IEC 61158-1:2014

IEC 61158-1 ®
Edition 1.0 2014-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Industrial communication networks – Fieldbus specifications –
Part 1: Overview and guidance for the IEC 61158 and IEC 61784 series

Réseaux de communication industriels – Spécifications des bus de terrain –
Partie 1: Présentation et lignes directrices des séries CEI 61158 et CEI 61784

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IEC 61158-1 ®
Edition 1.0 2014-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Industrial communication networks – Fieldbus specifications –

Part 1: Overview and guidance for the IEC 61158 and IEC 61784 series

Réseaux de communication industriels – Spécifications des bus de terrain –

Partie 1: Présentation et lignes directrices des séries CEI 61158 et CEI 61784

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX XC
ICS 25.040.40; 33.040; 35.100.05 ISBN 978-2-8322-1630-9

– 2 – IEC 61158-1:2014 © IEC 2014

CONTENTS
FOREWORD . 4

1 Scope . 6

2 Normative references . 6

3 Terms, definitions and abbreviated terms . 6

3.1 Terms and definitions . 6

3.2 Abbreviations . 7

Guidelines for implementers and users . 7
4.1 Background and purpose . 7
4.2 Supported options . 8
4.3 Benefits from using a common and formal style . 8
Concept of the IEC 61158 series . 9
6 Mapping onto the OSI Basic Reference Model . 11
6.1 Overview . 11
6.2 Physical layer service and protocol. 11
6.3 Data-link layer service . 12
6.4 Data-link layer protocol . 13
6.5 Application layer service . 13
6.6 Application layer protocol . 14
Structure of IEC 61158 and IEC 61784 series . 15
7.1 The IEC 61158 physical layer . 15
7.2 The IEC 61158 data-link layer . 15
7.3 The IEC 61158 application layer . 16
7.4 IEC 61784-1 and IEC 61784-2 fieldbus profiles . 16
7.5 IEC 61784-3 functional safety communication profiles . 20
7.6 IEC 61784-5 installation profiles . 22
7.7 Communication profiles for wireless communication networks . 24
8 Brief summary of the characteristics of service and protocol for each fieldbus type . 25
8.1 Summary of the physical layer service and protocol characteristics . 25
8.2 Summary of data-link layer service characteristics . 27
8.3 Summary of data-link layer protocol characteristics . 29
8.4 Summary of application layer service characteristics . 30

8.5 Summary of application layer protocol characteristics . 32
9 Application layer service description concepts . 34
9.1 Overview . 34
9.2 Architectural relationships . 34
9.3 Fieldbus application layer structure . 36
9.4 Fieldbus application layer naming and addressing . 48
9.5 Architecture summary . 49
9.6 Notional FAL service procedures . 50
9.7 Common FAL attributes . 51
9.8 Common FAL service parameters . 52
9.9 APDU size . 53
10 Data type ASE . 53
10.1 Overview . 53
10.2 Formal definition of data type objects . 55

11 Fieldbus system requirements . 57

11.1 General . 57

11.2 Industrial control network . 57

11.3 Communication between industrial control networks and other networks . 58

11.4 Quality of service features of an industrial control network . 58

11.5 Special requirements for wireless networks . 59

Annex A (informative) Trade name declarations . 60

Annex B (informative) Media selection for fieldbus systems . 62

B.1 General . 62

B.2 Cabled media . 62
B.3 Wireless media . 62
B.4 Media needing special consideration . 62
B.5 Performance characteristics of open and public networks . 63
Bibliography . 64

Figure 1 – Example of a fieldbus system . 9
Figure 2 – Concept of DL/AL to separate service and protocol parts . 10
Figure 3 – Basic fieldbus reference model . 11
Figure 4 – General model of physical layer . 12
Figure 5 – Relationship of the Data-link layer to other fieldbus layers and to users of
the fieldbus data-link service . 13
Figure 6 – Relationship of the fieldbus Application layer to other fieldbus layers and to
users of the fieldbus application service . 14
Figure 7 – Structure of communication profile families . 17
Figure 8 – Example of a CPF structure . 18
Figure 9 – Document structure of IEC 61918 and the CPF specific part of IEC 61784-5 . 24
Figure 10 – Relationship to the OSI Basic Reference Model . 35
Figure 11 – Architectural positioning of the fieldbus application layer . 35
Figure 12 – Client/server interactions . 38
Figure 13 – Pull model interactions . 39
Figure 14 – Push model interactions . 39
Figure 15 – APOs services conveyed by the FAL . 41
Figure 16 – Application entity structure . 43

