IEC 61158-3-4:2023

IEC 61158-3-4 ®
Edition 4.0 2023-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Industrial communication networks – Fieldbus specifications –
Part 3-4: Data-link layer service definition – Type 4 elements

Réseaux de communication industriels – Spécifications des bus de terrain –
Partie 3-4: Définition des services de la couche liaison de données – Éléments
de type 4
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IEC 61158-3-4 ®
Edition 4.0 2023-03
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Industrial communication networks – Fieldbus specifications –

Part 3-4: Data-link layer service definition – Type 4 elements

Réseaux de communication industriels – Spécifications des bus de terrain –

Partie 3-4: Définition des services de la couche liaison de données – Éléments

de type 4
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 25.040.40, 35.100.20, 35.110 ISBN 978-2-8322-7678-5

– 2 – IEC 61158-3-4:2023 © IEC 2023
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
1.1 General . 7
1.2 Specifications . 7
1.3 Conformance . 7
2 Normative references . 8
3 Terms, definitions, symbols, abbreviated terms and conventions . 8
3.1 Reference model terms and definitions . 8
3.2 Service convention terms and definitions . 9
3.3 Data-link service terms and definitions . 10
3.4 Symbols and abbreviations . 12
3.5 Conventions . 13
4 Data-link service and concepts . 14
4.1 Overview. 14
4.1.1 General . 14
4.1.2 Overview of DL-naming (addressing) . 14
4.2 Types and classes of data-link service . 15
4.3 Functional classes . 15
4.4 Facilities of the connectionless-mode data-link service . 15
4.5 Model of the connectionless-mode data-link service . 15
4.5.1 General . 15
4.5.2 Unconfirmed request . 15
4.5.3 Confirmed request . 16
4.6 Sequence of primitives . 16
4.6.1 Constraints on sequence of primitives . 16
4.6.2 Relation of primitives at the end-points of connectionless service . 17
4.6.3 Sequence of primitives at one DLSAP. 18
4.7 Connectionless-mode data transfer functions . 18
4.7.1 General . 18
4.7.2 Types of primitives and parameters . 18
5 DL-management service . 21
5.1 Scope and inheritance . 21
5.2 Facilities of the DL-management service . 21
5.3 Model of the DL-management service . 21
5.4 Constraints on sequence of primitives . 21
5.5 Set . 22
5.5.1 Function . 22
5.5.2 Types of parameters . 22
5.6 Get . 23
5.6.1 Function . 23
5.6.2 Types of parameters . 23
5.7 Action . 23
5.7.1 Function . 23
5.7.2 Types of parameters . 24
5.7.3 Sequence of primitives . 24

5.8 Event . 25
5.8.1 Function . 25
5.8.2 Types of parameters . 25
Bibliography . 26

Figure 1 – Relationship of PhE, DLE and DLS-users . 14
Figure 2 – Confirmed and unconfirmed UNITDATA request time-sequence diagram . 17
Figure 3 – Repeated confirmed request time-sequence diagram . 17
Figure 4 – State transition diagram for sequences of primitives at one DLSAP . 18
Figure 5 – Sequence of primitives for the DLM action service . 21

Table 1 – Summary of DL-connectionless-mode primitives and parameters . 17
Table 2 – Unitdata transfer primitives and parameters . 18
Table 3 – Control-status error codes . 20
Table 4 – Summary of DL-management primitives and parameters . 22
Table 5 – DLM-Set primitive and parameters . 22
Table 6 – DLM-Get primitive and parameters . 23
Table 7 – DLM-Action primitive and parameters . 24
Table 8 – DLM-Event primitive and parameters . 25

– 4 – IEC 61158-3-4:2023 © IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 3-4: Data-link layer service definition –
Type 4 elements
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
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Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
Attention is drawn to the fact that the use of the associated protocol type is restricted by its
intellectual-property-right holders. In all cases, the commitment to limited release of intellectual-
property-rights made by the holders of those rights permits a layer protocol type to be used with
other layer protocols of the same type, or in other type combinations explicitly
authorized by its intellectual-property-right holders.
NOTE Combinations of protocol Types are specified in the IEC 61784-1 series and the IEC 61784-2 series.
IEC 61158-3-4 has been prepared by subcommittee 65C: Industrial networks, of IEC technical
committee 65: Industrial-process measurement, control and automation. It is an International
Standard.
This fourth edition cancels and replaces the third edition published in 2019. This edition
constitutes a technical revision.

