Understanding DNP3 Data Objects, Groups, and Variations in SCADA Systems

The Distributed Network Protocol (DNP3) is a widely used SCADA communication protocol designed to enable reliable data exchange between master stations and outstations. It is optimized for transmitting small, structured data packets efficiently and reliably in industrial environments

A key strength of DNP3 lies in its object-based data model, which organizes data into object groups, variations, and indices. This structured approach enables flexible, efficient, and interoperable communication across devices from different vendors.

1. DNP3 Object-Based Data Model

DNP3 uses an object library at the application layer to represent all data exchanged between devices.

According to the DNP3 reference manual:

• Data is organized into object groups (types of data)
• Each object is further defined by variations (format of data)
• Individual data points are identified by indexes

This model allows:

• Standardized communication
• Efficient data transmission
• Support for multiple data types in a single message

2. DNP3 Data Objects

DNP3 data objects represent real-world measurements and control points stored in device databases.

Typical data types include:

• Binary inputs (e.g., breaker status)
• Analog inputs (e.g., voltage, temperature)
• Counters (e.g., energy usage)
• Control outputs

As explained in the DNP3 primer:

• These data points are stored as arrays in outstation databases
• Each element is identified by a zero-based index

This structure allows efficient access and transmission of large numbers of data points.

3. Object Groups (Data Categories)

DNP3 organizes objects into groups, where each group represents a category of data.

Examples:

• Binary Inputs
• Analog Inputs
• Counters
• Binary Outputs
• Analog Outputs

Groups define what type of data is being communicated, not how it is formatted.

The DNP3 manual describes this as:

Data objects are “broadly grouped together into Object Groups such as Binary Input Objects and Analog Input Objects”

4. Variations (Data Representation)

Each object group contains multiple variations, which define how the data is encoded.

Variations specify:

• Data format (integer, floating point)
• Size (16-bit, 32-bit, etc.)
• Presence of additional information (e.g., flags, timestamps)

For example, analog data may be transmitted as:

• Integer values
• Floating-point values
• With or without status flags

This flexibility allows DNP3 to:

• Optimize bandwidth
• Match device capabilities
• Provide detailed or compact data formats

5. Object Headers and Message Structure

DNP3 transmits data objects using object headers, which are part of the application layer message.

Each object header contains:

• Group number
• Variation number
• Qualifier field
• Range or count of objects

The diagram clearly shows:

• Object headers precede the actual data
• Multiple objects can be included in a single message

This structure allows:

• Efficient bulk data transfer
• Flexible addressing of data points

6. Indexing of Data Points

Each data object is identified by an index (point number).

• Indexes are zero-based
• Represent specific field devices or measurements
• Used to access data within object groups

From the primer:

“Element numbers are called point indexes… the lowest element is always identified as zero”

This enables scalable systems with thousands of data points.

7. Static and Event Objects

DNP3 defines two main types of data:

Static Objects

• Represent current values
• Retrieved using polling
• Belong to Class 0

Event Objects

• Represent changes in data
• Stored in event buffers
• Assigned to Class 1, 2, or 3 (priority levels)

From the DNP3 manual:

• Event data is returned when the master polls a specific class
• Only changed data is transmitted, improving efficiency

8. Class-Based Data Organization

DNP3 assigns data objects to classes:

• Class 0 → Static data
• Class 1 → High-priority events
• Class 2 → Medium-priority events
• Class 3 → Low-priority events

This allows:

• Priority-based polling
• Efficient bandwidth usage
• Faster reporting of critical events

9. Efficient Data Transmission

The object model supports efficient communication by:

• Sending only changed (event) data
• Allowing multiple object types in one message
• Supporting unsolicited responses

DNP3 systems can operate in:

• Polling mode (master requests data)
• Quiescent mode (report-by-exception)

In quiescent mode:

• Outstations send data only when changes occur
• Communication remains idle otherwise

10. Practical Example

In a substation SCADA system:

• Binary Input → Breaker ON/OFF
• Analog Input → Voltage level
• Counter → Energy consumption

Each data point:

• Belongs to a group
• Has a variation (format)
• Is identified by an index
• May be assigned a class

The master station retrieves:

• All static data (Class 0)
• Only changed event data (Class 1–3)

DNP3 Data Objects – Complete List of Groups and Variations

The table below presents a consolidated summary of all DNP3 object groups and variations defined in IEEE Std 1815-2012.

This summary includes every supported object group, its variation numbers, and the official object description. It serves as a quick-reference guide for DNP3 implementers, system integrators, and protocol analysts, showing the full range of static data, events, commands, counters, analog values, device attributes, and time-related objects available across all subset levels.

Objects are sorted by Group number and then by Variation for easy lookup. Variations that appear only in higher levels (e.g., floating-point, time-tagged events, or device attributes) are clearly identified by their presence in Level 4.