Figure 17 – Example FAL ASEs . 44
Figure 18 – FAL management of objects . 45
Figure 19 – ASE service conveyance . 46
Figure 20 – Defined and established AREPs . 48
Figure 21 – FAL architectural components . 50
Figure 22 – Data-type class hierarchy example . 53

Table 1 – OSI and IEC 61158 layers . 11
Table 2 – CPF, CP, and type relations . 19
Table 3 – Types of timeliness defined for publisher/subscriber interactions . 40
Table A.1 – Trade names of CPFs and CPs . 60

– 4 – IEC 61158-1:2014 © IEC 2014

INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
INDUSTRIAL COMMUNICATION NETWORKS –

FIELDBUS SPECIFICATIONS –
Part 1: Overview and guidance for the IEC 61158 and IEC 61784 series

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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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
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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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5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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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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expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
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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 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 layer
protocol type to be used with other layer protocols of the same type, or in other type
combinations explicitly authorized by their respective intellectual property right holders.
NOTE Combinations of protocol types are specified in IEC 61784-1 and IEC 61784-2.
International Standard IEC 61158-1 has been prepared by subcommittee 65C: Industrial
networks, of IEC technical committee 65: Industrial-process measurement, control and
automation.
This first edition cancels and replaces the third edition of IEC/TR 61158-1 published in 2010.
This edition constitutes a technical revision.

This first edition includes the following significant changes with respect to the previous

Technical Report:
• updates of the references to and information about the IEC 61158 series, IEC 61784-1,

IEC 61784-3, IEC 61784-5 series and IEC 61918 throughout the document;

• new Type 23 for profile family 8;

• new Type 24 and the related profile family CPF 19;

• new Subclause 7.7 Communication profiles for wireless communication networks;

• new Clause 11 Fieldbus system requirements;

• new Annex B Media selection for fieldbus systems.
The text of this standard is based on the following documents:
FDIS Report on voting
65C/757/FDIS 65C/767/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the 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 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.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a

colour printer.
– 6 – IEC 61158-1:2014 © IEC 2014

INDUSTRIAL COMMUNICATION NETWORKS –

FIELDBUS SPECIFICATIONS –
Part 1: Overview and guidance for the IEC 61158 and IEC 61784 series

1 Scope
This document specifies the generic concept of fieldbuses.
This document also presents an overview and guidance for the IEC 61158 series by:
• explaining the structure and content of the IEC 61158 series;
• relating the structure of the IEC 61158 series to the ISO/IEC 7498-1 OSI Basic Reference
Model;
• showing the logical structure of the IEC 61784 series;
• showing how to use parts of the IEC 61158 series in combination with the IEC 61784
series;
• providing explanations of some aspects of the IEC 61158 series that are common to the
type specific parts of the IEC 61158-5 including the application layer service description
concepts and the generic fieldbus data types.
2 Normative references
None.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1.1
communication system
arrangement of hardware, software and propagation media to allow the transfer of messages
from one application to another

3.1.2
fieldbus
communication system based on serial data transfer as typically used in industrial automation
and process control applications
3.1.3
fieldbus system
system using a fieldbus with connected devices
3.1.4
message
ordered series of octets intended to convey information
[SOURCE: ISO/IEC 2382-16:1996, 16.02.01, modified]

3.1.5
network
all of the media, connectors, repeaters, routers, gateways and associated node

communication elements by which a given set of communicating devices are interconnected

3.2 Abbreviations
For the purposes of this document, the following abbreviations, based partially on the

concepts developed in ISO/IEC 7498-1, apply:

AE application entity
AL application layer (N = 7)
APDU application layer protocol data unit
APO application process object
AR application relationship
AREP application relationship endpoint
ASE application service element
CP communication profile
CPF communication profile family
DL- data-link layer (as a prefix)
DLL data-link layer (N = 2)
FAL fieldbus application layer
FSCP functional safety communication profile
IETF Internet Engineering Task Force
IO input output
IP Internet protocol (see RFC 791)
kbit/s thousand bit per second
Mbit/s million bit per second
LME layer management entity
(n)-layer layer n of the OSI basic reference model
OSI open systems interconnection
Ph- physical layer (as a prefix)
PhL physical layer (N = 1)
SIL safety integrity level
4 Guidelines for implementers and users
4.1 Background and purpose
Communication in global markets requires a global understanding of a specification (standard
or not). ISO/OSI related specifications provide a common basis for understanding and
acceptance between international experts (manufacturers and end-users).
Examples are
• ISO/IEC 7498-1 for general layering and structuring;
• ISO/IEC 9545 for general application layer modeling;
• ISO/IEC 8886 for data-link layer modeling.

