IEC 61508-7:2010

IEC 61508-7 ®
Edition 2.0 2010-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Functional safety of electrical/electronic/programmable electronic safety-related
systems –
Part 7: Overview of techniques and measures

Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques
programmables relatifs à la sécurité –
Partie 7: Présentation de techniques et mesures

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IEC 61508-7 ®
Edition 2.0 2010-04
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Functional safety of electrical/electronic/programmable electronic safety-related
systems –
Part 7: Overview of techniques and measures

Sécurité fonctionnelle des systèmes électriques/électroniques/électroniques
programmables relatifs à la sécurité –
Partie 7: Présentation de techniques et mesures

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
PRICE CODE
INTERNATIONALE
XG
CODE PRIX
ICS 25.040.40; 35.240.50 ISBN 978-2-88910-530-4
– 2 – 61508-7 © IEC:2010
CONTENTS
FOREWORD.3
INTRODUCTION.5
1 Scope.7
2 Normative references .9
3 Definitions and abbreviations.9
Annex A (informative) Overview of techniques and measures for E/E/PE safety-related
systems: control of random hardware failures (see IEC 61508-2).10
Annex B (informative) Overview of techniques and measures for E/E/PE safety related
systems: avoidance of systematic failures (see IEC 61508-2 and IEC 61508-3) .27
Annex C (informative) Overview of techniques and measures for achieving software
safety integrity (see IEC 61508-3).54
Annex D (informative) A probabilistic approach to determining software safety integrity
for pre-developed software .107
Annex E (informative) Overview of techniques and measures for design of ASICs . 112
Annex F (informative) Definitions of properties of software lifecycle phases. 126
Annex G (informative) Guidance for the development of safety-related object oriented
software.132
Bibliography.134
Index .137

Figure 1 – Overall framework of IEC 61508.8

Table C.1 – Recommendations for specific programming languages .86
Table D.1 – Necessary history for confidence to safety integrity levels . 107
Table D.2 – Probabilities of failure for low demand mode of operation . 108
Table D.3 – Mean distances of two test points .109
Table D.4 – Probabilities of failure for high demand or continuous mode of operation .110
Table D.5 – Probability of testing all program properties .111
Table F.1 – Software Safety Requirements Specification . 126
Table F.2 – Software design and development: software architecture design . 127
Table F.3 – Software design and development: support tools and programming
language.128
Table F.4 – Software design and development: detailed design . 128
Table F.5 – Software design and development: software module testing and integration. 129
Table F.6 – Programmable electronics integration (hardware and software). 129
Table F.7 – Software aspects of system safety validation .130
Table F.8 – Software modification.130
Table F.9 – Software verification.131
Table F.10 – Functional safety assessment .131
Table G.1 – Object Oriented Software Architecture . 132
Table G.2 – Object Oriented Detailed Design.133
Table G.3 – Some Oriented Detailed terms .133

61508-7 © IEC:2010 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/
PROGRAMMABLE ELECTRONIC SAFETY-RELATED SYSTEMS –

Part 7: Overview of techniques and measures

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
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Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
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with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
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services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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.
International Standard IEC 61508-7 has been prepared by subcommittee 65A: System
aspects, of IEC technical committee 65: Industrial-process measurement, control and
automation.
This second edition cancels and replaces the first edition published in 2000. This edition
constitutes a technical revision.
This edition has been subject to a thorough review and incorporates many comments received
at the various revision stages.

