IEC 61158-5-21:2010

IEC 61158-5-21 ®
Edition 1.0 2010-08
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –
Part 5-21: Application layer service definition – Type 21 elements

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IEC 61158-5-21 ®
Edition 1.0 2010-08
INTERNATIONAL
STANDARD
Industrial communication networks – Fieldbus specifications –
Part 5-21: Application layer service definition – Type 21 elements

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
PRICE CODE
XB
ICS 25.04.40; 35.100.70; 35.110 ISBN 978-2-88912-114-4
– 2 – 61158-5-21 © IEC:2010(E)

CONTENTS
FOREWORD.4

INTRODUCTION.6

1 Scope.7

1.1 Overview .7

1.2 Specifications.8

1.3 Conformance.8

2 Normative references .8

3 Terms, definitions, symbols, abbreviations, and conventions .9
3.1 Terms and definitions from other ISO/IEC standards .9
3.2 Fieldbus data link layer terms.9
3.3 Fieldbus application layer specific definitions .10
3.4 Abbreviations and symbols.16
3.5 Conventions .16
4 Concepts .19
4.1 Common concepts.19
4.2 Type specific concepts .36
5 Data type ASE.39
5.1 General .39
5.2 Formal definition of data type objects .42
5.3 FAL defined data types.43
5.4 Data type ASE service specification .47
6 Communication model specification.47
6.1 ASEs.47
6.2 ARs .68
6.3 Summary of FAL classes .71
6.4 Permitted FAL services by AREP role.71
Bibliography.73

Figure 1 – Relationship to the OSI Basic Reference Model .20
Figure 2 – Architectural positioning of the fieldbus application layer.20
Figure 3 – Client/server interactions .23
Figure 4 – Pull model interactions.24

Figure 5 – Push model interactions .24
Figure 6 – APOs services conveyed by the FAL.26
Figure 7 – Application entity structure.28
Figure 8 – FAL management of objects.29
Figure 9 – ASE service conveyance.30
Figure 10 – Defined and established AREPs.32
Figure 11 – FAL architectural components .34
Figure 12 – Interaction between FAL and DLL .37
Figure 13 – Publisher-subscriber communication model.37
Figure 14 – Client-server communication model.38
Figure 15 – Object model.38
Figure 16 – ASEs of a Type 21 application .39

61158-5-21 © IEC:2010(E) – 3 –

Figure 17 – Data type class hierarchy example .40

Figure 18 – The AR ASE conveys APDUs between APs.61

Table 1 – Types of timeliness .25

Table 2 – Overall structure of the OD.38

Table 3 – Identify service.50

Table 4 – Status service .52

Table 5 – Access rights for object .54

Table 6 – Read service .55

Table 7 – Write service .57
Table 8 – TB-transfer.60
Table 9 – COS-transfer.60
Table 10 – Conveyance of service primitives by AREP role.62
Table 11 – Valid combinations of AREP roles involved in an AR .62
Table 12 – AR-unconfirmed send .66
Table 13 – AR-confirmed send.67
Table 14 – FAL class summary .71
Table 15 – Services by AREP role .72

– 4 – 61158-5-21 © IEC:2010(E)

INTERNATIONAL ELECTROTECHNICAL COMMISSION

____________
INDUSTRIAL COMMUNICATION NETWORKS –

FIELDBUS SPECIFICATIONS –
Part 5-21: Application layer service definition –

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,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
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
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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.
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.
International Standard IEC 61158-5-21:2010 has been prepared by subcommittee 65C:
Industrial networks, of IEC technical committee 65: Industrial-process measurement, control
and automation.
This standard cancels and replaces IEC/PAS 62573 published in 2008. This first edition
constitutes a technical revision.

61158-5-21 © IEC:2010(E) – 5 –

The text of this standard is based on the following documents:

FDIS Report on voting
65C/606/FDIS 65C/620/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 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.
NOTE The revision of this standard will be synchronized with the other parts of the IEC 61158 series.

– 6 – 61158-5-21 © IEC:2010(E)

INTRODUCTION
This part of IEC 61158 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/TR 61158-1.