This edition includes the following significant technical change with respect to the previous
edition:
a) Use of extended data size for DLS-user data. This extension is restricted to nodes operating
on a P-NET IP network.
The text of this International Standard is based on the following documents:
Draft Report on voting
65C/1201/FDIS 65C/1242/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all the parts of the IEC 61158 series, under the general title Industrial communication
networks – Fieldbus specifications, can be found on the IEC web site.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC 61158-3-4:2023 © IEC 2023
INTRODUCTION
This document is one of a series produced to facilitate the interconnection of automation system
components. It is related to other standards in the set as defined by the "three-layer" fieldbus
reference model described in IEC 61158-1.
Throughout the set of fieldbus standards, the term "service" refers to the abstract capability
provided by one layer of the OSI Basic Reference Model to the layer immediately above. Thus,
the data-link layer service defined in this document is a conceptual architectural service,
independent of administrative and implementation divisions.

INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 3-4: Data-link layer service definition –
Type 4 elements
1 Scope
1.1 General
This part of IEC 61158 provides common elements for basic time-critical messaging
communications between devices in an automation environment. The term "time-critical" is used
to represent the presence of a time-window, within which one or more specified actions are
required to be completed with some defined level of certainty. Failure to complete specified
actions within the time window risks failure of the applications requesting the actions, with
attendant risk to equipment, plant and possibly human life.
This document defines in an abstract way the externally visible services provided by the Type
4 fieldbus data-link layer in terms of
a) the primitive actions and events of the services;
b) the parameters associated with each primitive action and event, and the form which they
take; and
c) the interrelationship between these actions and events, and their valid sequences.
The purpose of this document is to define the services provided to
• the Type 4 fieldbus application layer at the boundary between the application and data-link
layers of the fieldbus reference model;
• systems management at the boundary between the data-link layer and systems
management of the fieldbus reference model.
1.2 Specifications
The principal objective of this document is to specify the characteristics of conceptual data-link
layer services suitable for time-critical communications, and thus supplement the OSI Basic
Reference Model in guiding the development of data-link protocols for time-critical
communications. A secondary objective is to provide migration paths from previously-existing
industrial communications protocols.
This document can be used as the basis for formal DL-Programming-Interfaces. Nevertheless,
it is not a formal programming interface, and any such interface will need to address
implementation issues not covered by this specification, including
a) the sizes and octet ordering of various multi-octet service parameters;
b) the correlation of paired request and confirm, or indication and response, primitives.
1.3 Conformance
This document does not specify individual implementations or products, nor does it constrain
the implementations of data-link entities within industrial automation systems.
There is no conformance of equipment to this data-link layer service definition standard.
Instead, conformance is achieved through implementation of the corresponding data-link
protocol that fulfills the Type 4 data-link layer services defined in this document.

– 8 – IEC 61158-3-4:2023 © IEC 2023
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
NOTE All parts of the IEC 61158 series, as well as the IEC 61784-1 series and the IEC 61784-2 series are
maintained simultaneously. Cross-references to these documents within the text therefore refer to the editions as
dated in this list of normative references.
ISO/IEC 7498-1, Information technology – Open Systems Interconnection – Basic Reference
Model: The Basic Model
ISO/IEC 7498-3, Information technology – Open Systems Interconnection – Basic Reference
Model: Naming and addressing
ISO/IEC 10731:1994, Information technology – Open Systems Interconnection – Basic
Reference Model – Conventions for the definition of OSI services
3 Terms, definitions, symbols, abbreviated terms and conventions
For the purposes of this document, the following terms, definitions, symbols, abbreviated terms
and conventions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1 Reference model terms and definitions
This document is based in part on the concepts developed in ISO/IEC 7498-1 and
ISO/IEC 7498-3, and makes use of the following terms defined therein.
3.1.1 DL-address [7498-3]
3.1.2 DL-address-mapping [7498-1]
3.1.3 called-DL-address [7498-3]
3.1.4 calling-DL-address [7498-3]
3.1.5 centralized multi-end-point-connection [7498-1]
3.1.6 correspondent (N)-entities [7498-1]
correspondent DL-entities (N=2)
correspondent Ph-entities (N=1)
3.1.7 DL-connection [7498-1]
3.1.8 DL-connection-end-point [7498-1]
3.1.9 DL-connection-end-point-identifier [7498-1]
3.1.10 DL-connection-mode transmission [7498-1]
3.1.11 DL-connectionless-mode transmission [7498-1]
3.1.12 DL-duplex-transmission [7498-1]