– 4 – 61508-7 © IEC:2010
The text of this standard is based on the following documents:
FDIS Report on voting
65A/554/FDIS 65A/578/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 parts of the IEC 61508 series, published under the general title Functional safety
of electrical / electronic / programmable electronic safety-related systems, can be found on
the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
61508-7 © IEC:2010 – 5 –
INTRODUCTION
Systems comprised of electrical and/or electronic elements have been used for many years to
perform safety functions in most application sectors. Computer-based systems (generically
referred to as programmable electronic systems) are being used in all application sectors to
perform non-safety functions and, increasingly, to perform safety functions. If computer
system technology is to be effectively and safely exploited, it is essential that those
responsible for making decisions have sufficient guidance on the safety aspects on which to
make these decisions.
This International Standard sets out a generic approach for all safety lifecycle activities for
systems comprised of electrical and/or electronic and/or programmable electronic (E/E/PE)
elements that are used to perform safety functions. This unified approach has been adopted
in order that a rational and consistent technical policy be developed for all electrically-based
safety-related systems. A major objective is to facilitate the development of product and
application sector international standards based on the IEC 61508 series.
NOTE 1 Examples of product and application sector international standards based on the IEC 61508 series are
given in the bibliography (see references [21], [22] and [37]).
In most situations, safety is achieved by a number of systems which rely on many
technologies (for example mechanical, hydraulic, pneumatic, electrical, electronic, programmable
electronic). Any safety strategy must therefore consider not only all the elements within an
individual system (for example sensors, controlling devices and actuators) but also all the
safety-related systems making up the total combination of safety-related systems. Therefore,
while this International Standard is concerned with E/E/PE safety-related systems, it may also
provide a framework within which safety-related systems based on other technologies may be
considered.
It is recognized that there is a great variety of applications using E/E/PE safety-related
systems in a variety of application sectors and covering a wide range of complexity, hazard
and risk potentials. In any particular application, the required safety measures will be
dependent on many factors specific to the application. This International Standard, by being
generic, will enable such measures to be formulated in future product and application sector
international standards and in revisions of those that already exist.
This International Standard
– considers all relevant overall, E/E/PE system and software safety lifecycle phases (for
example, from initial concept, through design, implementation, operation and maintenance
to decommissioning) when E/E/PE systems are used to perform safety functions;
– has been conceived with a rapidly developing technology in mind; the framework is
sufficiently robust and comprehensive to cater for future developments;
– enables product and application sector international standards, dealing with E/E/PE
safety-related systems, to be developed; the development of product and application
sector international standards, within the framework of this standard, should lead to a high
level of consistency (for example, of underlying principles, terminology etc.) both within
application sectors and across application sectors; this will have both safety and economic
benefits;
– provides a method for the development of the safety requirements specification necessary
to achieve the required functional safety for E/E/PE safety-related systems;
– adopts a risk-based approach by which the safety integrity requirements can be
determined;
– introduces safety integrity levels for specifying the target level of safety integrity for the
safety functions to be implemented by the E/E/PE safety-related systems;
NOTE 2 The standard does not specify the safety integrity level requirements for any safety function, nor does it
mandate how the safety integrity level is determined. Instead it provides a risk-based conceptual framework and
example techniques.
– 6 – 61508-7 © IEC:2010
– sets target failure measures for safety functions carried out by E/E/PE safety-related
systems, which are linked to the safety integrity levels;
– sets a lower limit on the target failure measures for a safety function carried out by a
single E/E/PE safety-related system. For E/E/PE safety-related systems operating in
– a low demand mode of operation, the lower limit is set at an average probability of a
–5
dangerous failure on demand of 10 ;
– a high demand or a continuous mode of operation, the lower limit is set at an average

–9 -1
frequency of a dangerous failure of 10 [h ];
NOTE 3 A single E/E/PE safety-related system does not necessarily mean a single-channel architecture.
NOTE 4 It may be possible to achieve designs of safety-related systems with lower values for the target safety
integrity for non-complex systems, but these limits are considered to represent what can be achieved for relatively
complex systems (for example programmable electronic safety-related systems) at the present time.
– sets requirements for the avoidance and control of systematic faults, which are based on
experience and judgement from practical experience gained in industry. Even though the
probability of occurrence of systematic failures cannot in general be quantified the
standard does, however, allow a claim to be made, for a specified safety function, that the
target failure measure associated with the safety function can be considered to be
achieved if all the requirements in the standard have been met;
– introduces systematic capability which applies to an element with respect to its confidence
that the systematic safety integrity meets the requirements of the specified safety integrity
level;
– adopts a broad range of principles, techniques and measures to achieve functional safety
for E/E/PE safety-related systems, but does not explicitly use the concept of fail safe.
However, the concepts of “fail safe” and “inherently safe” principles may be applicable and
adoption of such concepts is acceptable providing the requirements of the relevant
clauses in the standard are met.

