IEC 61158-4-21:2019

IEC 61158-4-21 ®
Edition 2.0 2019-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Industrial communication networks – Fieldbus specifications –
Part 4-21: Data-link layer protocol specification – Type 21 elements

Réseaux de communication industriels – Spécifications des bus de terrain –
Partie 4-21: Spécification du protocole de la couche liaison de données –
Éléments de type 21
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IEC 61158-4-21 ®
Edition 2.0 2019-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Industrial communication networks – Fieldbus specifications –

Part 4-21: Data-link layer protocol specification – Type 21 elements

Réseaux de communication industriels – Spécifications des bus de terrain –

Partie 4-21: Spécification du protocole de la couche liaison de données –

Éléments de type 21
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 25.040.40; 35.100.20; 35.110 ISBN 978-2-8322-9146-7

– 2 – IEC 61158-4-21:2019 © IEC 2019
CONTENTS
FOREWORD . 7
INTRODUCTION . 9
1 Scope . 10
1.1 General . 10
1.2 Specifications . 10
1.3 Procedures . 10
1.4 Applicability . 10
1.5 Conformance . 10
2 Normative references . 11
3 Terms, definitions, symbols and abbreviations . 11
3.1 Reference model terms and definitions . 11
3.2 Service convention terms and definitions . 13
3.3 Common terms and definitions . 14
3.4 Additional Type 21 definitions . 17
3.5 Common symbols and abbreviations . 17
3.6 Additional Type 21 symbols and abbreviations . 18
4 Overview of the data-link protocol . 19
4.1 General . 19
4.2 Overview of medium access control . 19
4.3 Service assumed from the physical layer . 19
4.4 DLL architecture . 20
4.4.1 General . 20
4.4.2 DLL management (DLM) interface support function . 21
4.5 Data type . 22
4.5.1 General . 22
4.5.2 Boolean . 22
4.5.3 Unsigned integer . 22
4.5.4 Signed integer . 23
4.5.5 Octet String . 23
4.5.6 Visible String . 23
4.5.7 Time of day . 24
4.6 Local parameters and variables . 24
4.6.1 General . 24
4.6.2 DLE configuration parameters . 24
4.6.3 Queues to support data transfer . 25
4.6.4 Variables to support SAP management . 26
4.6.5 Variables to support local device information management . 27
4.6.6 Variables and counter to support network information management . 31
4.6.7 Variables and counter to support a device path information
management . 35
4.6.8 Variables, counters, timers, and queues to support path table
management . 39
5 General structure and encoding . 39
5.1 Overview . 39
5.2 MAPDU structure and encoding . 39
5.3 Common MAC frame structure, encoding and elements of procedure . 40
5.3.1 MAC frame structure . 40

5.3.2 Elements of the MAC frame . 40
5.3.3 Elements of the Type 21 DLPDU . 41
5.4 Order of bit transmission . 49
5.5 Invalid DLPDU . 49
6 DLPDU structure and procedure . 49
6.1 General . 49
6.2 Common DLPDU Field . 49
6.2.1 General . 49
6.2.2 Version . 50
6.2.3 Length . 50
6.3 DL-DATA Transfer . 50
6.3.1 DT DLPDU . 50
6.4 DL-SPDATA Transfer . 53
6.4.1 SPDT DLPDU . 53
6.5 Network control messages . 54
6.5.1 General . 54
6.5.2 NCM_LA DLPDU . 54
6.5.3 NCM_AT DLPDU . 56
6.5.4 NCM_LS DLPDU . 57
6.5.5 NCM_RS DLPDU . 58
6.5.6 NCM_AR_DLPDU . 59
6.5.7 NCM_AR DLPDU structure . 59
7 DLE elements of procedure . 60
7.1 Overall structure . 60
7.2 DL-protocol machine (DLPM) . 61
7.2.1 Overview . 61
7.2.2 Primitive definitions . 61
7.2.3 DLPM state table . 64
7.2.4 DLPM functions . 68
7.3 DLL management Protocol . 69
7.3.1 Overview . 69
7.3.2 Primitive definitions . 69
7.3.3 DLM state table . 72
7.3.4 DLM functions . 96
8 Constants and error codes . 105
8.1 General . 105
8.2 Constants . 106
8.3 Data-link layer error codes . 107
Bibliography . 108

Figure 1 – Interaction of PhS primitives with DLE . 20
Figure 2 – Data-link layer architecture . 21
Figure 3 – Relationships of DLSAPs, DLSAP-addresses, and group DL-addresses . 26
Figure 4 – Common MAC frame format for Type 21 DLPDU . 40
Figure 5 – MAC frame format for other protocols . 40
Figure 6 – Version and Length field . 41
Figure 7 – DST_addr field . 42

