IEC 61784-3:2007

IEC 61784-3 ®
Edition 1.0 2007-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Industrial communication networks – Profiles –
Part 3: Functional safety fieldbuses – General rules and profile definitions

Réseaux de communications industriels – Profils –
Partie 3: Bus de terrain de sécurité fonctionnelle – Règles générales et
définitions de profils
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IEC 61784-3 ®
Edition 1.0 2007-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Industrial communication networks – Profiles –

Part 3: Functional safety fieldbuses – General rules and profile definitions

Réseaux de communications industriels – Profils –

Partie 3: Bus de terrain de sécurité fonctionnelle – Règles générales et

définitions de profils
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
CODE PRIX X
ICS 35.100.05 25.040.40 ISBN 978-2-83220-535-8

– 2 – 61784-3  IEC:2007
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 10
2 Normative references . 10
3 Terms, definitions, symbols, abbreviated terms and conventions . 11
3.1 Terms and definitions . 11
3.1.1 Common terms and definitions . 11
3.1.2 CPF 1: Additional terms and definitions . 16
3.1.3 CPF 2: Additional terms and definitions . 16
3.1.4 CPF 3: Additional terms and definitions . 16
3.1.5 CPF 6: Additional terms and definitions . 16
3.2 Symbols and abbreviated terms. 17
3.2.1 Common symbols and abbreviated terms . 17
3.2.2 CPF 1: Additional symbols and abbreviated terms . 17
3.2.3 CPF 2: Additional symbols and abbreviated terms . 17
3.2.4 CPF 3: Additional symbols and abbreviated terms . 17
3.2.5 CPF 6: Additional symbols and abbreviated terms . 17
4 Conformance . 18
5 Basics of safety-related fieldbus systems . 18
5.1 Safety function decomposition . 18
5.2 Communication system . 19
5.2.1 General . 19
5.2.2 IEC 61158 fieldbuses . 19
5.2.3 Communication channel types . 20
5.2.4 Safety function response time . 20
5.3 Communication errors . 21
5.3.1 General . 21
5.3.2 Corruption . 21
5.3.3 Unintended repetition . 21
5.3.4 Incorrect sequence . 21
5.3.5 Loss . 22
5.3.6 Unacceptable delay . 22
5.3.7 Insertion . 22
5.3.8 Masquerade . 22
5.3.9 Addressing . 22
5.4 Deterministic remedial measures . 22
5.4.1 General . 22
5.4.2 Sequence number . 23
5.4.3 Time stamp . 23
5.4.4 Time expectation . 23
5.4.5 Connection authentication . 23
5.4.6 Feedback message . 23
5.4.7 Data integrity assurance . 23
5.4.8 Redundancy with cross checking . 23
5.4.9 Different data integrity assurance systems . 24
5.5 Relationships between errors and safety measures . 24

61784-3  IEC:2007 – 3 –
5.6 Data integrity considerations . 25
5.6.1 Calculation of the residual error rate . 25
5.6.2 Residual error rate and SIL . 27
5.7 Relationship between functional safety and security . 27
5.8 Boundary conditions and constraints . 27
5.8.1 Electrical safety . 27
5.8.2 Electromagnetic compatibility (EMC) . 27
5.9 Installation guidelines . 28
5.10 Safety manual . 28
5.11 Safety policy . 28
6 Communication Profile Family 1 (FOUNDATION™ Fieldbus) – Profiles for functional
safety . 29
6.1 Functional Safety Communication Profile 1/1 . 29
6.2 Technical overview . 29
7 Communication Profile Family 2 (CIP™) – Profiles for functional safety . 30
7.1 Functional Safety Communication Profile 2/1 . 30
7.2 Technical overview . 30
8 Communication Profile Family 3 (PROFIBUS™, PROFINET™) – Profiles for
functional safety . 31
8.1 Functional Safety Communication Profile 3/1 . 31
8.2 Technical overview . 31
9 Communication Profile Family 6 (INTERBUS®) – Profiles for functional safety . 34
9.1 Functional Safety Communication Profile 6/7 . 34
9.2 Technical overview . 34
Annex A (informative) Example functional safety communication models . 36
A.1 General . 36
A.2 Model A . 36
A.3 Model B . 36
A.4 Model C . 37
A.5 Model D . 37
Annex B (informative) A safety communication channel model using CRC-based error
checking . 39
B.1 Overview . 39
B.2 Channel model for calculations . 39
B.3 Cyclic redundancy checking . 40
B.3.1 General . 40
B.3.2 Considerations concerning CRC polynomials . 42
Annex C (informative) Structure of technology-specific parts . 44
Bibliography . 46