– 8 – IEC 61158-1:2014 © IEC 2014

The IEC 61158 series specifies a number of different fieldbus types in each of its parts

(IEC 61158-2 and the type specific parts of IEC 61158-3-tt, IEC 61158-4-tt, IEC 61158-5-tt

and IEC 61158-6-tt). As a result of the editorial harmonization work done by IEC, each PhL,

DLL and AL specification within IEC 61158 is shown in a homogeneous way. The description

of each layer offers, as far as possible, common views, concepts, definitions, and descriptive

methods.
NOTE The list of IEC 61158 parts is abbreviated as IEC 61158-3-tt, IEC 61158-4-tt, IEC 61158-5-tt, or
IEC 61158-6-tt, where tt represents one or more type numbers.

This common approach has been adopted to assist users and implementers in understanding

the several specifications. It is also intended to assist in comparing available products and

their communications-related features.
4.2 Supported options
Most of the fieldbus types specified in the IEC 61158 series include a range of selectable and
configurable options within their detailed specifications. In general, only certain restricted
combinations of options will interwork or interoperate correctly.
The recommended combinations of options are collected in IEC 61784-1 and IEC 61784-2.
IEC 61784-1 and IEC 61784-2 provide users and implementers with details of supported
fieldbus specifications based on selected options that are intended to work together
consistently and correctly. In most cases, available product demonstrations and working plant
experience support these profiles.
Annex A of IEC 61784-1 and Annex A of IEC 61784-2 help select the needed fieldbus by
showing the key features of each of the profiled fieldbus protocol families.
As a result, the route map recommended to select a fieldbus is:
• Clause 5 to Clause 8 of this part of IEC 61158;
• IEC 61784-1, Annex A: Communication concepts;
• IEC 61784-2, Annex A: Performance indicator calculations;
• IEC 61784-1 and IEC 61784-2, Communication profile family;
• the parts of IEC 61158 as referenced in IEC 61784-1 and IEC 61784-2 for the selected
communication profile of interest.
4.3 Benefits from using a common and formal style

The benefits gained from using a common and formal style to specify the communication
system are:
• the common look and feel of a specification saves effort during evaluation;
• a common structure helps to identify and to specify common parts and contents;
• the common approach represents a first step to ensure long-term quality and stability;
• the missing parts and items of any specification are more readily identified by comparison
with the other specifications, leading to a simplified review and evaluation procedure;
• a common basis facilitates the development of test and certification procedures;
• the modular concepts support future enhancements, extensions and adaptation of new
technologies.
5 Concept of the IEC 61158 series

Conceptually, a fieldbus is an industrial digital communication network for integration of

industrial control and instrumentation devices into a system. Examples of such devices are

transducers, sensors, actuators and controllers.

The IEC 61158 series specifies a number of fieldbus protocol types. Each protocol type is

designed to permit multiple measurement and control devices to communicate. Devices

communicate directly only with other devices of the same protocol type. The basic

requirements of industrial communication networks for measurement and control are given in

Clause 11.
Devices which use the same lower-layer protocols in a compatible fashion but differ in their
higher-layer protocols may be able to share a lower-layer medium.
In all cases, a particular data-link layer protocol type may be used without restriction when
coupled with physical layer and application layer protocols of the same type or with other
combinations as specified in IEC 61784-1 and IEC 61784-2. Use of the various protocol types
in other combinations may require permission from their respective copyright holders.
The protocol types in IEC 61158 have been engineered to support information processing,
monitoring and control systems for any industrial sector and related domains. An example
application for high-integrity low-level communication between sensors, actuators and local
controllers in a process plant, together with the interconnection of programmable controllers,
is shown in Figure 1.
Information systems network
Gateway Gateway
Fieldbus
Process plant Factory shop floor
segment
Controllers
Programmable
ControlPlrerogrs ammable
Controllers
Programmable
Sensors
Controllers
Actuators
Fieldbus
Monitoring
segment
system
Bridge
Fieldbus
segment
Figure 1 – Example of a fieldbus system
A number of fieldbus types are specified in the IEC 61158 series using the following concepts
for decomposition.
a) First concept: The complex communication task is divided into different layers based on
an adaptation of ISO/IEC 7498-1, the ISO/OSI Basic Reference Model, thereby facilitating
well-structured functions and interfaces (see Clause 6). This has the following benefits:
– decomposition of complex tasks;
– modular structure to adapt different technologies.
b) Second concept: Each fieldbus type is composed of one or more layer specifications.