61508-7 © IEC:2010 – 7 –
FUNCTIONAL SAFETY OF ELECTRICAL/ELECTRONIC/
PROGRAMMABLE ELECTRONIC SAFETY-RELATED SYSTEMS –

Part 7: Overview of techniques and measures

1 Scope
1.1 This part of IEC 61508 contains an overview of various safety techniques and measures
relevant to IEC 61508-2 and IEC 61508-3.
The references should be considered as basic references to methods and tools or as
examples, and may not represent the state of the art.
1.2 IEC 61508-1, IEC 61598-2, IEC 61508-3 and IEC 61508-4 are basic safety publications,
although this status does not apply in the context of low complexity E/E/PE safety-related
systems (see 3.4.3 of IEC 61508-4). As basic safety publications, they are intended for use by
technical committees in the preparation of standards in accordance with the principles
contained in IEC Guide 104 and ISO/IEC Guide 51. IEC 61508-1, IEC 61508-2, IEC 61508-3
and IEC 61508-4 are also intended for use as stand-alone publications. The horizontal safety
function of this international standard does not apply to medical equipment in compliance with
the IEC 60601 series.
1.3 One of the responsibilities of a technical committee is, wherever applicable, to make
use of basic safety publications in the preparation of its publications. In this context, the
requirements, test methods or test conditions of this basic safety publication will not apply
unless specifically referred to or included in the publications prepared by those technical
committees.
1.4 Figure 1 shows the overall framework for parts 1 to 7 of IEC 61508 and indicates the role
that IEC 61508-7 plays in the achievement of functional safety for E/E/PE safety-related
systems.
– 8 – 61508-7 © IEC:2010
Technical Requirements Other Requirements
Part 4
Part 1
Definitions &
Development of the overall
abbreviations
safety requirements
(concept, scope, definition,
hazard and risk analysis)
7.1 to 7.5
Part 5
Example of methods
for the determination Part 1
of safety integrity Documentation
levels Clause 5 &
Part 1
Annex A
Allocation of the safety requirements
to the E/E/PE safety-related systems
7.6
Part 1
Management of
functional safety
Clause 6
Part 1
Specification of the system safety
requirements for the E/E/PE
safety-related systems
Part 1
Functional safety
7.10 assessm ent
Clause 8
Part 6
Guidelines for the
application of
Parts 2 & 3
Part 2 Part 3
Realisation phase Realisation phase
for E/E/PE for safety-related
safety-related software
systems
Part 7
Overview of
techniques and
measures
Part 1
Installation, commissioning
& safety validation of E/E/PE
safety-related systems
7.13 - 7.14
Part 1
Operation, maintenance,repair,
modification and retrofit,
decommissioning or disposal of
E/E/PE safety-related systems
7.15 - 7.17
Figure 1 – Overall framework of IEC 61508

