Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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16.2
SEL-400 Series Relays Instruction Manual Date Code 20171006
DNP3 Communication
Introduction to DNP3
Each level is a proper superset of the next lower-numbered level. A higher level
device can act as a master to a lower level device, but can only use the data types
and functions implemented in the lower level device. For example, a typical
SCADA master is a Level 3 device and can use Level 2 (or lower) functions to
poll a Level 2 (or lower) device by using only the data types and functions that
the lower-level device uses. A lower-level device can also poll a higher-level
device, but the lower level device can only access the features and data available
to its level.
Data Handling
Objects
DNP3 uses a system of data references called object types, commonly referred to
as objects. Each subset level specification requires a minimum implementation of
objects and also recommends several optional objects. DNP3 objects are specifi-
cations for the type of data the object carries. An object can include a single value
or more complex data. Some objects serve as shorthand references for collections
of data or even all data within the DNP3 device.
Each instance of the object includes an index that makes it unique. For example,
each binary status point (Object 1) has an index. If there are 16 binary status
points, these points are Object 1, Index 0 through Object 1, Index 15. Note that
index numbers are 0-based.
Each object also includes multiple versions called variations. For example,
Object 1 has three variations: 0, 1, and 2. Variation 0 is used to request Object 1
data from a DNP3 device using its default variation. Variation 1 is used to specify
binary input values only and Variation 2 is used to specify binary input values
with status information.
Each DNP3 device has both a list of objects and a map of object indices. The list
of objects defines the available objects, variations, and qualifier codes. The map
defines the indices for objects that have multiple instances and what data or con-
trol points correspond with each index.
A master initiates all DNP3 message exchanges except unsolicited data. DNP3
terminology describes all points from the perspective of the master. Binary points
for control that move from the master to the outstation are called binary outputs,
while binary status points within the outstation are called binary inputs.
Function Codes
Each DNP3 message includes a function code. Each object has a limited set of
function codes that a master may use to manipulate the object. The object listing
for the device shows the permitted function codes for each type of object. The
most common DNP3 function codes are listed in
Table 16.2 .
Table 16.2 Selected DNP3 Function Codes
Function Code Function Description
1 Read Request data from the outstation
2 Write Send data to the outstation
3 Select First part of a select-before-execute operate
4 Execute Second part of a select-before-execute operate
5 Direct operate One-step operation with acknowledgment
6 Direct operate, no ack. One-step operation with no acknowledgment