3.1.13 (N)-entity [7498-1]
DL-entity (N=2)
Ph-entity (N=1)
3.1.14 DL-facility
[7498-1]
3.1.15 flow control [7498-1]
3.1.16 (N)-layer [7498-1]
DL-layer (N=2)
Ph-layer (N=1)
3.1.17 layer-management [7498-1]
3.1.18 DL-local-view
[7498-3]
3.1.19 DL-name [7498-3]
3.1.20 naming-(addressing)-domain
[7498-3]
3.1.21 primitive name [7498-3]
3.1.22 DL-protocol
[7498-1]
3.1.23 DL-protocol-connection-identifier [7498-1]
3.1.24 DL-protocol-data-unit [7498-1]
3.1.25 DL-relay [7498-1]
3.1.26 Reset
[7498-1]
3.1.27 responding-DL-address [7498-3]
3.1.28 routing
[7498-1]
3.1.29 segmenting [7498-1]
3.1.30 (N)-service [7498-1]
DL-service (N=2)
Ph-service (N=1)
3.1.31 (N)-service-access-point [7498-1]
DL-service-access-point (N=2)
Ph-service-access-point (N=1)
3.1.32 DL-service-access-point-address
[7498-3]
3.1.33 DL-service-connection-identifier [7498-1]
3.1.34 DL-service-data-unit
[7498-1]
3.1.35 DL-simplex-transmission [7498-1]
3.1.36 DL-subsystem [7498-1]
3.1.37 systems-management [7498-1]
3.1.38 DLS-user-data
[7498-1]
3.2 Service convention terms and definitions
This document also makes use of the following terms defined in ISO/IEC 10731 as they apply
to the data-link layer:
– 10 – IEC 61158-3-4:2023 © IEC 2023
3.2.1 acceptor
3.2.2 confirm (primitive);
requestor.deliver (primitive)
3.2.3 deliver (primitive)
3.2.4 DL-confirmed-facility
3.2.5 DL-facility
3.2.6 DL-local-view
3.2.7 DL-mandatory-facility
3.2.8 DL-non-confirmed-facility
3.2.9 DL-service-primitive;
primitive
3.2.10 DL-service-provider
3.2.11 DL-service-user
3.2.12 DLS-user-optional-facility
3.2.13 indication (primitive);
acceptor.deliver (primitive)
3.2.14 request (primitive);
requestor.submit (primitive)
3.2.15 requestor
3.2.16 response (primitive);
acceptor.submit (primitive)
3.2.17 submit (primitive)
3.3 Data-link service terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.3.1
broadcast-node-address
address used to designate all DLEs on a link
Note 1 to entry: All DLEs on a link receive all DLPDUs where the first node-address is equal to the broadcast-node-
address. Such DLPDUs are always unconfirmed, and their receipt is never acknowledged. The value of the broadcast-
node-address is 126.
3.3.2
destination-DL-route
sequence of DL-route-elements, describing the complete route to the destination
Note 1 to entry: This includes both the destination DLSAP and a local component meaningful to the destination
DLS-user.
3.3.3
DL-route-element
octet holding a node DL-address or an address used by the DLS-user