– 4 – IEC 61158-4-21:2019 © IEC 2019
Figure 8 – SRC_addr field. 43
Figure 9 – Frame Control Field . 43
Figure 10 – Extension field . 46
Figure 11 – DSAP field . 47
Figure 12 – Source service access point field . 48
Figure 13 – Length of group mask and extension information . 48
Figure 14 – Group mask option field . 48
Figure 15 – Common DLPDU field . 50
Figure 16 – Building a DT DLPDU . 50
Figure 17 – DT DLPDU structure . 50
Figure 18 – SPDT DLPDU structure . 53
Figure 19 – NCM_LA DLPDU structure . 55
Figure 20 – DLL structure and elements . 60
Figure 21 – State transition diagram of the DLPM . 64
Figure 22 – State transition diagram of DLM . 73

Table 1 – DLL components . 20
Table 2 – UNSIGNEDn data type . 22
Table 3 – INTEGERn data type . 23
Table 4 – DLE configuration parameters . 25
Table 5 – Queues to support data transfer . 25
Table 6 – Variables to support SAP management . 26
Table 7 – Variables to support device information management . 27
Table 8 – DL–entity identifier . 28
Table 9 – Device Flags . 28
Table 10 – DLM state . 28
Table 11 – Device Unique Identification . 29
Table 12 – Unique identification of device connected to R-port1 . 29
Table 13 – Unique identification of device connected to R-port2 . 29
Table 14 – MAC address. 29
Table 15 – Port information . 30
Table 16 – Protocol version . 30
Table 17 – Device type . 31
Table 18 – Device description . 31
Table 19 – Hop count . 31
Table 20 – Variables to support managing network information . 32
Table 21 – Topology . 32
Table 22 – Collision count . 32
Table 23 – Device count . 33
Table 24 – Topology change count . 33
Table 25 – Last topology change time . 33
Table 26 – RNMP device UID . 33
Table 27 – RNMS device UID . 34

Table 28 – LNM device UID for R-port1 . 34
Table 29 – LNM device UID for R-port2 . 34
Table 30 – Network flags . 35
Table 31 – Variables and counter to support managing path information . 36
Table 32 – Hop count for R-port1 direction . 36
Table 33 – Hop count for R-port2 direction . 37
Table 34 – Preferred R-port . 37
Table 35 – Destination R-port . 37
Table 36 – In net count . 38
Table 37 – In net time . 38
Table 38 – Out net count . 39
Table 39 – Out net time . 39
Table 40 – Version and Length . 42
Table 41 – Destination DL–entity identifier . 42
Table 42 – Source DL–entity identifier . 43
Table 43 – Frame control . 44
Table 44 – Extension . 47
Table 45 – Destination service access point . 47
Table 46 – source service access point. 48
Table 47 – DT DLPDU parameters . 51
Table 48 – Primitives exchanged between DLS-user and DLE to send a DT DLPDU . 52
Table 49 – Primitives exchanged between DLS-user and DLEs to receive a DT DLPDU . 53
Table 50 – SPDT DLPDU Parameters . 53
Table 51 – Primitive exchanged between DLS-User and DLEs to send an SPDT
DLPDU . 54
Table 52 – Primitives exchanged between DLS-user and DLEs to receive an SPDT
DLPDU . 54
Table 53 – NCM_LA DLPDU parameters . 55
Table 54 – NCM_AT DLPDU parameters . 56
Table 55 – NCM_LS DLPDU parameters . 57
Table 56 – NCM_RS DLPDU parameters . 58
Table 57 – NCM_AR DLPDU parameters . 59
Table 58 – Primitives exchanged between DLPM and DLS-user . 61
Table 59 – Parameters exchanged between DLPM and DLS-user . 62
Table 60 – Primitives exchanged between DLPM and DLM . 63
Table 61 – Parameters used with primitives exchanged between DLPM and DLM . 64
Table 62 – DLPM state table . 65
Table 63 – DLPM functions table . 68
Table 64 – Primitives exchanged between DLM and DLS-user . 70
Table 65 – Parameters used with primitives exchanged between DLM and DLS-user. 71
Table 66 – Primitive exchanged between DLM and DMAC . 71
Table 67 – Parameters used with primitives exchanged between DLM and DMAC . 72
Table 68 – Primitive exchanged between DLM and DPHY . 72
Table 69 – Parameters used with primitives exchanged between DLM and DPHY . 72

– 6 – IEC 61158-4-21:2019 © IEC 2019
Table 70 – DLM state table . 74
Table 71 – DLM function table . 96
Table 72 – DLL constants . 106
Table 73 – Type 21 DLL error codes . 107

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 4-21: Data-link layer protocol specification –
Type 21 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 in addition to other activities, IEC publishes International Standards, Technical Specifications,
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Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
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with the International Organization for 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
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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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
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
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 IEC 61784-1 and IEC 61784-2.
International Standard IEC 61158-4-21 has been prepared by subcommittee 65C: Industrial
networks, of IEC technical committee 65: Industrial process measurement, control and
automation.
This second edition cancels and replaces the first edition published in 2010. This edition
constitutes a technical revision.

