IEC 61158-3-4:2019

IEC 61158-3-4 ®
Edition 3.0 2019-04
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 couche liaison de données – Éléments
de type 4
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IEC 61158-3-4 ®
Edition 3.0 2019-04
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 couche liaison de données – Éléments

de type 4
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 25.040.40; 35.100.20; 35.110 ISBN 978-2-8322-9110-8

– 2 – IEC 61158-3-4:2019 © IEC 2019
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, abbreviations and conventions . 8
3.1 Reference model terms and definitions . 8
3.2 Service convention terms and definitions . 10
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 . 16
4.5 Model of the connectionless-mode data-link service . 16
4.5.1 General . 16
4.5.2 Unconfirmed request . 16
4.5.3 Confirmed request . 16
4.6 Sequence of primitives . 17
4.6.1 Constraints on sequence of primitives . 17
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 . 23
5.7.3 Sequence of primitives . 24

5.8 Event . 24
5.8.1 Function . 24
5.8.2 Types of parameters . 24
Bibliography . 26

Figure 1 – Relationship of PhE, DLE and DLS-users . 15
Figure 2 – Confirmed and unconfirmed UNITDATA request time-sequence diagram . 17
Figure 3 – Repeated confirmed request time-sequence diagram . 18
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 . 19
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 61158-3-4:2019 © IEC 2019
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
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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
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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
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
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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 IEC 61784-1 and IEC 61784-2.
International Standard IEC 61158-3-4 has been prepared by subcommittee 65C: Industrial
networks, of IEC technical committee 65: Industrial-process measurement, control and
automation.
This third edition cancels and replaces the second edition published in 2014. This edition
constitutes a technical revision.

This edition includes the following significant technical changes with respect to the previous
edition:
a) additional user parameters to services;
b) additional services to support distributed objects;
c) additional secure services;
The text of this International Standard is based on the following documents:
FDIS Report on voting
65C/945/FDIS 65C/954/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This publication has been drafted in accordance with ISO/IEC Directives, Part 2.
A list of all the parts of the IEC 61158 series, published under the general title Industrial
communication networks – Fieldbus specifications can be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC 61158-3-4:2019 © IEC 2019
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 International Standard 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 specification may 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 1 data-link layer services defined in this document.

– 8 – IEC 61158-3-4:2019 © IEC 2019
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 IEC 61784-1 and IEC 61784-2 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, abbreviations and conventions
For the purposes of this document, the following terms, definitions, symbols, abbreviations
and conventions apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://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 DL-connection
[7498-1]
3.1.7 DL-connection-end-point [7498-1]
3.1.8 DL-connection-end-point-identifier
[7498-1]
3.1.9 DL-connection-mode transmission [7498-1]
3.1.10 DL-connectionless-mode transmission
[7498-1]
3.1.11 correspondent (N)-entities [7498-1]
correspondent DL-entities  (N=2)
correspondent Ph-entities  (N=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]
– 10 – IEC 61158-3-4:2019 © IEC 2019
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:
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 may be
performed, as a result of Wait acknowledges from the remote DLE or DLS-user

– 12 – IEC 61158-3-4:2019 © IEC 2019
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
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 their receipt may or may not be acknowledged. 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 may or may not be provided 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

– 14 – IEC 61158-3-4:2019 © IEC 2019
(=) 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.
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 may, 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.

Application
Layer
DLS-user DLS-user
DLSAP DLSAP
Data Link
DLE DLE
Layer
Physical
PhE PhE
Layer
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).
– 16 – IEC 61158-3-4:2019 © IEC 2019
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
FALSE.
parameter of this indication is
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
specifies “WAIT”. In this case, the original requesting DLE requeues the original request
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
ESPONSE COMES LATER / ACKNOWLEDGE”. In this case, the original requesting
specifies “R
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 may occur. The constraints determine the order in which primitives occur,
but do not fully specify when they may 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
Initiator   Responder
DL-UNITDATA request
DL-UNITDATA indication
Figure 2 – Confirmed and unconfirmed UNITDATA request time-sequence diagram

– 18 – IEC 61158-3-4:2019 © IEC 2019
4.6.2.3 Repeated confirmed UNITDATA request
Initiator   Responder
DL-UNITDATA request
DL-UNITDATA indication
DL-UNITDATA response
DL-UNITDATA indication
Figure 3 – Repeated confirmed request time-sequence diagram
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.
Idle
DL-UNITDATA request, response or indication

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 pr
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