– 10 – IEC 61158-1:2014 © IEC 2014

Most types include a number of services and protocol options that require an appropriate

selection to support a working system. Compatible selections of options and services

within one of the IEC 61158 fieldbus types are specified as standardized communication

profiles in IEC 61784-1 and IEC 61784-2. Most of these profiles are supported by

consortia or trade associations which are identified in the profile specification.

c) Third concept: The physical, data-link and application layers are described in

complementary ways, in terms of the offered services and the protocol which provides

those services.
Figure 2 illustrates the differences between service and protocol viewpoints of the data-link
and application layers. The protocol parts show the layer implementer’s oriented view and the

service parts show the layer user’s oriented view.

Layer user Layer implementer
Oriented view Oriented view
AL services AL protocol
Part 5 of IEC 61158 Part 6 of IEC 61158
- Model and concepts - Syntax definition and coding
- Data type definitions - Application relationships
- Application objects procedures
- Service description - Protocol machines (state machines)
- Communication end-point
management.
DL services DL protocol
Part 3 of IEC 61158 Part 4 of IEC 61158
- Model and concepts - Coding
- Service description - Medium access
- Management services - Protocol machines (state machines)

Figure 2 – Concept of DL/AL to separate service and protocol parts
The application layer structure is as follows:
– the "what“ is described by application layer service elements (ASE) in the type specific
parts of IEC 61158-5; and
– the "how“ is described by application layer relationships (AR) in the type specific parts of
IEC 61158-6.
The data-link layer structure is as follows:
– the "what“ is described by data-link layer services and models in the type specific parts of
IEC 61158-3; and
– the "how“ is described by data-link layer protocol machines and medium access principles
in the type specific parts of IEC 61158-4.
The physical layer is structured similarly, but, because its services are readily described, they
are described in IEC 61158-2 together with the definitions of the physical protocols:
– the "what“ is described by physical layer services and models, and
– the "how“ is described by physical layer electrical and mechanical specifications.

6 Mapping onto the OSI Basic Reference Model

6.1 Overview
IEC 61158 protocol types are described using the principles, methodology and model of

ISO/IEC 7498-1. The OSI model provides a layered approach to communications standards,

whereby the layers can be developed and modified independently. IEC 61158 specifies

functionality from top to bottom of a full OSI stack and, potentially, some functions for the

users of the stack. Functions of the intermediate OSI layers, layers 3 through 6, may be
consolidated into either the IEC 61158 data-link layer or the IEC 61158 application layer, or
may be realized by a separate layer. Likewise, some features common to users of the fieldbus

application layer may be provided by the IEC 61158 application layer to simplify user

operation.
Table 1 shows the OSI layers, their functions, and the equivalent layers in the IEC 61158
basic fieldbus reference model (see Figure 3).
Table 1 – OSI and IEC 61158 layers
OSI layer Function IEC 61158 layer
7 Application Translates demands placed on the communications stack into a form Application
understood by the lower layers and vice versa (IEC 61158-5-tt,
IEC 61158-6-tt)
6 Presentation
Converts data to/from standardized network formats ↑
5 Session Creates and manages dialogue among lower layers
↑
4 Transport Provides transparent reliable data transfer (end-to-end transfer across a
↓ or ↑
network which may include multiple links)
3 Network Performs message routing ↓ or ↑
2 Data-link Controls access to the communication medium. Performs error Data-link
detection, (point-to-point transfer on a link) (IEC 61158-3-tt,
IEC 61158-4-tt)
1 Physical Encodes/decodes signals for transmission/reception in a form
Physical
appropriate to the communications medium. Specifies communication
(IEC 61158-2)
media characteristics
NOTE -tt is a placeholder for the part numbers representing types.
NOTE ↓ and ↑ indicate that the functionality of this layer, when present, is included in the fieldbus layer that is
nearest in the direction of the arrow. Thus it is possible that the network and transport functionality are included in
either the data-link or application layers, and it is possible that the session and presentation functionality are
included in the application layer but not in the data-link layer.

Application layer
Data-link layer
Physical layer
Medium
Figure 3 – Basic fieldbus reference model
6.2 Physical layer service and protocol
IEC 61158-2 comprises physical layer specifications corresponding to many of the different
DL-Layer protocol types specified in the type specific parts of IEC 61158-4.
Systems
management
– 12 – IEC 61158-1:2014 © IEC 2014

NOTE 1 The type numbers used are consistent throughout the IEC 61158 series.

NOTE 2 Not all types have a physical layer specification in IEC 61158-2. In that case the communication profile in
IEC 61784-1 or IEC 61784-2 provides appropriate references to other standards.

