidth
than polled report-by-exception because that method does not require polling
messages from the master. To properly evaluate which access method provides
optimum performance for your application, you must also consider overall
system size and the volume of data communication expected.
Binary Control
Operations
DNP3 masters use Object 12, control device output block, to perform DNP3
binary control operations. The control device output block has both a
trip/close selection and a code selection. The trip/close selection allows a
single DNP3 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 DNP3
master implementations.
The control device output block code selection specifies either a latch or pulse
operation on the point. In many cases, DNP3 outstations have only a limited
subset of the possible combinations of the code field. Sometimes, DNP3
outstations assign special operation characteristics to the latch and pulse
selections. Table D.13 describes control point operation for the SEL-751.
Conformance Testing
In addition to the protocol specifications, the DNP Users Group has approved
conformance-testing requirements for Level 1 and Level 2 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 outstation
are fully interoperable (that is, work together properly for all implemented
features). Conformance testing does help to standardize the testing procedure
and move the DNP3 implementers toward a higher level of interpretability.
DNP3 Serial Network
Issues
Data Link Layer Operation
DNP3 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.
A feature called data link confirmation is a mechanism that provides positive
confirmation of message receipt by the receiving DNP3 device. While this
feature helps you recognize a failed device or failed communications link
quickly, it also adds significant overhead to the DNP3 conversation. You
should consider whether you require this link integrity function in your
application at the expense of overall system speed and performance.
Ta b l e D. 3 DN P 3 Acc ess M et h od s
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
Outstation devices send unsolicited event data to the mas-
ter, and the master occasionally polls for Class 0 data
Quiescent Master never polls and relies on unsolicited reports only
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