3.3.4
DLSAP
distinctive point at which DL-services are provided by a single DL-entity to a single higher-layer
entity
3.3.5
DL(SAP)-address
individual DLSAP-address, designating a single DLSAP of a single DLS-user
3.3.6
DLS-user address
uniquely identifies a DLS-user locally
3.3.7
frame
denigrated synonym for DLPDU
3.3.8
full DL-route
combination of a destination-DL-route and a source-DL-route
3.3.9
local link
single DL-subnetwork in which any of the connected DLEs may communicate directly, without
any intervening DL-relaying, whenever all of those DLEs that are participating in an instance of
communication are simultaneously attentive to the DL-subnetwork during the period(s) of
attempted communication
3.3.10
maximum-indication-delay
time value that indicates to the DLS-user the maximum time interval for the DLS-user to prepare
a response after receiving an indication requiring a response
Note 1 to entry: If the DLS-user is unable to prepare a response within maximum-indication-delay, the DLS-user is
required to issue a DL-UNITDATA request with a DLSDU type indicating ACKNOWLEDGE. As a result the DLE will
transmit an acknowledging DLPDU on the link.
3.3.11
maximum-retry-time
time value that indicates to the DLE for how long time retransmission of the request can be
performed, as a result of Wait acknowledges from the remote DLE or DLS-user
3.3.12
no-confirm-node-address
node address which indicates that a request or response is unconfirmed
Note 1 to entry: The value of the no-confirm-node-address is 0.
3.3.13
node
single DL-entity as it appears on one local link
3.3.14
node-address
value that uniquely identifies a DLE on a link
Note 1 to entry: The value of a Node-address is in the range of 0-127. The values 0, 126 and 127 are reserved for
special purposes.
– 12 – IEC 61158-3-4:2023 © IEC 2023
3.3.15
normal class device
device which replies to requests from other normal class devices, and initiates transmissions
Note 1 to entry: Such a device can act as a server (responder) and as a client (requestor) – this is also called a
peer.
3.3.16
receiving DLS-user
DL-service user that acts as a recipient of DLS-user-data
Note 1 to entry: A DL-service user can be concurrently both a sending and receiving DLS-user.
3.3.17
sending DLS-user
DL-service user that acts as a source of DLS-user-data
3.3.18
service-node-address
address reserved for service purposes only
Note 1 to entry: All DLEs on a link receive all DLPDUs where the first Node-address is equal to the service-node-
address. Such DLPDUs can be Confirmed or Unconfirmed, and it is possible that their receipt can be acknowledged,
or not. The service-node-address can be used on links with only two DLEs – the requesting Normal class DLE and
the responding simple-class or normal-class DLE. The value of the service-node-address is 127.
3.3.19
simple-class device
device which replies to requests from normal class devices
Note 1 to entry: Such a device can act as a server or responder only.
3.3.20
source-DL-route
holds a sequence of DL-route-elements, describing the complete route back to the source
3.4 Symbols and abbreviations
NOTE Many symbols and abbreviations are common to more than one protocol Type; they are not necessarily used
by all protocol Types.
DL- Data-link layer (as a prefix)
DLC DL-connection
DLCEP DL-connection-end-point
DLE DL-entity (the local active instance of the data-link layer)
DLL DL-layer
DLPCI DL-protocol-control-information
DLPDU DL-protocol-data-unit
DLM DL-management
DLME DL-management Entity (the local active instance of DL-management)
DLMS DL-management Service
DLS DL-service
DLSAP DL-service-access-point
DLSDU DL-service-data-unit
FIFO First-in first-out (queuing method)
OSI Open systems interconnection

Ph- Physical layer (as a prefix)
PhE Ph-entity (the local active instance of the physical layer)
PhL Ph-layer
QoS Quality of service
3.5 Conventions
This document uses the descriptive conventions given in ISO/IEC 10731.
The service model, service primitives, and time-sequence diagrams used are entirely abstract
descriptions; they do not represent a specification for implementation.
Service primitives, used to represent service user/service provider interactions (see
ISO/IEC 10731), convey parameters that indicate information available in the user/provider
interaction.
This document uses a tabular format to describe the component parameters of the DLS
primitives. The parameters that apply to each group of DLS primitives are set out in tables
throughout the remainder of this document. Each table consists of up to six columns, containing
the name of the service parameter, and a column each for those primitives and parameter-
transfer directions used by the DLS:
– the request primitive’s input parameters;
– the request primitive’s output parameters;
– the indication primitive’s output parameters;
– the response primitive’s input parameters; and
– the confirm primitive’s output parameters.
NOTE The request, indication, response and confirm primitives are also known as requestor.submit,
acceptor.deliver, acceptor.submit, and requestor.deliver primitives, respectively (see ISO/IEC 10731).
One parameter (or part of it) is listed in each row of each table. Under the appropriate service
primitive columns, a code is used to specify the type of usage of the parameter on the primitive
and parameter direction specified in the column:
M parameter is mandatory for the primitive.
U parameter is a User option, and can be provided or not depending on the
dynamic usage of the DLS-user. When not provided, a default value for the
parameter is assumed.
C  parameter is conditional upon other parameters or upon the environment of
the DLS-user.
(blank) parameter is never present.
Items in brackets further qualify some entries. These may be
a) a parameter-specific constraint
(=) indicates that the parameter is semantically equivalent to the parameter in the service
primitive to its immediate left in the table.
b) an indication that some note applies to the entry
(n) indicates that the following note n contains additional information pertaining to the
parameter and its use.
In any particular interface, not all parameters need be explicitly stated. Some may be implicitly
associated with the DLSAP at which the primitive is issued.