Table 1 – Overview of the effectiveness of the various measures on the possible errors . 25
Table 2 – Definition of items used for calculation of the residual error rate . 26
Table 3 – Relationship of residual error rate to SIL level . 27
Table 4 – Overview of profile identifier usable for FSCP 6/7. 34
Table B.1 – Example dependency d and block length n . 42
min
Table C.1 – Common subclause structure for technology-specific parts . 44

– 4 – 61784-3  IEC:2007
Figure 1 – Relationships of IEC 61784-3 with other standards (machinery) . 7
Figure 2 – Relationships of IEC 61784-3 with other standards (process) . 8
Figure 3 – Safety communication as a part of a safety function . 19
Figure 4 – Example model of a functional safety communication system . 20
Figure 5 – Example of safety function response time components . 21
Figure 6 – Example application . 26
Figure 7 – Scope of FSCP 1/1 . 29
Figure 8 – Relationship of Safety Validators . 30
Figure 9 – Basic communication preconditions for FSCP 3/1 . 32
Figure 10 – Structure of a FSCP 3/1 safety PDU . 33
Figure 11 – Safe communication modes. 33
Figure 12 – FSCP 6/7 communication preconditions . 35
Figure A.1 – Model A . 36
Figure A.2 – Model B . 37
Figure A.3 – Model C . 37
Figure A.4 – Model D . 38
Figure B.1 – Communication channel with perturbation . 39
Figure B.2 – Binary symmetric channel (BSC) . 40
Figure B.3 – Example of a block with message and CRC bits (redundancy code) . 41
Figure B.4 – Block codes for error detection . 41
Figure B.5 – Proper and improper CRC polynomials . 42