Complete List of DNP3 Objects and Variations

Group	Variation	Description
0	209	Device Attributes— Secure authentication version
0	210	Device Attributes— Number of security statistics per association
0	211	Device Attributes— Identifier of support for user-specific attributes
0	212	Device Attributes— Number of master-defined data set prototypes
0	213	Device Attributes— Number of outstation-defined data set prototypes
0	214	Device Attributes— Number of master-defined data sets
0	215	Device Attributes— Number of outstation-defined data sets
0	216	Device Attributes— Max number of binary outputs per request
0	217	Device Attributes— Local timing accuracy
0	218	Device Attributes— Duration of timing accuracy
0	219	Device Attributes— Support for analog output events
0	220	Device Attributes— Max analog output index
0	221	Device Attributes— Number of analog outputs
0	222	Device Attributes— Support for binary output events
0	223	Device Attributes— Max binary output index
0	224	Device Attributes— Number of binary outputs
0	225	Device Attributes— Support for frozen counter events
0	226	Device Attributes— Support for frozen counters
0	227	Device Attributes— Support for counter events
0	228	Device Attributes— Max counter index
0	229	Device Attributes— Number of counter points
0	230	Device Attributes— Support for frozen analog inputs
0	231	Device Attributes— Support for analog input events
0	232	Device Attributes— Maximum analog input index
0	233	Device Attributes— Number of analog input points
0	234	Device Attributes— Support for double-bit binary input events
0	235	Device Attributes— Maximum double-bit binary input index
0	236	Device Attributes— Number of double-bit binary input points
0	237	Device Attributes— Support for binary input events
0	238	Device Attributes— Max binary input index
0	239	Device Attributes— Number of binary input points
0	240	Device Attributes— Max transmit fragment size
0	241	Device Attributes— Max receive fragment size
0	242	Device Attributes— Device manufacturer’s software version
0	243	Device Attributes— Device manufacturer’s hardware version
0	245	Device Attributes— User-assigned location name
0	246	Device Attributes— User assigned ID code/number
0	247	Device Attributes— User-assigned device name
0	248	Device Attributes— Device serial number
0	249	Device Attributes— DNP3 subset and conformance
0	250	Device Attributes— Device manufacturer’s product name and model
0	252	Device Attributes— Device manufacturer’s name
0	254	Device Attributes— Non-specific all attributes request
0	255	Device Attributes— List of attribute variations
1	0	Binary Input— Any Variation
1	1	Binary Input— Packed format
1	2	Binary Input— With flags
2	0	Binary Input Event— Any Variation
2	1	Binary Input Event— Without time
2	2	Binary Input Event— With absolute time
2	3	Binary Input Event— With relative time
3	0	Double-bit Binary Input— Any Variation
3	1	Double-bit Binary Input— Packed format
3	2	Double-bit Binary Input— With flags
4	0	Double-bit Binary Input Event— Any Variation
4	1	Double-bit Binary Input Event— Without time
4	2	Double-bit Binary Input Event— With absolute time
4	3	Double-bit Binary Input Event— With relative time
10	0	Binary Output— Any Variation
10	2	Binary Output— Output status with flags
11	0	Binary Output Event— Any Variation
11	1	Binary Output Event— Status without time
11	2	Binary Output Event— Status with time
12	0	Binary Command— Control relay output block (CROB)
12	1	Binary Command— Control relay output block (CROB)
13	0	Binary Output Command Event— Any Variation
13	1	Binary Output Command Event— Command status without time
13	2	Binary Output Command Event— Command status with time
20	0	Counter— Any Variation
20	1	Counter— 32-bit with flag
20	2	Counter— 16-bit with flag
20	5	Counter— 32-bit without flag
20	6	Counter— 16-bit without flag
21	0	Frozen Counter— Any Variation
21	1	Frozen Counter— 32-bit with flag
21	2	Frozen Counter— 16-bit with flag
21	5	Frozen Counter— 32-bit with flag and time
21	6	Frozen Counter— 16-bit with flag and time
21	9	Frozen Counter— 32-bit without flag
21	10	Frozen Counter— 16-bit without flag
22	0	Counter Event— Any Variation
22	1	Counter Event— 32-bit with flag
22	2	Counter Event— 16-bit with flag
23	0	Frozen Counter Event— Any Variation
23	1	Frozen Counter Event— 32-bit with flag
23	2	Frozen Counter Event— 16-bit with flag
23	5	Frozen Counter Event— 32-bit with flag and time
23	6	Frozen Counter Event— 16-bit with flag and time
30	0	Analog Input— Any Variation
30	1	Analog Input— 32-bit with flag
30	2	Analog Input— 16-bit with flag
30	3	Analog Input— 32-bit without flag
30	4	Analog Input— 16-bit without flag
30	5	Analog Input— Single-prec flt-pt with flag
32	0	Analog Input Event— Any Variation
32	1	Analog Input Event— 32-bit without time
32	2	Analog Input Event— 16-bit without time
32	3	Analog Input Event— 32-bit with time
32	4	Analog Input Event— 16-bit with time
32	5	Analog Input Event— Single-prec flt-pt without time
32	7	Analog Input Event— Single-prec flt-pt with time
34	0	Analog Input Deadband— Any Variation
34	1	Analog Input Deadband— 16-bit
34	2	Analog Input Deadband— 32-bit
40	0	Analog Output Status— Any Variation
40	1	Analog Output Status— 32-bit with flag
40	2	Analog Output Status— 16-bit with flag
41	1	Analog Output— 32-bit
41	2	Analog Output— 16-bit
50	1	Time and Date— Absolute time
51	1	Time and Date CTO— Absolute time, synchronized
51	2	Time and Date CTO— Absolute time, unsynchronized
52	1	Time Delay— Coarse
52	2	Time Delay— Fine
60	1	Class Objects— Class 0 data
60	2	Class Objects— Class 1 data
60	3	Class Objects— Class 2 data
60	4	Class Objects— Class 3 data
80	1	Internal Indications— Packed format

Conclusion

The DNP3 data model—based on objects, groups, variations, and indices—provides a structured and flexible framework for representing industrial data. This model enables:

• Efficient communication
• Scalable system design
• Interoperability across vendors
• Reliable data handling in SCADA systems

By separating data type (groups) from data format (variations), DNP3 achieves both flexibility and efficiency, making it one of the most effective protocols for industrial automation.