– 14 – IEC 61158-3-4:2023 © IEC 2023
In the diagrams that illustrate these interfaces, dashed lines indicate cause-and-effect or time-
sequence relationships, and wavy lines indicate that events are roughly contemporaneous.
4 Data-link service and concepts
4.1 Overview
4.1.1 General
The DLS provides for the transparent transfer of data between DLS-users. It makes the way
that supporting communications resources are utilized invisible to these DLS-users.
In particular, the DLS provides for the following:
a) Transparency of transferred information. The DLS provides for the transparent transfer of
DLS-user-data. It does not restrict the content, format or coding of the DLSDUs, nor does it
interpret the structure or meaning of that information. It can, however, restrict the amount
of information that can be transferred as an indivisible unit.
NOTE It is possible for a DLS-user to segment arbitrary-length data into limited-length DLSDUs before making
DLS requests, and afterwards reassemble received DLSDUs into these larger data units.
b) Reliable transfer of data. The DLS relieves the DLS-user from concerns regarding insertion,
corruption, loss or duplication of data.
c) Prioritized data transfer. The DLS provides DLS-users with a means to prioritize requests.
d) Queue. The DLS provides the requesting DLS-user with a prioritized FIFO queue, where
each queue item can hold a single DLSDU.
4.1.2 Overview of DL-naming (addressing)
A DLE is implicitly connected to a single PhE, and (separately) to a single DLSAP and
associated DLS-user. A DLE always delivers received DLSDUs at the same DLSAP, and hence
to the same DLS-user. This concept is illustrated in Figure 1.

Figure 1 – Relationship of PhE, DLE and DLS-users
Each DLE has a node DL-address. Node DL-addresses uniquely identify DLEs within the local
Link.
A DL-route-element is an octet, which can hold either a node DL-address or a higher-layer
address used by the DLS-user.
A destination-DL-route holds a sequence of DL-route-elements, describing the complete route
to the destination DLSAP plus a local component meaningful to the destination DLS-user.
A source-DL-route holds a sequence of DL-route-elements, describing the complete route back
to the source DLSAP plus a local component meaningful to the source DLS-user.
A full DL-route is defined as a destination-DL-route and a source-DL-route.
4.2 Types and classes of data-link service
There are two types of DLS as follows:
– a connectionless-mode data transfer service, providing confirmed and unconfirmed data
transfer (defined in 4.5.2 and 4.5.3);
– a management service. The Type 4 management service provides services for reading and
writing managed objects (DLM-SET and DLM-GET requests), as defined in Clause 5.
4.3 Functional classes
The functional class of a DLE determines its capabilities, and thus the complexity of conforming
implementations. Two functional classes are defined as follows:
a) simple-class, including only responder functionality (server);
b) normal-class, including initiator and responder functionality (client and server, also called
peer).
4.4 Facilities of the connectionless-mode data-link service
The DLS provides a means of transferring DLSDUs of limited length from one source DLS-user
to one or more destination DLS-users. The transfer of DLSDUs is transparent, in that the
boundaries of DLSDUs and the contents of DLSDUs are preserved unchanged by the DLS, and
there are no constraints on the DLSDU (other than limited length) imposed by the DLS.
4.5 Model of the connectionless-mode data-link service
4.5.1 General
A defining characteristic of data-link connectionless-mode unitdata transmission is the
independent nature of each invocation of the DLS.
Only one type of object, the unitdata object, can be submitted to the DLS-provider for
transmission.
The DLS-user issuing a request primitive specifies whether the request is to be confirmed by
the remote DLS-user, or not. This is specified in the destination-DL-route and source-DL-route
parameters of the DL-UNITDATA request primitive. If the remote DLS-user confirms a request, it
does this by issuing a new, independent DL-UNITDATA request primitive.
4.5.2 Unconfirmed request
The DLE of the requesting DLS-user forms a DLPDU, which includes the submitted DLSDU and
sends the DLPDU to the receiving DLE. The receiving DLE delivers the received DLSDU to the
DLS-user by a DL-UNITDATA indication primitive. The value of the confirmation-expected
parameter of this indication is FALSE.