61784-3  IEC:2007 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INDUSTRIAL COMMUNICATION NETWORKS – PROFILES

Part 3: Functional safety fieldbuses – General rules and profile definitions

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
in the subject dealt with may participate in this preparatory work. International, governmental and non-
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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
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
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.
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 functional safety communication profiles for families
1, 2, 3 and 6 given in IEC 61784-3-1, IEC 61784-3-2, IEC 61784-3-3 and IEC 61784-3-6.
IEC takes no position concerning the evidence, validity and scope of these patent rights.The holders of these
patent rights have assured the IEC that they are willing to negotiate licences under reasonable and non-
discriminatory terms and conditions with applicants throughout the world. In this respect, the statements of the
holders of these patent rights are registered with IEC.
NOTE Patent details and corresponding contact information are provided in IEC 61784-3-1, IEC 61784-3-2, IEC
61784-3-3 and IEC 61784-3-6.
International Standard IEC 61784-3 has been prepared by subcommittee 65C: Industrial
networks, of IEC technical committee 65: Industrial process measurement, control and
automation.
This bilingual version (2012-12) corresponds to the monolingual English version, published in
2007-12.
– 6 – 61784-3  IEC:2007
The text of this standard is based on the following documents:
FDIS Report on voting
65C/470/FDIS 65C/481/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.
The French version of this standard has not been voted upon.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result 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.
The list of all parts of the IEC 61784-3 series, under the general title Industrial
communications networks – Profiles – Functional safety fieldbuses, can be found on the IEC
website.
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.
61784-3  IEC:2007 – 7 –
INTRODUCTION
The IEC 61158 fieldbus standard together with its companion standards IEC 61784-1 and
IEC 61784-2 defines a set of communication protocols that enable distributed control of
automation applications. Fieldbus technology is now considered well accepted and well
proven. Thus many fieldbus enhancements are emerging, addressing not yet standardized
areas such as real time, safety-related and security-related applications.
This standard explains the relevant principles for functional safety communications with
reference to IEC 61508 series and specifies several safety communication layers (profiles and
corresponding protocols) based on the communication profiles and protocol layers of
IEC 61784-1, IEC 61784-2 and the IEC 61158 series. It does not cover electrical safety and
intrinsic safety aspects.
Figure 1 shows the relationships between this standard and relevant safety and fieldbus
standards in a machinery environment.
Product standards
ISO 12100-1 and ISO 14121
ISO 12100-1 and ISO 14121
IEC 61496 IEC 61131-6 IEC 61800-5-2 Safety of machinery – Principles for
Safety of machinery – Principles for
IEC 61496 IEC 61131-6 IEC 61800-5-2
Safety f. e.g. SafSaetfye tfyorf PorLC Safety functions design and risk assessment
design and risk assessment
Safety f. e.g. Safety for Safety functions
light curtains (under cPonsLCideration) for drives
light curtains PLC for drives
IEC 61784-4 IEC 62443 Design of safety-related electrical, electronic and programmable
IEC 61784-4 IEC 62443
electronic control systems (SRECS) for machinery
Security Security
Security Security
(profile-specific) (common part)
(profile-specific) (common part)
SIL based PL based
IEC 61784-5 IEC 61918
IEC 61784-5 IEC 61918
Design objective
Installation guide Installation guide
Installation guide Installation guide
(profile-specific) (common part) Applicable standards
(profile-specific) (common part)
ISO 13849-1, -2
ISO 13849-1, -2
IEC 60204-1
IEC 60204-1
Safety-related parts
Safety-related parts
Safety of electrical
Safety of electrical
IEC 61326-3-1 of machinery
IEC 61326-3-1 of machinery
equipment
equipment
EMC and (SRPCS)
EMC and (SRPCS)
functional safety
functional safety
IEC 61784-3
IEC 61784-3
Non-electrical
Non-electrical
Functional safety
Functional safety
communication
communication
Electrical
Electrical
US: NFPA 79
US: NFPA 79
profiles
profiles
(2006)
(2006)
IEC 62061
IEC 62061
Functional safety
Functional safety
IEC 61158 series /
IEC 61158 series /
for machinery
for machinery
IEC 61784-1, -2 IEC 61508 series
IEC 61784-1, -2 IEC 61508 series (SRECS)
(SRECS)
Functional safety
Fieldbus for use in Functional safety (including EMI for
Fieldbus for use in
(including EMI for
(basic standard)
industrial environment)
industrial control systems (basic standard)
industrial environment)
industrial control systems
Key
(yellow) safety-related standards
(blue) fieldbus-related standards
(dashed yellow) this standard
Figure 1 – Relationships of IEC 61784-3 with other standards (machinery)

– 8 – 61784-3  IEC:2007
Figure 2 shows the relationships between this standard and relevant safety and fieldbus
standards in a process environment.
Product standards
IEC 61496 IEC 61131-6 IEC 61800-5-2
IEC 61496 IEC 61131-6 IEC 61800-5-2
Safety f. e.g. Safety for Safety functions
Safety for PLC
Safety f. e.g. Safety for Safety functions
light curtains (under cPonsLCideration) for drives
light curtains PLC for drives
IEC 61784-4 IEC 62443
IEC 61784-4 IEC 62443
Security Security
Security Security
(profile-specific) (common part)
(profile-specific) (common part)
See safety standards for machinery
(Figure 1)
IEC 61784-5 IEC 61918
IEC 61784-5 IEC 61918
Installation guide Installation guide
Installation guide Installation guide
(profile-specific) (common part)
(profile-specific) (common part)
Valid also in process industries,
whenever applicable
a)
a)
IEC 61326-3-2
IEC 61326-3-2
EMC and
EMC and
functional safety
functional safety
IEC 61784-3
IEC 61784-3
Functional safety
Functional safety
communication
communication
profiles
profiles
US:
US:
ISA-84.00.01
ISA-84.00.01
b)
b)
(3 parts = modified
IEC 61511 series
IEC 61511 series (3 parts = modified
Functional safety – IEC 61511)
IEC 61158 series / Functional safety – IEC 61511)
IEC 61158 series /
Safety instrumented
Safety instrumented
IEC 61784-1, -2 IEC 61508 series
IEC 61784-1, -2 IEC 61508 series
systems for the
systems for the
Functional safety
Fieldbus for use in
Functional safety
Fieldbus for use in DE: VDI 2180
DE: VDI 2180
process industry sector
process industry sector
(basic standard)
(basic standard)
industrial control systems Part 1-4
industrial control systems
Part 1-4
Key
(yellow) safety-related standards
(blue) fieldbus-related standards
(dashed yellow) this standard
a
For specified electromagnetic environments; otherwise IEC 61326-3-1.
b
EN ratified.
Figure 2 – Relationships of IEC 61784-3 with other standards (process)
Safety communication layers which are implemented as parts of safety-related systems
according to IEC 61508 series provide the necessary confidence in the transportation of
messages (information) between two or more participants on a fieldbus in a safety-related
system, or sufficient confidence of safe behaviour in the event of fieldbus errors or failures.
Safety communication layers specified in this standard do this in such a way that a fieldbus
can be used for applications requiring functional safety up to the Safety Integrity Level (SIL)
specified by its corresponding functional safety communication profile.
The resulting SIL claim of a system depends on the implementation of the selected functional
safety communication profile within this system – implementation of a functional safety
communication profile in a standard device is not sufficient to qualify it as a safety device.