61508-7 © IEC:2010 – 9 –
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 61508-4:2010 Functional safety of electrical/electronic/programmable electronic safety-
related systems – Part 4: Definitions and abbreviations
3 Definitions and abbreviations
For the purposes of this document, the definitions and abbreviations given in IEC 61508-4
apply.
– 10 – 61508-7 © IEC:2010
Annex A
(informative)
Overview of techniques and measures for E/E/PE safety-related systems:
control of random hardware failures
(see IEC 61508-2)
A.1 Electric
Global objective: To control failures in electromechanical components.
A.1.1 Failure detection by on-line monitoring
NOTE This technique/measure is referenced in Tables A.2, A.3, A.7 and A.13 to A.18 of IEC 61508-2.
Aim: To detect failures by monitoring the behaviour of the E/E/PE safety-related system in
response to the normal (on-line) operation of the equipment under control (EUC).
Description: Under certain conditions, failures can be detected using information about (for
example) the time behaviour of the EUC. For example, if a switch, which is part of the E/E/PE
safety-related system, is normally actuated by the EUC, then if the switch does not change
state at the expected time, a failure will have been detected. It is not usually possible to
localise the failure.
A.1.2 Monitoring of relay contacts
NOTE This technique/measure is referenced in Tables A.2 and A.14 of IEC 61508-2.
Aim: To detect failures (for example welding) of relay contacts.
Description: Forced contact (or positively guided contact) relays are designed so that their
contacts are rigidly linked together. Assuming there are two sets of changeover contacts, a
and b, if the normally open contact, a, welds, the normally closed contact, b, cannot close
when the relay coil is next de-energised. Therefore, the monitoring of the closure of the
normally closed contact b when the relay coil is de-energised may be used to prove that
the normally open contact a has opened. Failure of normally closed contact b to close
indicates a failure of contact a, so the monitoring circuit should ensure a safe shut-down, or
ensure that shut-down is continued, for any machinery controlled by contact a.
References:
Zusammenstellung und Bewertung elektromechanischer Sicherheitsschaltungen für Ver-
riegelungseinrichtungen. F. Kreutzkampf, W. Hertel, Sicherheitstechnisches Informations- und
Arbeitsblatt 330212, BIA-Handbuch. 17. Lfg. X/91, Erich Schmidt Verlag, Bielefeld.
www.BGIA-HANDBUCHdigital.de/330212
A.1.3 Comparator
NOTE This technique/measure is referenced in Tables A.2, A.3, A.4 of IEC 61508-2.
Aim: To detect, as early as possible, (non-simultaneous) failures in an independent
processing unit or in the comparator.
Description: The signals of independent processing units are compared cyclically or
continuously by a hardware comparator. The comparator may itself be externally tested, or it
may use self-monitoring technology. Detected differences in the behaviour of the processors
lead to a failure message.
61508-7 © IEC:2010 – 11 –
A.1.4 Majority voter
NOTE This technique/measure is referenced in Tables A.2, A.3 and A.4 of IEC 61508-2.
Aim: To detect and mask failures in one of at least three hardware channels.
Description: A voting unit using the majority principle (2 out of 3, 3 out of 3, or m out of n) is
used to detect and mask failures. The voter may itself be externally tested, or it may use self-
monitoring technology.
References:
Guidelines for Safe Automation of Chemical Processes. CCPS, AIChE, New York, 1993,
ISBN-10: 0-8169-0554-1, ISBN-13: 978-0-8169-0554-6
A.1.5 Idle current principle (de-energised to trip)
NOTE This technique/measure is referenced in Table A.16 of IEC 61508-2.
Aim: To execute the safety function if power is cut or lost.
Description: The safety function is executed if the contacts are open and no current flows.
For example, if brakes are used to stop a dangerous movement of a motor, the brakes are
opened by closing contacts in the safety-related system and are closed by opening the
contacts in the safety-related system.
Reference:
Guidelines for Safe Automation of Chemical Processes. CCPS, AIChE, New York, 1993,
ISBN-10: 0-8169-0554-1, ISBN-13: 978-0-8169-0554-6
A.2 Electronic
Global objective: To control failure in solid-state components.
A.2.1 Tests by redundant hardware
NOTE This technique/measure is referenced in Tables A.3, A.15, A.16 and A.18 of IEC 61508-2.
Aim: To detect failures using hardware redundancy, i.e. using additional hardware not
required to implement the process functions.
Description: Redundant hardware can be used to test at an appropriate frequency the
specified safety functions. This approach is normally necessary for realising A.1.1 or A.2.2.
A.2.2 Dynamic principles
NOTE This technique/measure is referenced in Table A.3 of IEC 61508-2.
Aim: To detect static failures by dynamic signal processing.
Description: A forced change of otherwise static signals (internally or externally generated)
helps to detect static failures in components. This technique is often associated with
electromechanical components.
Reference:
– 12 – 61508-7 © IEC:2010
Elektronik in der Sicherheitstechnik. H. Jürs, D. Reinert, Sicherheitstechnisches Informations-
und Arbeitsblatt 330220, BIA-Handbuch, Erich-Schmidt Verlag, Bielefeld, 1993.
http://www.bgia-handbuchdigital.de/330220
A.2.3 Standard test access port and boundary-scan architecture
NOTE This technique/measure is referenced in Tables A.3, A.15 and A.18 of IEC 61508-2.
Aim: To control and observe what happens at each pin of an IC.
Description: Boundary-scan test is an IC design technique which increases the testability of
the IC by resolving the problem of how to gain access to the circuit test points within it. In a
typical boundary-scan IC, comprised of core logic and input and output buffers, a shift-register
stage is placed between the core logic and the input and output buffers adjacent to each IC
pin. Each shift-register stage is contained in a boundary-scan cell. The boundary-scan cell
can control and observe what happens at each input and output pin of an IC, via the standard
test access port. Internal testing of the IC core logic is accomplished by isolating the on-chip
core logic from stimuli received from surrounding components, and then performing an
internal self-test. These tests can be used to detect failures in the IC.
Reference:
IEEE 1149-1:2001, IEEE standard test access port and boundary-scan architecture, IEEE
Computer Society, 2001, ISBN: 0-7381-2944-5
A.2.4 (Not used)
A.2.5 Monitored redundancy
NOTE This technique/measure is referenced in Table A.3 of IEC 61508-2.
Aim: To detect failure, by providing several functional units, by monitoring the behaviour of
each of these to detect failures, and by initiating a transition to a safe condition if any
discrepancy in behaviour is detected.
Description: The safety function is executed by at least two hardware channels. The outputs
of these channels are monitored and a safe condition is initiated if a fault is detected (i.e. if
the output signals from all channels are not identical).
References:
Elektronik in der Sicherheitstechnik. H. Jürs, D. Reinert, Sicherheitstechnisches Informations-
und Arbeitsblatt 330220, BIA-Handbuch, Erich-Schmidt Verlag, Bielefeld, 1993.
http://www.bgia-handbuchdigital.de/330220
Dependability of Critical Computer Systems 1. F. J. Redmill, Elsevier Applied Science, 1988,
ISBN 1-85166-203-0
A.2.6 Electrical/electronic components with automatic check
NOTE This technique/measure is referenced in Table A.3 of IEC 61508-2.
Aim: To detect faults by periodic checking of the safety functions.
Description: The hardware is tested before starting the process, and is tested repeatedly at
suitable intervals. The EUC continues to operate only if each test is successful.
References:
61508-7 © IEC:2010 – 13 –
Elektronik in der Sicherheitstechnik. H. Jürs, D. Reinert, Sicherheitstechnisches Informations-
und Arbeitsblatt 330220, BIA-Handbuch, Erich-Schmidt Verlag, Bielefeld, 1993.
http://www.bgia-handbuchdigital.de/330220
Dependability of Critical Computer Systems 1. F. J. Redmill, Elsevier Applied Science, 1988,
ISBN 1-85166-203-0
A.2.7 Analogue signal monitoring
NOTE This technique/measure is referenced in Tables A.3 and A.13 of IEC 61508-2.
Aim: To improve confidence in measured signals.
Description: Wherever there is a choice, analogue signals are used in preference to digital
on/off states. For example, trip or safe states are represented by analogue signal levels,
usually with signal level tolerance monitoring. The technique provides continuity monitoring
and a higher level of confidence in the transmitter, reducing the necessary proof-test
frequency of the transmitter sensing function. External interfaces, for example impulse lines,
will also require testing.
A.2.8 De-rating
NOTE This technique/measure is referenced in 7.4.2.13 of IEC 61508-2.
Aim: To increase the reliability of hardware components.
Description: Hardware components are operated at levels which are guaranteed by the
design of the system to be well below the maximum specification ratings. De-rating is the
practice of ensuring that under all normal operating circumstances, components are operated
well below their maximum stress levels.
A.3 Processing units
Global objective: To recognise failures which lead to incorrect results in processing units.
A.3.1 Self-test by software: limited number of patterns (one-channel)
NOTE This technique/measure is referenced in Table A.4 of IEC 61508-2.
Aim: To detect, as early as possible, failures in the processing unit.
Description: The hardware is built using standard techniques which do not take any special
safety requirements into account. The failure detection is realised entirely by additional
software functions which perform self-tests using at least two complementary data patterns
(for example 55hex and AAhex).
A.3.2 Self-test by software: walking bit (one-channel)
NOTE This technique/measure is referenced in Table A.4 of IEC 61508-2.
Aim: To detect, as early as possible, failures in the physical storage (for example registers)
and instruction decoder of the processing unit.
Description: The failure detection is realised entirely by additional software functions which
perform self-tests using a data pattern (for example walking-bit pattern) which tests the
physical storage (data and address registers) and the instruction decoder. However, the
diagnostic coverage is only 90 %.