The application service is provided by the application protocol making use of the services

available from the data-link or other immediately lower layer. This standard defines the

application service characteristics that fieldbus applications and/or system management may

exploit.
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 application layer service defined in this standard is a conceptual architectural service,
independent of administrative and implementation divisions.

61158-5-21 © IEC:2010(E) – 7 –

INDUSTRIAL COMMUNICATION NETWORKS –

FIELDBUS SPECIFICATIONS –
Part 5-21: Application layer service definition –

Type 21 elements
1 Scope
1.1 Overview
The Fieldbus Application Layer (FAL) provides user programs with a means to access the
fieldbus communication environment. In this respect, the FAL can be considered a window
between corresponding application programs.
This standard provides the common elements for basic time-critical and non-time-critical
messaging communications between application programs in an automation environment as
well as material specific to the Type 21 protocol. 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 standard defines, in an abstract way, the externally visible service provided by the FAL in
terms of:
a) an abstract model for defining application resources (objects) capable of being
manipulated by users via the FAL service;
b) the primitive actions and events of the service;
c) the parameters associated with each primitive action and event, and the form that they
take;
d) the interrelationship between these actions and events, and their valid sequences.
The purpose of this standard is to define the services provided to:
a) the FAL-user at the boundary between the user and the application layer of the
fieldbus Reference Model;
b) systems management at the boundary between the application layer and systems
management of the fieldbus Reference Model.

This standard describes the structure and services of the IEC FAL, in conformance with the
OSI Basic Reference Model (ISO/IEC 7498) and the OSI Application layer Structure
(ISO/IEC 9545).
FAL services and protocols are provided by FAL application entities (AEs) contained in the
application processes. The FAL AE is composed of a set of object-oriented Application
Service Elements (ASEs) and a Layer Management Entity (LME) that manages the AE. The
ASEs provide communication services that operate on a set of related application process
object (APO) classes. One of the FAL ASEs is a management ASE that provides a common
set of services for management of the instances of FAL classes.
Although these services specify how requests and responses are issued and delivered from
the perspective of applications, they do not include a specification of what the requesting and
responding applications are to do with them. That is, these services only define what requests
and responses applications can send or receive, not the functions of the applications
themselves. This permits greater flexibility to the FAL-users in standardizing such object

– 8 – 61158-5-21 © IEC:2010(E)

behavior. In addition to these services, some supporting services are also defined in this

standard to provide access to the FAL to control certain aspects of its operation.

1.2 Specifications
The principal objective of this standard is to specify the characteristics of conceptual

application layer services suitable for time-critical communications, and thus supplement the

OSI Basic Reference Model in guiding the development of application layer protocols for time-

critical communications.
A secondary objective is to provide migration paths from previously existing industrial

communications protocols. This latter objective gives rise to the diversity of services
standardized as the various types of IEC 61158, and the corresponding protocols
standardized in subparts of IEC 61158-6.
This standard may be used as the basis for formal application programming interfaces.
Nevertheless, it is not a formal programming interface, and any such interface must address
implementation issues not covered by this standard, including:
a) sizes and octet ordering of various multi-octet service parameters;
b) correlation of paired primitives for request and confirmation, or indication and response.
1.3 Conformance
This standard does not specify individual implementations or products, nor does it constrain
the implementations of application layer entities in industrial automation systems.
There is no conformance of equipment to this application layer service definition standard.
Instead, conformance is achieved through the implementation of conforming application layer
protocols that fulfill any given type of application layer services as defined in this standard.
2 Normative references
The following documents are indispensable for the application 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,.
IEC 60559, Binary floating-point arithmetic for microprocessor systems
IEC 61158-2:2010 , Industrial communication networks – Fieldbus specifications – Part 2:

Physical layer specification and service definition
IEC 61158-3-21:2010 , Industrial communication networks – Fieldbus specifications –
Part 3-21: Data-link layer service definition – Type 21 elements
IEC 61158-4-21:2010 , Industrial communication networks – Fieldbus specifications –
Part 4-21: Data-link layer protocol specification – Type 21 elements
IEC 61158-6-21:2010 , Industrial communication networks – Fieldbus specifications –
Part 6-21: Application layer protocol specification – Type 21 elements
ISO/IEC 7498-1, Information technology – Open Systems Interconnection – Basic Reference
Model: The Basic Model
—————————
To be published.
61158-5-21 © IEC:2010(E) – 9 –

