EN 60848:2002

SLOVENSKI STANDARD
01-maj-2003
GRAFCET specification language for sequential function charts (IEC 60848:2002)
GRAFCET specification language for sequential function charts
Entwurfssprache GRAFCET für Ablauf-Funktionspläne
Langage de spécification GRAFCET pour diagrammes fonctionnels en séquence
Ta slovenski standard je istoveten z: EN 60848:2002
ICS:
01.100.25 5LVEHVSRGURþMD Electrical and electronics
HOHNWURWHKQLNHLQHOHNWURQLNH engineering drawings
35.060 Jeziki, ki se uporabljajo v Languages used in
informacijski tehniki in information technology
tehnologiji
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN 60848
NORME EUROPÉENNE
EUROPÄISCHE NORM April 2002
ICS 29.020
English version
GRAFCET specification language for sequential function charts
(IEC 60848:2002)
Langage de spécification GRAFCET Entwurfssprache GRAFCET
pour diagrammes fonctionnels en séquence für Ablauf-Funktionspläne
(CEI 60848:2002) (IEC 60848:2002)
This European Standard was approved by CENELEC on 2002-04-01. CENELEC members are bound to
comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and
notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Czech Republic,
Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands,
Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2002 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60848:2002 E
Foreword
The text of document 3B/344/FDIS, future edition 2 of IEC 60848, prepared by SC 3B, Documentation,
of IEC TC 3, Information structures, documentation and graphical symbols, was submitted to the
IEC-CENELEC parallel vote and was approved by CENELEC as EN 60848 on 2002-04-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2003-01-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2005-04-01
Annexes designated "normative" are part of the body of the standard.
Annexes designated "informative" are given for information only.
In this standard, annex ZA is normative and annexes A, B and C are informative.
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 60848:2002 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following note has to be added for the standard indicated:
IEC 61131-3 NOTE Harmonized as EN 61131-3:1993 (not modified).
__________
- 3 - EN 60848:2002
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references the latest edition of the publication referred to applies (including
amendments).
NOTE When an international publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
Publication Year Title EN/HD Year
IEC 60050-351 1998 International Electrotechnical --
Vocabulary
Part 351: Automatic control
IEC 60617-12 1997 Graphical symbols for diagrams EN 60617-12 1998
Part 12: Binary logic elements

NORME
CEI
INTERNATIONALE IEC
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
2002-02
Langage de spécification GRAFCET
pour diagrammes fonctionnels en séquence
GRAFCET specification language
for sequential function charts
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électronique ou mécanique, y compris la photocopie et les photocopying and microfilm, without permission in writing from
microfilms, sans l'accord écrit de l'éditeur. the publisher.
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Telephone: +41 22 919 02 11 Telefax: +41 22 919 03 00 E-mail: inmail@iec.ch  Web: www.iec.ch
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Pour prix, voir catalogue en vigueur
For price, see current catalogue

60848 © IEC:2002 – 3 –
CONTENTS
FOREWORD.5
INTRODUCTION.7
1 Scope and object .9
2 Normative references.9
3 Terms and definitions .9
4 General principles.13
4.1 Context.13
4.2 GRAFCET, a behaviour specification language .15
4.3 GRAFCET, short presentation.15
4.4 Syntax rule .19
4.5 Evolution rules.19
4.6 Input events.21
4.7 Internal events.21
4.8 Output modes.23
4.9 Application of the evolution rules.23
4.10 Comparison between the two output modes .29
5 Graphical representation of the elements .31
6 Graphical representation of sequential structures .57
6.1 Basic structures.57
6.2 Particular structures .63
7 Structuring.69
7.1 Partition of a grafcet .69
7.2 Structuring using the forcing of a partial grafcet .73
7.3 Structuring using the enclosure.75
7.4 Structuring using the macro-steps.81
Annex A (informative) Example of control of a press.83
Annex B (informative) Example: Automatic weighing-mixing.85
Annex C (informative) Relations between the GRAFCET of IEC 60848
and the SFC of IEC 61131-3.97
Bibliography . 101