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A.3.3 Self-test supported by hardware (one-channel)
NOTE This technique/measure is referenced in Table A.4 of IEC 61508-2.
Aim: To detect, as early as possible, failures in the processing unit, using special hardware
that increases the speed and extends the scope of failure detection.
Description: Additional special hardware facilities support self-test functions to detect
failure. For example, this could be a hardware unit which cyclically monitors the output of a
certain bit pattern according to the watch-dog principle.
A.3.4 Coded processing (one-channel)
NOTE This technique/measure is referenced in Table A.4 of IEC 61508-2.
Aim: To detect, as early as possible, failures in the processing unit.
Description: Processing units can be designed with special failure-recognising or failure-
correcting circuit techniques. So far, these techniques have been applied only to relatively
simple circuits and are not widespread; however, future developments should not be
excluded.
References:
Le processeur codé: un nouveau concept appliqué à la sécurité des systèmes de transports.
Gabriel, Martin, Wartski, Revue Générale des chemins de fer, No. 6, June 1990
Vital Coded Microprocessor Principles and Application for Various Transit Systems. P. Forin,
IFAC Control Computers Communications in Transportation, 79-84, 1989
A.3.5 Reciprocal comparison by software
NOTE This technique/measure is referenced in Table A.4 of IEC 61508-2.
Aim: To detect, as early as possible, failures in the processing unit, by dynamic software
comparison.
Description: Two processing units exchange data (including results, intermediate results and
test data) reciprocally. A comparison of the data is carried out using software in each unit and
detected differences lead to a failure message.
A.4 Invariable memory ranges
Global objective: The detection of information modifications in the invariable memory.
A.4.1 Word-saving multi-bit redundancy (for example ROM monitoring
with a modified Hamming code)
NOTE 1 This technique/measure is referenced in Table A.5 of IEC 61508-2.
NOTE 2 See also A.5.6 “RAM monitoring with a modified Hamming code, or detection of data failures with error-
detection-correction codes (EDC)” and C.3.2 “Error detecting and correcting codes”.
Aim: To detect all single-bit failures, all two-bit failures, some three-bit failures, and some all-
bit failures in a 16-bit word.
Description: Every word of memory is extended by several redundant bits to produce a
modified Hamming code with a Hamming distance of at least 4. Every time a word is read,
checking of the redundant bits can determine whether or not a corruption has taken place. If a
difference is found, a failure message is produced. The procedure can also be used to detect