ISO/IEC 7498-3, Information technology – Open Systems Interconnection – Basic Reference

Model: Naming and addressing
ISO/IEC 8822, Information technology – Open Systems Interconnection – Presentation

service definition
ISO/IEC 9545, Information technology – Open Systems Interconnection – Application layer

structure
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
3.1 Terms and definitions from other ISO/IEC standards
3.1.1 ISO/IEC 7498-1 terms
a) application entity
b) application process
c) application protocol data unit
d) application service element
e) application entity invocation
f) application process invocation
g) application transaction
h) real open system
i) transfer syntax
3.1.2 ISO/IEC 8822 terms
a) abstract syntax
b) presentation context
3.1.3 ISO/IEC 9545 terms
a) application-association
b) application-context
c) application context name
d) application-entity-invocation

e) application-entity-type
f) application-process-invocation
g) application-process-type
h) application-service-element
i) application control service element
3.2 Fieldbus data link layer terms
For the purposes of this document, the following terms as defined in IEC 61158-3-21:2010
and IEC 61158-4-21:2010 apply.
a) DL-Time
b) DL-Scheduling-policy
c) DLCEP
– 10 – 61158-5-21 © IEC:2010(E)

d) DLC
e) DL-connection-oriented mode

f) DLPDU
g) DLSDU
h) DLSAP
k) link
l) ISO/IEC 8802-3:2000 MAC address

m) DL–entity identifier
3.3 Fieldbus application layer specific definitions
3.3.1
application
function or data structure for which data are consumed or produced
3.3.2
application objects
multiple object classes that manage and provide a runtime exchange of messages across the
network and within the network device
3.3.3
application process
part of a distributed application on a network, which is located on one device and addressed
unambiguously
3.3.4
application process identifier
distinguishes multiple application processes used in a device
3.3.5
application process object
component of an application process that is identifiable and accessible through an FAL
application relationship
NOTE Application process object definitions are composed of a set of values for the attributes of their class (see
the definition for “application process object class”). Application process object definitions may be accessed
remotely using the services of the FAL Object Management ASE. FAL Object Management services can be used to
load or update object definitions, to read object definitions, and to create and delete application objects and their
corresponding definitions dynamically.
3.3.6
application process object class
class of application process objects defined in terms of the set of their network-accessible
attributes and services
3.3.7
application relationship
cooperative association between two or more application-entity-invocations for the purpose of
exchange of information and coordination of their joint operation
NOTE This relationship is activated either by the exchange of application-protocol-data-units or as a result of
preconfiguration activities.
3.3.8
application relationship application service element
application-service-element that provides the exclusive means for establishing and
terminating all application relationships

61158-5-21 © IEC:2010(E) – 11 –

3.3.9
application relationship endpoint

context and behavior of an application relationship as seen and maintained by one of the

application processes involved in the application relationship

NOTE Each application process involved in the application relationship maintains its own application relationship
endpoint.
3.3.10
attribute
description of an externally visible characteristic or feature of an object

NOTE The attributes of an object contain information about variable portions of an object. Typically, they provide
status information or govern the operation of an object. Attributes may also affect the behavior of an object.
Attributes are divided into class attributes and instance attributes.
3.3.11
behavior
indication of how an object responds to particular events
3.3.12
channel
single physical or logical link of an input or output application object of a server to the process
3.3.13
class
set of objects, all of which represent the same type of system component
NOTE A class is a generalization of an object, a template for defining variables and methods. All objects in a
class are identical in form and behavior, but usually contain different data in their attributes.
3.3.14
class attributes
attribute shared by all objects within the same class
3.3.15
class code
unique identifier assigned to each object class
3.3.16
class-specific service
service defined by a particular object class to perform a required function that is not
performed by a common service
NOTE A class-specific object is unique to the object class that defines it.
3.3.17
client
a) object that uses the services of another (server) object to perform a task
b) initiator of a message to which a server reacts
3.3.18
consume
act of receiving data from a producer
3.3.19
consumer
node or sink that receives data from a producer