60848 © IEC:2002 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
GRAFCET SPECIFICATION LANGUAGE
FOR SEQUENTIAL FUNCTION CHARTS
FOREWORD
1) The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of the 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, the IEC publishes International Standards. 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-governmental organizations liaising
with the IEC also participate in this preparation. The IEC collaborates closely 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 the 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 National Committees.
3) The documents produced have the form of recommendations for international use and are published in the form
of standards, technical specifications, technical reports or guides and they are accepted by the National
Committees in that sense.
4) In order to promote international unification, IEC National Committees undertake to apply IEC International
Standards transparently to the maximum extent possible in their national and regional standards. Any
divergence between the IEC Standard and the corresponding national or regional standard shall be clearly
indicated in the latter.
5) The IEC provides no marking procedure to indicate its approval and can not be rendered responsible for any
equipment declared to be in conformity with one of its standards.
6) Attention is drawn to the possibility that some of the elements of this International Standard may be the subject
of patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60848 has been prepared by subcommittee 3B: Documentation, of
IEC technical committee 3: Information structures, documentation and graphical symbols.
This second edition cancels and replaces the first edition published in 1988 and constitutes a
global technical revision with the addition of the following main concepts: input event, internal
event, assignation, allocation, forcing, macro-step and enclosure.
The text of this standard is based on the following documents:
FDIS Report on voting
3B/344/FDIS 3B/346/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 3.
Annexes A, B and C are for information only.
The committee has decided that the contents of this publication will remain unchanged until
2006. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
60848 © IEC:2002 – 7 –
INTRODUCTION
The main reason for the revision of this standard is the desire of the users to increase the
standardised specification language with new concepts, allowing a structured and hierarchical
description.
Otherwise, in addition to the descriptive and functional aspects of the first edition, it now seems
necessary to add the formal and behavioural aspects, which are essential for the definition of a
real specification language.
For all these reasons, an overall review of the document is required.
This standard is mainly for people (design engineers, realisation engineers, maintenance
engineers, etc.) who need to specify the behaviour of a system (control-command of an
automatic machine, safety component, etc.). This specification language should also serve as
a communication means between designers and users of automated systems.

60848 © IEC:2002 – 9 –
GRAFCET SPECIFICATION LANGUAGE
FOR SEQUENTIAL FUNCTION CHARTS
1 Scope and object
1)
This International Standard defines the GRAFCET specification language for the functional
description of the behaviour of the sequential part of a control system.
This standard specifies the symbols and the rules for the graphical representation of this
language, as well as for its interpretation.
This standard has been prepared for automated production systems of industrial applications.
However no particular area of application is excluded.
Methods of development of a specification that makes use of GRAFCET are beyond the scope
of this standard. One method is for example the "SFC language" specified in IEC 61131-3,
which defines a set of programming languages for programmable controllers.
NOTE See annex C for further information on the relations between IEC 60848 and implementation languages
such as the SFC of IEC 61131-3.
2 Normative references
The following normative documents contain provisions, which, through reference in this text,
constitute provisions of this International Standard. For dated references, subsequent
amendments to, or revisions of, any of these publications do not apply. However, parties to
agreements based on this International Standard are encouraged to investigate the possibility
of applying the most recent editions of the normative documents indicated below. For undated
references, the latest edition of the normative document referred to applies. Members of IEC
and ISO maintain registers of currently valid International Standards.
IEC 60050-351:1998, International Electrotechnical Vocabulary (VEI – Part 351: Automatic
control
IEC 60617-12:1997, Graphical symbols for diagrams – Part 12: Binary logical elements
3 Terms and definitions
For the purposes of this International Standard, the following definitions apply. The definitions
of the terms preceded by an asterisk apply only in the context of the GRAFCET specification
language. The chosen order is the alphabetic one.
3.1
* action
GRAFCET language element associated with a step, indicating an activity to be performed on
output variables
———————
1)
GRAFCET , GRAphe Fonctionnel de Commande Etape Transition.

60848 © IEC:2002 – 11 –
3.2
chart, graph
graphical presentation describing the behaviour of a system, for example the relations between
two or more variable quantities, operations or states
3.3
* directed link
GRAFCET language element indicating the evolution paths between steps by connecting steps
to transitions and transitions to steps
3.4
* grafcet chart
function chart using GRAFCET
NOTE The “grafcet chart” can, in short form, be called “grafcet”.
3.5
* input event
event characterized by the change of at least one value of all input variables of the sequential
part of the system
3.6
* internal event
event characterized by an input event associated with the situation of the sequential part of the
system
3.7
* interpretation
part of the GRAFCET enabling the linkage of:
– the input variables and the structure, by the means of the transition-condition;
– the output variables and the structure, by the means of the actions
3.8
* situation
name of the state of the system described by grafcet and characterised by the active steps at a
given instant
3.9
* step
GRAFCET language element used for the definition of the state of the sequential part of the
system
NOTE 1 A step can be active or inactive.
NOTE 2 The set of active steps represents the situation of the system.
3.10
* structure
part of the GRAFCET enabling the description of the possible evolution between situations
3.11
system
a set of interrelated elements considered in a defined context as a whole and separated from
their environment
[IEV 351-11-01]
NOTE 1 Such elements may be material objects and concepts as well as their results (e.g. forms of organisation,
mathematical methods, programming languages).

