DNP3 Protocol

Supported device types and versions
Communication line configuration
Communication station configuration
I/O tag configuration
Tell commands
Literature
Changes and modifications
Document revisions

Supported device types and versions

Protokol DNP3 is a telemetry protocol designed for communication of a master station (usually SCADA) with outstations (usually PLC, RTU or other IED - Intelligent Electronic Devices). It was developed while IEC only worked on IEC60870-5 standards (IEC 870-5-101 and IEC 870-5-104).

DNP3 defines multiple object Groups (e.g. Binary Input, Analog Input, Counter) and multiple Variations for each object group (e.g. for Analog Input there are variations 32-bit with flag, 16-bit with flag, 32-bit without flag, 16-bit without flag, Single-precision floating point with flag ..). Each object is defined by a Group and Index (a non-negative number with the size of 1, 2, or 4 bytes).

At the same time DNP3 protocol classifies objects into 4 classes (Class 0 - static objects; Class 1..3 - dynamic objects with change registration). Similar to IEC 870-5-101 and IEC 870-5-104, DNP3 supports requesting all values ​​of a particular class (Poll request), in addition to explicit reading of a particular group of objects with a specified variation (or possibly using variation 0 - automatic variation). When reading, it is possible to request the data of the whole group, or to specify an index or the entire range of objects.

The following application functions are supported for the objects:

• Read (1) - reading objects.
• Write (2) - Single-phase write of objects (with numeric code confirming successful write or talking about write error).
• Select (3) + Operate (4) - a two-phase write of objects that virtually excludes the possibility of error due to undetected interference on the line. Each operation returns a group, index, and write value in addition to the return code.
• Direct Operate (5) - single-phase write of objects (except the numeric code, the operation returns a group, index, and write value).
• Direct Operate Non Return (6) - single-phase write of an object without confirmation (least secure).
• Enable Unsolicited Responses (20) - activation of sending spontaneous changes by subordinate stations.

The protocol also supports sending spontaneous changes by outstations - using application function Unsolicited Response (130).

Data is sent as Data Link Layer packets, also called transport segments. One or more transport segments form an Application Layer fragment. The fragment can be sent as confirmed (Data Link Layer function code 3 - CONFIRMED_USER_DATA) or unconfirmed (Data Link Layer function code 4 - UNCONFIRMED_USER_DATA). The transport segments within one fragment are always confirmed - with the exception of the last one (Data Link Layer function code 0 - ACK).

Table 1: D2000 implementation supports the following groups and their variations:

* Note: Operation Analysis means that D2000 KOM can analyze the message and assign values to the appropriate I/O tag. For example, for a Binary Input Event (2) group, values are assigned to the I/O tags from the Binary Input (1) group.

** Note: The outstation can send object values from the Time and Date CTO (51) group, defining the Common Time of Occurence (CTO) for subsequent events. Afterwards, values with relative time are sent - Binary Input Event (2) and Double-bit Binary Input Event (4), using With relative time (3) variations.

Note: D2000 KOM supports sending a message with the writing type outside the range allowed by the standard - e.g. for the Analog Output (41) group, a write operation with Write (2) function can be configured.

The D2000 implementation also supports time synchronization (according to the parameters configured on the station) and browsing. Secure authentication is not supported.

Communication line configuration

Category of communication line:

• Serial
• SerialOverUDP Device Redundant
• MOXA IP Serial Library
• TCP / IP-UDP: The IP address and UDP port of the outstation are configured on the line (the default port for the DNP3 protocol is 20000). The UDP port on the side of D2000 KOM process is dynamically assigned. If it needs to be fixed, use the SerialOverUDP Device Redundant line.
• TCP/IP-TCP: The IP address and TCP port of the outstation are configured on the line (the default port for the DNP3 protocol is 20000). Parameter Line number is not used, set it to 0.
  Note: if a TCP/IP-TCP line has all stations set to StOFF, TCP connection will be closed. Thus it is possible to control TCP communication from event using a tell command STSTAT.