61508-7 © IEC:2010 – 15 –
addressing failures, by calculating the redundant bits for the concatenation of the data word
and its address.
References:
Prüfbare und korrigierbare Codes. W. W. Peterson, München, Oldenburg, 1967
Error detecting and error correcting codes. R. W. Hamming, The Bell System Technical
Journal 29 (2), 147-160, 1950
A.4.2 Modified checksum
NOTE This technique/measure is referenced in Table A.5 of IEC 61508-2.
Aim: To detect all odd-bit failures, i.e. approximately 50 % of all possible bit failures.
Description: A checksum is created by a suitable algorithm which uses all the words in a
block of memory. The checksum may be stored as an additional word in ROM, or an
additional word may be added to the memory block to ensure that the checksum algorithm
produces a predetermined value. In a later memory test, a checksum is created again using
the same algorithm, and the result is compared with the stored or defined value. If a
difference is found, a failure message is produced.
A.4.3 Signature of one word (8-bit)
NOTE This technique/measure is referenced in Table A.5 of IEC 61508-2.
Aim: To detect all one-bit failures and all multi-bit failures within a word, as well as
approximately 99,6 % of all possible bit failures.
Description: The contents of a memory block is compressed (using either hardware or
software) using a cyclic redundancy check (CRC) algorithm into one memory word. A typical
CRC algorithm treats the whole contents of the block as byte-serial or bit-serial data flow, on
which a continued polynomial division is carried out using a polynomial generator. The
remainder of the division represents the compressed memory contents – it is the "signature"
of the memory – and is stored. The signature is computed once again in later tests and
compared with one already stored. A failure message is produced if there is a difference.
A.4.4 Signature of a double word (16-bit)
NOTE This technique/measure is referenced in Table A.5 of IEC 61508-2.
Aim: To detect all one-bit failures and all multi-bit failures within a word, as well as
approximately 99,998 % of all possible bit failures.
Description: This procedure calculates a signature using a cyclic redundancy check (CRC)
algorithm, but the resulting value is at least two words in size. The extended signature is
stored, recalculated and compared as in the single-word case. A failure message is produced
if there is a difference between the stored and recalculated signatures.
A.4.5 Block replication (for example double ROM with hardware or
software comparison)
NOTE This technique/measure is referenced in Table A.5 of IEC 61508-2.
Aim: To detect all bit failures.
Description: The address space is duplicated in two memories. The first memory is operated
in the normal manner. The second memory contains the same information and is accessed in
parallel to the first. The outputs are compared and a failure message is produced if a