61784-3  IEC:2007 – 9 –
This standard describes:
 basic principles for implementing the requirements of IEC 61508 for safety-related data
communications, including possible transmission faults, remedial measures and
considerations affecting data integrity;
 individual description of functional safety profiles for several communication profile
families in IEC 61784-1 and IEC 61784-2;
 safety layer extensions to the communication service and protocols sections of the
IEC 61158 series.
– 10 – 61784-3  IEC:2007
INDUSTRIAL COMMUNICATION NETWORKS – PROFILES

Part 3: Functional safety fieldbuses – General rules and profile definitions

1 Scope
This part of the IEC 61784-3 series explains some common principles than can be used in the
transmission of safety-relevant messages among participants within a distributed network
using fieldbus technology in accordance with the requirements of IEC 61508 series for
functional safety. These principles can be used in various industrial applications such as
process control, manufacturing automation and machinery.
This part and the IEC 61784-3-x parts specify several functional safety communication
profiles based on the communication profiles and protocol layers of the fieldbus technologies
in IEC 61784-1, IEC 61784-2 and the IEC 61158 series.
NOTE 1 Other safety-related communication systems meeting the requirements of IEC 61508 series may exist
that are not included in this standard.
NOTE 2 It does not cover electrical safety and intrinsic safety aspects. Electrical safety relates to hazards such
as electrical shock. Intrinsic safety relates to hazards associated with potentially explosive atmospheres.
All systems are exposed to unauthorized access at some point of their life cycle. Additional
measures need to be considered in any safety-related application to protect fieldbus systems
against unauthorized access. IEC 62443 will address many of these issues; the relationship
with IEC 62443 is detailed in a dedicated subclause of this part.
NOTE 3 Additional profile specific requirements for security may also be specified in the future IEC 61784-4.
NOTE 4 Implementation of a functional safety communication profile according to this part in a device is not
sufficient to qualify it as a safety device, as defined in IEC 61508 series.
NOTE 5 The resulting SIL claim of a system depends on the implementation of the selected functional safety
communication profile within this system.
2 Normative references
The following referenced 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 61131-2, Programmable controllers – Part 2: Equipment requirements and tests
IEC 61158 (all parts), Industrial communication networks – Fieldbus specifications
IEC 61326-3-1, Electrical equipment for measurement, control and laboratory use – EMC
requirements – Part 3-1: Immunity requirements for safety-related systems and for equipment
intended to perform safety-related functions (functional safety) – General industrial
applications
IEC 61326-3-2, Electrical equipment for measurement, control and laboratory use – EMC
requirements – Part 3-2: Immunity requirements for safety-related systems and for equipment
intended to perform safety-related functions (functional safety) – Industrial applications with
specified EM environment
—————————
In the following pages of this standard, “this part” will be used for “this part of the IEC 61784-3 series”.
To be published.
61784-3  IEC:2007 – 11 –
IEC 61508 (all parts), Functional safety of electrical/electronic/programmable electronic
safety-related systems
IEC 61508-1, Functional safety of electrical/electronic/programmable electronic safety-related
systems – Part 1: General requirements
IEC 61508-2, Functional safety of electrical/electronic/programmable electronic safety-related
systems – Part 2: Requirements for electrical/electronic/programmable electronic safety-
related systems
IEC 61784-1, Industrial communication networks – Profiles – Part 1: Fieldbus profiles
IEC 61784-2, Industrial communication networks – Profiles – Part 2: Additional fieldbus
profiles for real-time networks based on ISO/IEC 8802-3
IEC 61784-3-1, Industrial communication networks – Profiles – Part 3-1: Functional safety
fieldbuses – Additional specifications for CPF 1
IEC 61784-3-2, Industrial communication networks – Profiles – Part 3-2: Functional safety
fieldbuses – Additional specifications for CPF 2
IEC 61784-3-3, Industrial communication networks – Profiles – Part 3-3: Functional safety
fieldbuses – Additional specifications for CPF 3
IEC 61784-3-6, Industrial communication networks – Profiles – Part 3-6: Functional safety
fieldbuses – Additional specifications for CPF 6
IEC 61784-5 (all parts), Industrial communication networks – Profiles – Part 5: Installation of
fieldbuses – Installation profiles for CPF x
IEC 61918, Industrial communication networks – Installation of communication networks in
industrial premises
IEC 62280-1:2002, Railway applications – Communication, signalling and processing systems
– Part 1: Safety-related communication in closed transmission systems
3 Terms, definitions, symbols, abbreviated terms and conventions
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1.1 Common terms and definitions
3.1.1.1
absolute time stamp
time stamp referenced to a global time which is common for a group of devices using a
fieldbus
[IEC 62280-2, modified]
3.1.1.2
availability
probability for an automated system that for a given period of time there are no unsatisfactory
system conditions such as loss of production
NOTE Availability depends on MTBF (mean time between failure) and MDT (mean down time):
Availability = MTBF / (MTBF + MDT).