Communication line protocol parameters

Configuration line dialog box - tab Protocol parameters.
They influences some of the optional protocol parameters. The following line parameters can be set:

Communication station configuration

• Communication protocol DNP3.
• Station address: 16-bit number from range 0-65 519 (other addresses have special meanings).

Communication station protocol parameters

The following station parameters can be set:

I/O tag configuration

Possible value types of I/O tag: Ai, Ao, Di, Dout, Ci, Co, Qi, TxtI, TxtO, TiA, ToA, TiR, ToR.

Introduction

The I/O tag corresponds to one DNP3 object.

The minimum I/O tag configuration requires to configure the Group and Index parameters. Such an I/O tag must be placed in one of the dynamic classes Class 1-3, so that changes of its value are sent spontaneously or in a response to the Poll request.

Explicit Read must be configured for static data (assigned to Class 0) or for objects that are not assigned to any of the classes Class 1-3. For instance, according to the standard, objects from the Binary-Coded Decimal Integer (101), Unsigned Integer (102), and Octet String (110) groups may not be in Class 0, and the Analog Input Reporting Deadband (34) is not in Class 0, so its objects must be explicitly read.

When configuring explicit reading, a particular Variation can be specified (or keep the default value <Automatic>, which means that variation 0 is used). Furthermore, it is possible to configure the required address range with the Range parameter and optionally with the From and To parameters (a request generated by a single I/O tag can read data for multiple I/O tags that do not have explicit reading configured, or for items of a structured variable). Using the Read Period parameter, it is possible to configure a custom reading period different from the period specified in the station parameters.

If the I/O tag is of output type, writing can be configured with Write parameter. The Write Type parameter then specifies the application function, the Write Group parameter specifies the group and the Write Variation specifies the variation used for writing. If a writing is configured for I/O tag, the Range parameter must be specified, and if a particular value of this parameter requires entering From and To parameters, they must have the same value.

The Disable parameter can be used to disable the I/O tag.

Figure - configuration of I/O tag address for DNP3 protocol

Detailed description of individual parameters:

Address group

Group: object group to which the I/O tag belongs. The list of supported groups according to the standard is in Table 1 (they are supported if Reading is in the Supported Operations column).

Index: index of an object in the object group. The index is a non-negative number with a size of 1, 2, or 4 bytes. The Group + Index combination specifies the object address in the DNP3 protocol.

Disable: enables to disable the I/O tag, so that it will not acquire values ​​and, in the case of output I/O tags, writing will be disabled too.

Read / write parameters group

Explicit Read: enable explicit reading by the Read (1) application function. It is also necessary to configure the Variation, Range and, depending on the value of Range parameter, optionally the From and To indices.

Read Period: a positive real number that indicates the explicit reading period in seconds. If not specified, the period defined on the station is used.

Variation: a preferred variation (group-specific format) used for explicit reading. The <Automatic> variation is sent as a Variation 0 (according to the standard, the master announces that it does not have the preferred format and leaves the selection to outstation).

Range: range of objects specified for explicit reading or writing. With explicit reading, it is also possible to retrieve all objects in a particular group, or a range of objects. The values ​​read in this way will then also be assigned to the I/O tags that do not have explicit reading configured, if the Group + Index matches retrieved data. Also, the read values can be assigned to items of a structured variable, if it is configured on Destination tab.
The following ranges are supported:

From, To: specifies range of indices or virtual addresses for certain values of the Range parameter (0-5). The value of From parameter must be is less or equal to the value of To parameter. If writing is configured for the I/O tag, and the Range value requires From and To parameter to be specified, then their values must be the same.
Note: In practice, we have encountered DNP3 implementations that only supported writing with Range = 7, 8, and 9 (1/2/4-octet count of objects), i.e. they did not support writing specifying Range equal to 0, 1, and 2 (1/2/4-octet start / stop index) using From and To indices, even if they were set to the same value.