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difference is detected. In order to detect certain kinds of bit errors, the data must be stored
inversely in one of the two memories and inverted once again when read.
A.5 Variable memory ranges
Global objective: Detecting failures during addressing, writing, storing and reading.
NOTE Soft-errors are listed in Table A.1, IEC 61508-2 as faults to be detected during operation or to be analysed
in the derivation of the safe failure fraction. Causes of soft errors are: (1) Alpha particles from package decay, (2)
Neutrons, (3) external EMI noise, (4) Internal cross-talk. External EMI noise is covered by other requirements of
this international standard.
The effect of Alpha particles and Neutrons should be mastered by safety integrity measures at runtime. Safety
integrity measures effective for hard errors may not be effective for soft errors, e.g. RAM tests, such as walk-path,
galpat, etc. are not effective, whereas monitoring techniques such as Parity and ECC with recurring read of the
memory cells are.
A soft error occurs when a radiation event causes enough of a charge disturbance to reverse or flip the data state
of a low energized semiconductor memory cell, register, latch, or flip-flop. The error is called “soft” because the
circuit itself is not permanently damaged by the radiation. Soft-errors are classified in Single Bit Upsets (SBU) or
Single Event Upsets (SEU) and Multi-Bit Upsets (MBU).
If the disturbed circuit is a storage element like memory cell or flip-flop, the state is stored until the next (intended)
write operation. The new data will be stored correctly. In a combinatory circuit the effect is rather a glitch because
there is a continuous energy flow from the component driving this node. On connecting wires and communication
lines the effect could also be a glitch. However due to the larger capacitance the effect by Alpha particles and
Neutrons is considered negligible.
Soft-errors may be relevant to variable memory of any kind, i.e., to DRAM, SRAM, register banks in µP, cache,
pipelines, configuration registers of devices such as ADC, DMA, MMU, Interrupt controller, complex timers.
Sensitivity to alpha and neutron particles is a function of both core voltage and geometry. Smaller geometries at
2,5 V core voltage and especially below 1,8 V would require more evaluation and more effective protective
measures.
The soft error rate has been reported (see a) and i) below) to be in a range of 700 Fit/MBit to 1 200 Fit/MBit for
(embedded) memories. This is a reference value to be compared with data coming from the silicon process with
which the device is implemented. Until recently SBU were considered to be dominant, but the latest forecast (see
a) below) reports a growing percentage of MBU of the overall soft-error rate (SER) for technologies from 65 nm
down.
The following literature and sources give details about soft-errors:
a) Altitude SEE Test European Platform (ASTEP) and First Results in CMOS 130 nm SRAM. J-L. Autran,
P. Roche, C. Sudre et al. Nuclear Science, IEEE Transactions on Volume 54, Issue 4, Aug. 2007
Page(s):1002 - 1009
b) Radiation-Induced Soft Errors in Advanced Semiconductor Technologies, Robert C. Baumann, Fe
...