– 12 – 61158-5-21 © IEC:2010(E)

3.3.20
consuming application
application that consumes data

3.3.21
conveyance path
unidirectional flow of APDUs across an application relationship

3.3.22
cyclic
repetitive in a regular manner

3.3.23
data consistency
means for coherent transmission and access of the input- or output-data object between and
within client and server
3.3.24
device
physical hardware connected to the link
NOTE A device may contain more than one node.
3.3.25
device profile
a collection of device-dependent information and functionality providing consistency between
similar devices of the same device type
3.3.26
diagnostic information
all data available at the server for maintenance purposes
3.3.27
end node
producing or consuming node
3.3.28
endpoint
one of the communicating entities involved in a connection
3.3.29
error
discrepancy between a computed, observed, or measured value or condition and the specified
or theoretically correct value or condition
3.3.30
error class
general grouping for related error definitions and corresponding error codes
3.3.31
error code
identification of a specific type of error within an error class
3.3.32
event
instance of a change of conditions

61158-5-21 © IEC:2010(E) – 13 –

3.3.33
FIFO variable
variable object class composed of a set of homogeneously typed elements, where the first

written element is the first element that can be read

NOTE In a fieldbus system, only one complete element can be transferred as a result of one service invocation.

3.3.34
frame
simplified synonym for data link protocol data unit (DLPDU)

3.3.35
group
a) (General): a general term for a collection of objects
b) (Addressing): when describing an address, an address that identifies more than one
entity
3.3.36
invocation
act of using a service or other resource of an application process
NOTE Each invocation represents a separate thread of control that may be described by its context. Once the
service completes, or use of the resource is released, the invocation ceases to exist. For service invocations, a
service that has been initiated but not yet completed is referred to as an outstanding service invocation. For
service invocations, an Invoke ID may be used to identify the service invocation unambiguously and differentiate it
from other outstanding service invocations.
3.3.37
index
address of an object within an application process
3.3.38
instance
actual physical occurrence of an object within a class that identifies one of many objects in
the same object class
EXAMPLE California is an instance of the object class US-state.
NOTE The terms object, instance, and object instance are used to refer to a specific instance.
3.3.39
instance attributes
attribute that is unique to an object instance and not shared by the object class

3.3.40
instantiated
object that has been created in a device
3.3.41
logical device
specific FAL class that abstracts a software component or a firmware component as an
autonomous self-contained facility of an automation device
3.3.42
manufacturer ID
identification of each product manufacturer by a unique number

– 14 – 61158-5-21 © IEC:2010(E)

3.3.43
management information
network-accessible information that supports management of the operation of the fieldbus

system, including the application layer

NOTE Managing includes functions, such as controlling, monitoring, and diagnosis.

3.3.44
network
set of nodes connected by some type of communication medium, including any intervening

repeaters, bridges, routers, and lower-layer gateways

3.3.45
object
abstract representation of a particular component within a device, usually a collection of
related data in the form of variables, and methods (procedures) for operating on that data that
have clearly defined interface and behavior
3.3.46
object dictionary
collection of definitions, communication-specific attributes and parameters, and application-
dependent data
3.3.47
object-specific service
service unique to the object class that defines it
3.3.48
physical device
automation or other network device
3.3.49
point-to-point connection
connection that exists between exactly two application objects
3.3.50
pre-established AR endpoint
AR endpoint placed in an established state during configuration of the AEs that control its
endpoints
3.3.51
process data
object(s) that are already pre-processed and transferred cyclically for the purpose of
information or further processing
3.3.52
produce
act of sending data to be received by a consumer
3.3.53
producer
node that is responsible for sending data
3.3.54
property
general term for descriptive information about an object

61158-5-21 © IEC:2010(E) – 15 –

3.3.55
provider
source of a data connection
3.3.56
publisher
role of an AR endpoint that transmits APDUs onto the fieldbus for consumption by one or

more subscribers
NOTE A publisher may not be aware of the identity or number of subscribers.