Write: enable writing operations function for output I/O tags. The parameters Range, Write Type, Write Group, Write Variation, and optionally - depending on value of the Range parameter - also From and To must be specified.

Write Type: application function used for writing. The following functions are supported:

Table 1 in the Supported Operations column for Write operation indicates which application functions are in compliance with the standard. Our implementation permits to use application functions beyond the standard if a specific device supports them.

Write Group: object group used for writing. The list of supported groups according to the standard is in Table 1 (writing is supported if Writing is in the Supported Operations column).

Write Variation: variation used for writing. The list of supported variations for individual groups according to the standard is shown in Table 1 in the Variations column (writing is supported if Writing is in the Supported Operations column).

Browse

For the I/O tags, it is possible to discover the list of objects and their data types, as long as the KOM process is running and communication with an outstation is established.
Clicking the Browse button opens the DNP3 Item Browser window and displays a list of objects that have been read so far. The object list is created dynamically as a result of received messages (responses to Read and Poll requests as well as spontaneously arrived values).

At the same time - if the Browse All Groups parameter is set to YES, the first browsing (as well as browsing when the Refresh button is pressed) generates requests to read all supported object groups.

The list of objects is dynamic, i.e. when a new value arrives in the KOM process or when a new object is detected, it is updated.

Double-clicking on a particular line will cause the Group and Index parameters to be inserted into the configuration of I/O tag from which the DNP3 Item Browser window was opened.

Figure - DNP3 Item Browser window

The window displays the following information:

Group: object group. In addition to the groups configurable for the I/O tags, "event" groups (such as Binary Input Event (2)) may also appear in the list, as long as the KOM process has received a message reporting a change in object value.

Index: index of an object in a group

Source: source of data:

• READ - data came in response to data read request - Read (1) function
• POLLED - data came in response to a Poll request - reading object group Class Objects (60)
• SPONTANEOUS - data came spontaneously - using Unsolicited Response (130) function

Value: the current value, optionally - depending on a particular object group and variation - with flags and timestamp

Point: the name of the I/O tag that is configured for this object. Names can also be repeated:

• if a column of a structured variable is specified for the Destination column parameter on the Destination tab in the I/O tag configuration
• if an "event" object group and an object group configurable for I/O tag are mapped to the same I/O tag (in the figure above, Analog Output Status (40) and Analog Output Event (42), both with Index=0, display I/O tag M.DNP3_OutSim_AnalogOutput41_2).

Filtering in the list of objects: the browse window allows you to filter by the values ​​displayed in each column. The values don't have to be entered completely. The notation *FILTERED EXPRESSION* is sufficient, where the asterisks represent any text before the start and end of the expression (e.g., *put* covers both input or output).

Total number of objects: the total number of objects in all object groups is displayed at the bottom left of the window.

Meaning of individual options and buttons:

Copy all to clipboard

Copies all displayed objects and details to the Windows Clipboard.

Refresh

The Refresh button can be used to force the object list to be re-read from the device. By default, the KOM process reads the list of objects and their data types only for the first request and stores it in memory. Depending on the number of objects and the speed of the device, this reading may take a longer time. It then sends these stored list to the CNF process(es), so that the next reading of the list in the DNP3 Item Browser window is fast.

Cancel

D2000 Cnf has a recycling of browser window implemented. If the window is closed by the Cancel button or after selecting an object, it is actually only hidden and is available for browsing for another I/O tag within the same station, so that the list the objects being viewed is preserved. Clicking on the cross at the top right corner will cause the window to actually close.

Tell commands

Literature

IEEE Standard for Electric Power Systems Communications - Distributed Network Protocol (DNP3).

Changes and modifications

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Document revisions

• Ver. 1.0 - May 28, 2019 - creation of document

Requirements