60848 © IEC:2002 – 13 –
NOTE 2 The system is considered to be separated from the environment and from the other external systems by
an imaginary surface, which cuts the links between them and the system.
NOTE 3 The language GRAFCET can be use to describe the logical behaviour of any kind of system.
3.12
* transient evolution
evolution characterized by the clearing of several successive transitions on the occurrence of a
single input event
3.13
* transition
GRAFCET language element indicating a possible evolution of the activity between two or more
steps
NOTE The possible evolution is realised by clearing the transition.
3.14
* transition-condition
GRAFCET language element associated with a transition indicating the result of a boolean
expression
NOTE The transition-condition can be either true or false.
4 General principles
4.1 Context
The implementation of an automated system requires, in particular, a description relating cause
and effect. To do this, the logical aspect of the desired behaviour of the system will be described.
The sequential part of the system, which is accessed via Boolean input and output variables, is
the logical aspect of this physical system. The behaviour indicates the way in which the output
variables depend on the input variables (see note to figure 1). The object of the GRAFCET is to
specify the behaviour of the sequential part of the systems.
SYSTEM
SEQUENTIAL PART OF THE SYSTEM
fi
fr
auto
Boolean Boolean
P/P
hg
inputs outputs
fi
Dcy
0 fr
hg
Pv
(auto Pv ) + (P/P Dcy)
⋅ ⋅
mr fr
Non boolean
[C > 6]
[C > 6]
C
outputs
Test
Non boolean mr
inputs
PID
NOTE  The sequential part of the system is characterised by its input variables, its output variables, and its
behaviour. This sequential part comprises only input and output boolean variables. However the GRAFCET
specification language by extension allows the description of the behaviour of the non Boolean variables (for
example: evaluation of an assertion or allocation of a numeric value for a variable).
Figure 1 – Graphical representation of the sequential part of a system

60848 © IEC:2002 – 15 –
4.2 GRAFCET, a behaviour specification language
The GRAFCET specification language enables a grafcet to be created showing the expected
behaviour of a given sequential system. This language is characterized mainly by its graphic
elements, which, associated with an alphanumerical expression of variables, provides a
synthetic representation of the behaviour, based on an indirect description of the situation of
the system.
The behaviour description on states is the following: the "monomarked" states correspond to
the GRAFCET situations, which implies the uniqueness of the situation at a given instant. The
states are connected to each other by means of an evolution condition, which allows the
passage from one situation to another one to be described.
For reasons of convenience, the behaviour description based on states is better replaced by a
description based on steps called GRAFCET. In the GRAFCET, several steps may be active
simultaneously, the situation being then characterized by the set of active steps at the
considered moment. The evolution of one set of steps to another are translated by one or
several transitions, each characterized by:
• its preceding steps,
• its succeeding steps,
• its associated transition-condition.
NOTE These reasons lead to the syntax rule enforcing the alternation step-transition.
4.3 GRAFCET, short presentation
The GRAFCET is used for the design of grafcet charts to provide a graphical and synthetic
representation of the sequential systems behaviour. The representation (figure 2) distinguishes:
• the structure, which allows possible evolutions between the situations to be described,
• the interpretation, which enables the relationship between input, output variables and the
structure (evolution, assignation and allocation rules are necessary to achieve this
interpretation).
4.3.1 The structure comprises the following basic items
• Step (definition: 3.9, symbol 1). A step is either active or inactive, the set of the active
steps of a grafcet chart at any given instant represents the situation of this grafcet at this
instant.
• Transition (definition: 3.13, symbol 7). A transition indicates that an evolution of the activity
between two or more steps may evolve. This evolution is realized by the clearing of the
transition.
• Directed link (definition: 3.3, symbol 10). A directed link connects one or several steps to a
transition, or a transition to one or several steps.
4.3.2 The following elements are used for the interpretation
• Transition-condition (definition: 3.14, symbol 13). Associated with each transition, the
transition-condition is a logical expression which is true or false and which is composed of
input variables and/or internal variables.
• Action (definition: 3.1). The action indicates, in a rectangle, what shall be done to the
output variable, either by assignation (continuous action, symbol 20), or allocation (stored
action, symbol 26).
60848 © IEC:2002 – 17 –
QuickDescent
On
SEQUENTIAL Boolean
HighPosition DriftRotation
Boolean
ouput
PART OF A
input
SlowDescent variables
LowPosition
variables
SYSTEM
EndApproch Ascent
Input variables forming the
transition condition with the
logic operator : « AND »
Steps 1 and 2
(1)
On AND HighPosition
2 QuickDescent DriftRotation
Ouput variables assigned in
Directed links
(2)
EndApproch
the associated actions to
the step 2
3 SlowDescent DriftRotation
(3)
Continuous actions
LowPosition
associated to the steps
3 and 4
Ascent
Transitions 3 and 4
Transition condition
(4)
HighPosition
associated to transition 4
STRUCTURE
INTERPRETATION
Figure 2 – Structure and interpretation elements used in a grafcet chart to describe
the behaviour of a sequential part of the system defined by its input and output variables