– 12 – 61784-3  IEC:2007
3.1.1.3
black channel
communication channel without available evidence of design or validation according to
IEC 61508 series
3.1.1.4
bridge
abstract device that connects multiple network segments along the data link layer
3.1.1.5
communication channel
logical connection between two end-points within a communication system
3.1.1.6
communication system
arrangement of hardware, software and propagation media to allow the transfer of messages
(ISO/IEC 7498 application layer) from one application to another
3.1.1.7
connection
logical binding between two application objects within the same or different devices
3.1.1.8
Cyclic Redundancy Check (CRC)
redundant data derived from, and stored or transmitted together with, a block of data
in order to detect data corruption
procedure used to calculate the redundant data
NOTE 1 Terms “CRC code” and "CRC signature" , and labels such as CRC1, CRC2, may also be used in this
standard to refer to the redundant data.
NOTE 2 See also [26], [27] .
3.1.1.9
diversity
different means of performing a required function
EXAMPLE Diversity may be achieved by different physical methods or different design approaches.
[IEC 61508-4:1998]
3.1.1.10
error
discrepancy between a computed, observed or measured value or condition and the true,
specified or theoretically correct value or condition
NOTE 1 An error can be caused by a faulty item, e.g. a computing error made by faulty computer equipment.
[IEV 191-05-24], [IEC 61508-4:1998], [IEC 61158]
NOTE 2 Errors may be due to design mistakes within hardware/software and/or corrupted information due to
electromagnetic interference and/or other effects.
NOTE 3 Errors do not necessarily result in a failure or a fault.
3.1.1.11
failure
termination of the ability of a functional unit to perform a required function
—————————
Figures in square brackets refer to the bibliography.

61784-3  IEC:2007 – 13 –
NOTE 1 The definition in IEV 191-04-01 is the same, with additional notes.
[IEC 61508-4:1998], [ISO/IEC 2382-14.01.11]
NOTE 2 Failure may be due to an error (for example, problem with hardware/software design or message
disruption)
3.1.1.12
fault
abnormal condition that may cause a reduction in, or loss of, the capability of a functional unit
to perform a required function
NOTE IEV 191-05-01 defines “fault” as a state characterized by the inability to perform a required function,
excluding the inability during preventative maintenance or other planned actions, or due to lack of external
resources.
[IEC 61508-4:1998], [ISO/IEC 2382-14.01.10]
3.1.1.13
fieldbus
communication system based on serial data transfer and used in industrial automation or
process control applications
3.1.1.14
fieldbus system
system using a fieldbus with connected devices
3.1.1.15
frame
denigrated synonym for DLPDU
3.1.1.16
Frame Check Sequence (FCS)
redundant data derived from a block of data within a DLPDU (frame), using a hash function,
and stored or transmitted together with the block of data, in order to detect data corruption
NOTE 1 An FCS can be derived using for example a CRC or other hash function.
NOTE 2 See also [26], [27].
3.1.1.17
hash function
(mathematical) function that maps values from a (possibly very) large set of values into a
(usually) smaller range of values
NOTE 1 Hash functions can be used to detect data corruption.
NOTE 2 Common hash functions include parity, checksum or CRC.
[IEC 62210, modified]
3.1.1.18
hazard
state or set of conditions of a system that, together with other related conditions will inevitably
lead to harm to persons, property or environment
3.1.1.19
master
active communication entity able to initiate and schedule communication activities by other
stations which may be masters or slaves