3.3.57
publishing manager
role of an AR endpoint in which it issues one or more confirmed service request application
protocol data units (APDUs) to a publisher to request that a specified object be published.
Two types of publishing managers are defined by this standard, pull publishing managers and
push publishing managers, each of which is defined separately.
3.3.58
push publisher
type of publisher that publishes an object in an unconfirmed service request APDU
3.3.59
push publishing manager
type of publishing manager that requests that a specified object be published using an
unconfirmed service
3.3.60
push subscriber
type of subscriber that recognizes received unconfirmed service request APDUs as published
object data
3.3.61
server
a) role of an application relationship endpoint (AREP) in which it returns a confirmed service
response APDU to the client that initiated the request
b) object that provides services to another (client) object
3.3.62
service
operation or function than an object and/or object class performs upon request from another
object and/or object class
3.3.63
station
host of one AP, identified by a unique data link connection endpoint (DLCEP)-address
3.3.64
subscriber
role of an AREP in which it receives APDUs produced by a publisher

– 16 – 61158-5-21 © IEC:2010(E)

3.4 Abbreviations and symbols
AE Application Entity
AL Application Layer
ALME Application Layer Management Entity

ALP Application Layer Protocol

APO Application Object
AP Application Process
APDU Application Protocol Data Unit

AR Application Relationship
AREP Application Relationship End Point
ASCII American Standard Code for Information Interchange
ASE Application Service Element
Cnf Confirmation
DL- (as a prefix) Data Link -
DLCEP Data Link Connection End Point
DLL Data Link Layer
DLM Data Link Management
DLSAP Data Link Service Access Point
DLSDU DL-service-data-unit
DNS Domain Name Service
FAL Fieldbus Application Layer
Ind Indication
Req Request
Rsp Response
3.5 Conventions
3.5.1 Overview
The FAL is defined as a set of object-oriented ASEs. Each ASE is specified in a separate
subclause. Each ASE specification is composed of two parts: its class specification and its
service specification.
The class specification defines the attributes of the class. Access to these attributes is
beyond the scope of this document except where specified. The service specification defines
the services provided by the ASE.
3.5.2 General conventions
This standard uses the descriptive conventions given in ISO/IEC 10731.
3.5.3 Conventions for class definitions
Class definitions are described using templates. Each template consists of a list of attributes
for the class. The general form of the template is as shown below:

61158-5-21 © IEC:2010(E) – 17 –

FAL ASE: ASE name
CLASS:  Class name
CLASS ID: #
PARENT CLASS: Parent class name

ATTRIBUTES:
1 (o) Key Attribute: numeric identifier

2 (o) Key Attribute: name
3 (m) Attribute: attribute name(values)

4 (m) Attribute: attribute name(values)

4.1 (s) Attribute: attribute name(values)

4.2 (s) Attribute: attribute name(values)
4.3 (s) Attribute: attribute name(values)
5 (c) Constraint: constraint expression
5.1 (m) Attribute: attribute name(values)
5.2 (o) Attribute: attribute name(values)
6 (m) Attribute: attribute name(values)
6.1 (s) Attribute: attribute name(values)
6.2 (s) Attribute: attribute name(values)
SERVICES:
1 (o) OpsService: service name
2 (c) Constraint: constraint expression
2.1 (o) OpsService: service name
3 (m) MgtService: service name

(1) The FAL ASE: entry is the name of the FAL ASE that provides the services for the class
being specified.
(2) The CLASS: entry is the name of the class being specified. All objects defined using this
template will be an instance of this class. The class may be specified by this standard, or
by a user of this standard.
(3) The CLASS ID: entry is a number that identifies the class being specified. This number is
not used for Type 21 elements.
(4) The PARENT CLASS: entry is the name of the parent class for the class being specified.
All attributes defined for the parent class and inherited by it are inherited for the class
being defined, and therefore do not have to be redefined in the template for this class.
NOTE The parent-class TOP indicates that the class being defined is an initial class definition. The parent class
TOP is used as a starting point from which all other classes are defined. The use of TOP is reserved for classes
defined by this standard.
(5) The ATTRIBUTES label indicates that the following entries are attributes defined for the