NOTE 3 For ease of reference, type numbers are given in clause names. This means that the specification given
therein applies to this type but does not exclude its use for other types.

NOTE 4 It is up to the user of IEC 61158 to select interoperating sets of provisions. Refer to the IEC 61784-1 and

IEC 61784-2 for standardized communication profiles based on IEC 61158 .

A general model of the physical layer is shown in Figure 4.

DLL
DL - Ph Interface
Systems Systems
Ph DIS
Management
DCE Independent Sublayer
Management
Ph interface
–
DTE - DCE
Interface
Ph MDS
Medium Dependent Sublayer
MDS MAU Interface
-
Ph MAU
Medium Attachment Unit
Medium Interface
Medium
Figure 4 – General model of physical layer
NOTE 5 The protocol types use a subset of the structure elements.
NOTE 6 Since Type 8 uses a more complex DIS than the other types, it uses the term MIS to differentiate.
The common characteristics for all variants and types are as follows:
– digital data transmission; and

– either half-duplex communication (bi-directional but in only one direction at a time) or full-
duplex communication.
6.3 Data-link layer service
The data-link service is provided by the data-link protocol making use of the services
available from the physical layer. This and related parts of the IEC 61158 series defines the
data-link service characteristics that the immediately higher-level protocol may exploit. The
relationship between the international standards for fieldbus data-link service, fieldbus data-
link protocol, fieldbus application protocol and systems management is illustrated in Figure 5.
NOTE Systems management, as used in the IEC 61158 series, is a local mechanism for managing the layer
protocols.
Alternative
Ph MAU variant
Alternative
PH MAU variant
Alternative
Ph MAU variant
Alternative
Ph MAU variant
DCE DTE
MDS MIS
Physical Layer
Application layer
Data-link
Data-link
services
Data-link layer
management
services
Physical layer
Medium
Figure 5 – Relationship of the Data-link layer

to other fieldbus layers and to users of the fieldbus data-link service
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,
a data-link service defined in IEC 61158 is a conceptual architectural service, independent of
administrative and implementation divisions.
6.4 Data-link layer protocol
The data-link protocol provides the data-link service by making use of the services available
from the physical layer. The relationship between the International Standards for fieldbus
data-link service, fieldbus data-link protocol, fieldbus physical service and systems
management is illustrated in Figure 5.
NOTE 1 Systems management, as used in the IEC 61158 series, is a local mechanism for managing the layer
protocols.
NOTE 2 Not all types have a data-link layer specification. In that case the communication profile in IEC 61784-1
or IEC 61784-2 provides appropriate references to other standards.
The primary aim of the data-link protocol standards 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 implementers 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.
These data-link protocol standards are concerned, in particular, with the communication and
interworking of sensors, effectors and other automation devices, using these standards,

together with other standards positioned within the OSI or fieldbus reference models;
otherwise, incompatible systems may work together in any combination.
6.5 Application layer service
The application service is provided by the application protocol making use of the services
available from the data-link or other immediately lower layer. Each part of the IEC 61158-5
series defines the application service characteristics that any immediately higher-level
protocols may exploit. The relationship between the international standards for fieldbus
application service, fieldbus application protocol and systems management is illustrated in
Figure 6.
NOTE Systems management, as used in the IEC 61158 series, is a local mechanism for managing the layer
protocols.
Sys tems
Manag ement
– 14 – IEC 61158-1:2014 © IEC 2014

Application
S M
services
FIELDBUS
Y A
Application
S N
APPLICATION
management
T A
LAYER
services
E G
Data-link
M E
services
M
DATA-LINK
E
LAYER
N
T
PHYSICAL
LAYER
MEDIUM
Figure 6 – Relationship of the fieldbus Application layer
to other fieldbus layers and to users of the fieldbus application service
6.6 Application layer protocol
The application protocol provides the application service by making use of the services
available from the data-link layer or other immediately lower layer. The relationship between
the International Standards for fieldbus application service, fieldbus application protocol,
fieldbus data-link service and system management is illustrated in Figure 6.
NOTE Systems management, as used in the IEC 61158 series, is a local mechanism for managing the layer
protocols.
An application process uses the fieldbus application layer services to exchange information
with other application processes. The services define the abstract interface between the
application process and the application layer.
The application layer protocol is the set of rules that governs the format and meaning of the
information exchange between the application layers in various devices. The application layer
uses the protocol to implement the application layer services definitions.
The protocol machine defines the various states of an application layer and the valid
transitions between the states. It may be c
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