– 16 – IEC 61158-3-4:2023 © IEC 2023
4.5.3 Confirmed request
The DLE of the requesting DLS-user forms a DLPDU, which includes the submitted DLSDU and
sends the DLPDU to the receiving DLE. The receiving DLE delivers the received DLSDU to the
DLS-user by a DL-UNITDATA indication primitive. The value of the confirmation-expected
parameter of this indication is TRUE.
If the receiving DLS-user is unable to handle the indication immediately, the receiving DLS-user
should issue a DL-UNITDATA response primitive within the time specified by maximum-
indication-delay.
If the receiving DLS-user either
NITDATA response primitive or a DL-UNITDATA request primitive
a) does not reply with a DL-U
within the interval maximum-indication-delay from receipt of the triggering DL-UNITDATA
indication primitive, or
b) does reply with a DL-UNITDATA response primitive within the interval maximum-indication-
NITDATA indication primitive,
delay from receipt of the triggering DL-U
then the receiving DLE transmits an acknowledging DLPDU to the original requesting DLE. The
following actions depend on whether the replying DLE is of simple-class or normal-class.
1) If the replying DLE is of simple-class, the acknowledge DLPDU from the replying DLE
AIT". In this case, the original requesting DLE requeues the original request
specifies "W
DLPDU at the lowest possible priority for retransmission at the next opportunity. When the
replying DLS-user has prepared the response, it should await the repeated request from the
original requesting DLE, and this time reply by issuing a DL-UNITDATA request primitive
within the time interval maximum-indication-delay.
The action in the original requesting DLE of requeuing the original request for retransmission
is repeated as long as the replying DLE keeps responding with "WAIT" acknowledges, or
until retransmission has been attempted for the time interval specified in the maximum-retry-
time configuration parameter.
2) If the replying DLE is of Normal class, the acknowledge DLPDU from the replying DLE
specifies "RESPONSE COMES LATER / ACKNOWLEDGE". In this case, the original requesting
DLE does nothing further. When the DLS-user at the replying DLE has prepared the
response, it should reply by issuing a DL-UNITDATA request primitive. The replying DLE
forms an appropriate DLPDU and queues it for transmission at the first opportunity.
4.6 Sequence of primitives
4.6.1 Constraints on sequence of primitives
Subclause 4.6.1 defines the constraints on the sequence in which the primitives defined in 4.6.2
and Table 1 can occur. The constraints determine the order in which primitives occur, but do
not fully specify when they can occur.

Table 1 – Summary of DL-connectionless-mode primitives and parameters
Service Service subtype Primitive Parameter
Data Transfer Unitdata DL-UNITDATA request (in Destination-DL-route,
Source-DL-route,
Priority,
Maximum-retry-time,
Control status,
Data field format,
DLSDU)
DL-UNITDATA indication (out Destination-DL-route,
Source-DL-route,
Confirmation-expected,
Control status,
Data field format,
DLSDU)
DL-UNITDATA response (in Destination-DL-route,
Source-DL-route)
4.6.2 Relation of primitives at the end-points of connectionless service
4.6.2.1 General
A request primitive issued at one DLSAP will have consequences at one or more other DLSAPs.
These relations are summarized in Figure 2 and Figure 3.
4.6.2.2 Confirmed and unconfirmed UNITDATA request

Figure 2 – Confirmed and unconfirmed UNITDATA request time-sequence diagram
4.6.2.3 Repeated confirmed UNITDATA request

Figure 3 – Repeated confirmed request time-sequence diagram

– 18 – IEC 61158-3-4:2023 © IEC 2023
4.6.3 Sequence of primitives at one DLSAP
The possible overall sequences of primitives at one DLSAP are defined in the state transition
diagram of Figure 4.
NOTE Since there is no conformance to this document, the use of a state transition diagram to describe the
allowable sequences of service primitives does not impose any requirements or constraints on the internal
organization of any implementation of the service.

Figure 4 – State transition diagram for sequences of primitives at one DLSAP
4.7 Connectionless-mode data transfer functions
4.7.1 General
DL-connectionless-mode unitdata service primitives are used to transmit independent DLSDUs
from one DLS-user to one or more other DLS-users. Each DLSDU is transmitted in a single
DLPDU. The DLSDU is independent in the sense that it bears no relationship to any other
DLSDU transmitted through another invocation of the DL-service by the same DLS-user. The
DLSDU is self-contained in that all the information required to deliver the DLSDU is presented
to the DL-provider, together with the user data to be transmitted, in a single service access.
4.7.2 Types of primitives and parameters
4.7.2.1 General
Table 2 indicates the types of primitives and the parameters needed for the DL-connectionless-
mode unitdata service.
Table 2 – Unitdata transfer primitives and parameters
DL-UNITDATA
...