– 14 – 61784-3  IEC:2007
3.1.1.20
message
ordered series of octets intended to convey information
[ISO/IEC 2382-16.02.01, modified]
3.1.1.21
message sink
part of a communication system in which messages are considered to be received
[ISO/IEC 2382-16.02.03]
3.1.1.22
message source
part of a communication system from which messages are considered to originate
[ISO/IEC 2382-16.02.02]
3.1.1.23
nuisance trip
spurious trip with no harmful effect
NOTE Internal abnormal errors can be caused in communication systems such as wireless transmission, for
example by too many retries in the presence of interferences.
3.1.1.24
protective extra-low-voltage (PELV)
electrical circuit in which the voltage cannot exceed a.c. 30 V r.m.s., 42,4 V peak or d.c. 60 V
in normal and single-fault condition, except earth faults in other circuits
NOTE A PELV circuit is similar to an SELV circuit that is connected to protective earth.
[IEC 61131-2]
3.1.1.25
redundancy
existence of means, in addition to the means which would be sufficient for a functional unit to
perform a required function or for data to represent information
EXAMPLE Duplicated functional components and the addition of parity bits are both instances of redundancy.
NOTE 1 Redundancy is used primarily to improve reliability or availability.
NOTE 2 The definition in IEV 191-15-01 is less complete.
[IEC 61508-4:1998], [ISO/IEC 2382-14.01.12]
3.1.1.26
relative time stamp
time stamp referenced to the local clock of an entity
NOTE In general, there is no relationship to clocks of other entities.
[IEC 62280-2, modified]
3.1.1.27
reliability
probability that an automated system can perform a required function under given conditions
for a given time interval (t1, t2)
NOTE 1 It is generally assumed that the automated system is in a state to perform this required function at the
beginning of the time interval.
NOTE 2 The term "reliability" is also used to denote the reliability performance quantified by this probability.

61784-3  IEC:2007 – 15 –
NOTE 3 Within the MTBF or MTTF period of time, the probability that an automated system will perform a
required function under given conditions is decreasing.
NOTE 4 Reliability differs from availability.
[IEC 62059-11, modified]
3.1.1.28
risk
combination of the probability of occurrence of harm and the severity of that harm
[IEC 61508-4:1998]
3.1.1.29
safety communication layer (SCL)
communication layer that includes all the necessary measures to ensure safe transmission of
data in accordance with the requirements of IEC 61508
3.1.1.30
safety connection
connection that utilizes the safety protocol for communications transactions
3.1.1.31
safety data
data transmitted across a safety network using a safety protocol
NOTE The Safety Communication Layer does not ensure safety of the data itself, only that the data is transmitted
safely.
3.1.1.32
safety device
device designed in accordance with IEC 61508 and which implements the functional safety
communication profile
3.1.1.33
safety extra-low-voltage (SELV)
electrical circuit in which the voltage cannot exceed a.c. 30 V r.m.s., 42,4 V peak or d.c. 60 V
in normal and single-fault condition, including earth faults in other circuits
NOTE An SELV circuit is not connected to protective earth.
[IEC 61131-2]
3.1.1.34
safety function
function to be implemented by an E/E/PE safety-related system, other technology safety-
related system or external risk reduction facilities, which is intended to achieve or maintain a
safe state for the EUC, in respect of a specific hazardous event
[IEC 61508-4:1998]
3.1.1.35
safety function response time
worst case elapsed time following an actuation
...