class.
a) Each of the attribute entries contains a line number in column 1; a mandatory (m),
optional (o), conditional (c), or selector (s) indicator in column 2; an attribute type label
in column 3; a name or a conditional expression in column 4; and an optional list of
enumerated values in column 5. In the column following the list of values, the default
value for the attribute may be specified.
b) Objects are normally identified by a numeric identifier or by an object name, or by both.
In the class templates, these key attributes are defined under the key attribute.
c) The line number defines the sequence and the level of nesting of the line. Each
nesting level is identified by period. The numbers below refer to the general template
form above. Nesting is used to specify:
i) fields of a structured attribute (4.1, 4.2, 4.3);
ii) attributes conditional on a constraint statement. Attributes may be mandatory (5.1)
or optional (5.2) if the constraint is true. Not all optional attributes require
constraint statements as does the attribute defined in (5.2);

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iii) the selection fields of a choice type attribute (6.1 and 6.2).

(6) The SERVICES label indicates that the following entries are services defined for the class.

a) An (m) in column 2 indicates that the service is mandatory for the class, while an (o)

indicates that it is optional. A (c) in this column indicates that the service is conditional.

When all services defined for a class are defined as optional, at least one has to be
selected when an instance of the class is defined.

b) The label “OpsService” designates an operational service (1).

c) The label “MgtService” designates a management service (2).

d) The line number defines the sequence and the level of nesting of the line. Each

nesting level is identified by period. Nesting within the list of services is used to specify

services conditional on a constraint statement.
3.5.4 Conventions for service definitions
3.5.4.1 General
The service model, service primitives, and time-sequence diagrams used are entirely abstract
descriptions; they do not represent a specification for implementation.
3.5.4.2 Service parameters
Service primitives are used to represent interactions between service user and service
provider (ISO/IEC 10731). They convey parameters that indicate information available in the
user/provider interaction. In any particular interface, not all parameters must be stated
explicitly.
The service definition of this standard uses a tabular format to describe the component
parameters of the ASE service primitives. The parameters that apply to each group of service
primitives are set out in tables. Each table consists of up to five columns:
a) parameter name;
b) request primitive;
c) indication primitive;
d) response primitive;
e) confirmation primitive.
One parameter, or a component, 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 specified in the column:
M The parameter is mandatory for the primitive.
U The parameter is a user option, and may or may not be provided depending on
dynamic usage of the service user. When not provided, a default value for the
parameter is assumed.
C The parameter is conditional upon other parameters or upon the environment of the
service user.
— (blank) The parameter is never present.
S The parameter is a selected item.
Some entries are further qualified by items in parentheses. 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:

61158-5-21 © IEC:2010(E) – 19 –

“(n)” indicates that the following note “n” contains additional information pertaining to

the parameter and its use.
3.5.4.3 Service procedures
The service procedures are defined in terms of:

a) The interactions between application entities through the exchange of fieldbus APDUs;

b) The interactions between an application layer service provider and an application layer

service user in the same system through the invocation of application layer service

primitives.
These procedures are applicable to instances of communication between systems that
support time-constrained communications services within the fieldbus application layer.
4 Concepts
4.1 Common concepts
4.1.1 Overview
The fieldbus is intended to be used in factories and process plants to interconnect primary
automation devices (e.g., sensors, actuators, local display devices, annunciators,
programmable logic controllers, small single loop controllers, and standalone field controls)
with control and monitoring equipment located in control rooms.
Primary automation devices are associated with the lowest levels of the industrial automation
hierarchy and perform a limited set of functions within a definite time window. Some of these
functions include diagnostics, data validation, and handling of multiple inputs and outputs.
These primary automation devices, also called field devices, are located close to the process
fluids, the fabricated part, the machine, the operator, and the environment. This use positions
the fieldbus at the lowest levels of the computer integrated manufacturing (CIM) architecture.
Some of the expected benefits in using fieldbus systems are reductions in wiring, increases in
the amount of data exchanged, a wider distribution of control between the primary automation
devices and the control room equipment, and satisfaction of time-critical constraints.
This subclause d
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