– 8 – IEC 61158-4-21:2019 © IEC 2019
This edition includes the following significant technical changes with respect to the previous
edition:
• added Frame control Value, DLM function and DLL constants;
• changed NCM_RETRY_RNMS to NCM_CHECK_NET_INTEGRITY_REQ;
• updated DLM state table;
• miscellaneous editorial corrections.
The text of this International Standard is based on the following documents:
FDIS Report on voting
65C/946/FDIS 65C/955/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 parts of the IEC 61158 series, published 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 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.
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.
The data-link protocol provides the data-link service by making use of the services available
from the physical layer. The primary aim of this document is to provide a set of rules for
communication expressed in terms of the procedures to be carried out by peer data-link
entities (DLEs) at the time of communication. These rules for communication are intended to
provide a sound basis for development in order to serve a variety of purposes:
a) as a guide for implementors and designers;
b) for use in the testing and procurement of equipment;
c) as part of an agreement for the admittance of systems into the open systems environment;
d) as a refinement to the understanding of time-critical communications within OSI.
This document is concerned, in particular, with the communication and interworking of
sensors, effectors and other automation devices. By using this document together with other
standards positioned within the OSI or fieldbus reference models, otherwise incompatible
systems may work together in any combination.
NOTE Use of some of the associated protocol types is restricted by their 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 particular data-link layer protocol type to be used with physical layer and application layer protocols in type
combinations as specified explicitly in the profile parts. Use of the various protocol types in other combinations
may require permission of their respective intellectual-property-right holders.
The International Electrotechnical Commission (IEC) draws attention to the fact that it is
claimed that compliance with this document may involve the use of patents concerning
Type 21 elements and possibly other types given in Subclause 4.1, 4.2 and 7.3 as follows:
KR 0789444 [LS] A communication packet processing apparatus and method for
ring topology Ethernet network capable of preventing permanent
packet looping
KR 0732510 [LS] Network system
KR 0870670 [LS] Method for determining a Ring Manager Node
IEC takes no position concerning the evidence, validity and scope of these patent rights.
The holder of these patent rights has assured the IEC that he/she is willing to negotiate
licences either free of charge or under reasonable and non-discriminatory terms and
conditions with applicants throughout the world. In this respect, the statement of the holder of
these patent rights is registered with IEC. Information may be obtained from:
[LS]: LS Industrial Systems Co., Ltd.
LS Tower 1026-6
Hogye-dong, Dongan-gu,
Anyang-si, Gyeonggi-do 431-848
Republic of Korea
Attention is drawn to the possibility that some of the elements of this document may be the
subject of patent rights other than those identified above. IEC shall not be held responsible for
identifying any or all such patent rights.
ISO (www.iso.org/patents) and IEC (http://www.iec.ch/tctools/patent_decl.htm) maintain on-
line data bases of patents relevant to their standards. Users are encouraged to consult the
data bases for the most up to date information concerning patents.

– 10 – IEC 61158-4-21:2019 © IEC 2019
INDUSTRIAL COMMUNICATION NETWORKS –
FIELDBUS SPECIFICATIONS –
Part 4-21: Data-link layer protocol specification –
Type 21 elements
1 Scope
1.1 General
The DLL provides basic time-critical data communications between devices in an automated
environment. Type 21 provides priority-based cyclic and acyclic data communication using an
internal collision-free, full-duplex dual-port Ethernet switch technology. For wide application in
various automation applications, Type 21 does not restrict the cyclic/acyclic scheduling policy
in the DLL.
1.2 Specifications
This document describes:
a) procedures for the timely transfer of data and control information from one data link user
entity to a peer user entity, and among the data link entities forming the distributed data
link service provider;
b) procedures for giving communication opportunities based on ISO/IEC/IEEE 8802-3 MAC,
with provisions for nodes to be added or removed during normal operation;
c) structure of the fieldbus data link protocol data units (DLPDUs) used for the transfer of
data and control information by the protocol of this document, and their representation as
physical interface data units.
1.3 Procedures
The procedures are defined in terms of:
a) the interactions between peer data link entities (DLEs) through the exchange of fieldbus
DLPDUs;
b) the interactions between a data link service (DLS) provider and a DLS-user in the same
system through the exchange of DLS primitives;
c) the interactions between a DLS-provider and a physical layer service provider in the same
system through the exchange of Ph-service primitives.
1.4 Applicability
These procedures are applicable to instances of communication between systems that
support time-critical communications services in the data link layer of the OSI or fieldbus
reference models, and that require the ability to interconnect in an open systems
interconnection environment. Profiles provide a simple multi-attribute means of summarizing
an implementation’s capabilities, and thus its applicability to various time-deterministic
communications needs.
1.5 Conformance
This document also specifies conformance requirements for systems implementing these
procedures. This document does not contain tests to demonstrate compliance with such
requirements.
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.
IEC 61158-3-21:2019, Industrial Communication Networks – Fieldbus specifications –
Part 3-21: Data-link layer service definition – Type 21 elements
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/IEEE 8802-3:2017, Information technology – Telecommunications and information
exchange between systems – Local and metropolitan area networks – Specific requirements –
Part 3: Standard for Ethernet
ISO/IEC 10731, Information technology – Open Systems Interconnection – Basic Reference
Model – Conventions for the definition of OSI services
3 Terms, definitions, symbols and abbreviations
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.

– 12 – IEC 61158-4-21:2019 © IEC 2019
3.1.1 called-DL-address [ISO/IEC 7498-3]
3.1.2 calling-DL-address
[ISO/IEC 7498-3]
3.1.3 centralized multi-end-point-connection [ISO/IEC 7498-1]
3.1.4 correspondent (N)-entities [ISO/IEC 7498-1]
correspondent DL-entities  (N=2)
correspondent Ph-entities  (N=1)
3.1.5 demultiplexing [ISO/IEC 7498-1]
3.1.6 DL-address
[ISO/IEC 7498-3]
3.1.7 DL-address-mapping [ISO/IEC 7498-1]
3.1.8 DL-connection
[ISO/IEC 7498-1]
3.1.9 DL-connection-end-point [ISO/IEC 7498-1]
3.1.10 DL-connection-end-point-identifier
[ISO/IEC 7498-1]
3.1.11 DL-connection-mode transmission [ISO/IEC 7498-1]
3.1.12 DL-connectionless-mode transmission [ISO/IEC 7498-1]
3.1.13 DL-data-sink [ISO/IEC 7498-1]
3.1.14 DL-data-source
[ISO/IEC 7498-1]
3.1.15 DL-duplex-transmission [ISO/IEC 7498-1]
3.1.16 DL-facility
[ISO/IEC 7498-1]
3.1.17 DL-local-view [ISO/IEC 7498-3]
3.1.18 DL-name
[ISO/IEC 7498-3]
3.1.19 DL-protocol [ISO/IEC 7498-1]
3.1.20 DL-protocol-connection-identifier
[ISO/IEC 7498-1]
3.1.21 DL-protocol-control-information [ISO/IEC 7498-1]
3.1.22 DL-protocol-data-unit
[ISO/IEC 7498-1]
3.1.23 DL-protocol-version-identifier [ISO/IEC 7498-1]
3.1.24 DL-relay
[ISO/IEC 7498-1]
3.1.25 DL-service-connection-identifier [ISO/IEC 7498-1]
3.1.26 DL-service-data-unit
[ISO/IEC 7498-1]
3.1.27 DL-simplex-transmission [ISO/IEC 7498-1]
3.1.28 DL-subsystem
[ISO/IEC 7498-1]
3.1.29 DL-user-data [ISO/IEC 7498-1]
3.1.30 flow control [ISO/IEC 7498-1]
3.1.31 layer-management [ISO/IEC 7498-1]
3.1.32 multiplexing
[ISO/IEC 7498-3]
3.1.33 naming-(addressing)-authority [ISO/IEC 7498-3]
3.1.34 naming-(addressing)-domain
[ISO/IEC 7498-3]
3.1.35 naming-(addressing)-subdomain [ISO/IEC 7498-3]
3.1.36 (N)-entity [ISO/IEC 7498-1]
DL-entity
Ph-entity
3.1.37 (N)-interface-data-unit [ISO/IEC 7498-1]
DL-service-data-unit  (N=2)
Ph-interface-data-unit  (N=1)
3.1.38 (N)-layer
[ISO/IEC 7498-1]
DL-layer  (N=2)
Ph-layer  (N=1)
3.1.39 (N)-service [ISO/IEC 7498-1]
DL-service  (N=2)
Ph-service  (N=1)
3.1.40 (N)-service-access-point
[ISO/IEC 7498-1]
DL-service-access-point  (N=2)
Ph-service-access-point  (N=1)
3.1.41 (N)-service-access-point-address [ISO/IEC 7498-1]
DL-service-access-point-address  (N=2)
Ph-service-access-point-address  (N=1)
3.1.42 peer-entities
[ISO/IEC 7498-1]
3.1.43 Ph-interface-control-information [ISO/IEC 7498-1]
3.1.44 Ph-
...