60848 © IEC:2002 – 19 –
4.4 Syntax rule
Step transition and transition step alternation shall always be respected whatever the sequence.
Consequences:
• Two steps shall never be connected directly by a directed link.
• The directed link shall only connect a step to a transition or a transition to a step.
4.5 Evolution rules
As each situation is characterized by the set of active steps at a given instant, the GRAFCET
evolution rules only affect the application, on the steps, of the evolution principle between the
situations of the sequential part of the system.
4.5.1 Initial situation
The initial situation is the situation at the initial time. Therefore it is described by the set of
steps active at this time. The choice of the situation at the initial time depends on the
methodology relating to the type of sequential part of the system considered.
Rule 1: The initial situation, chosen by the designer, is the situation at the initial time.
4.5.2 Clearing of a transition
Rule 2: A transition is said to be enabled when all immediately preceding steps linked to this
transition are active. The clearing of a transition occurs:
• when the transition is ENABLED,
• AND WHEN its associated transition-condition is TRUE.
4.5.3 Evolution of active steps
Rule 3: The clearing of a transition simultaneously provokes the activation of all the immediate
succeeding steps and the deactivation of all the immediate preceding steps.
4.5.4 Simultaneous evolutions
The evolution between two active situations implies that no other intermediate situation is
possible, the change from one representation of the situation by a set of steps to another
representation is instantaneous.
Rule 4: Several transitions, which can be cleared simultaneously, are simultaneously cleared.
4.5.5 Simultaneous activation and deactivation of a step
If a step is included in the description of the preceding situation and in that of the following one,
it can therefore only remain active.
Rule 5: If during the operation, an active step is simultaneously activated and deactivated, it
remains active.
60848 © IEC:2002 – 21 –
4.6 Input events
The evolution rules show that only a change in the values of the input variables may cause the
evolution of the grafcet. This change called "input event" shall be defined by the preceding
value and the succeeding value of all the input variables to characterise this single event. In
practice, a set of input events is specified only by the characterised state change (rising edge
or falling edge) of one or several boolean input variables.
NOTE The rising edge of a logical variable, indicated by the sign "↑" in front of a boolean variable, indicates that
this rising edge is only true for the change from value 0 to value 1 of the variable concerned. The falling edge of a
logical variable noted by the sign "↓" in front of a boolean variable, indicates that this falling edge is only true for
the change from value 1 to value 0 of the variable concerned.
It is said that "the event occurs" at the date of the change of state of the input variables which
characterize it.
4.6.1 Input events specification
The input events specification is implemented by a logical expression of one or several
characteristic variables, often in a transition-condition. More rarely, it may also directly specify
an internal event (see 4.7).
EXAMPLE 1:
The expression "↑a" describes the set of all input events for which the preceding
↑↑a
↑↑
value of the input variable a is 0 and its succeeding value is 1, regardless of the
value of the other input variables of the system.
EXAMPLE 2:
The expression "a ⋅↑b" describes the set of all input events for which the succeeding
a ⋅⋅⋅↑⋅ ↑↑↑b
value of the input variable a is 1, and the preceding value of the input variable b is 0
and its succeeding value is 1, regardless of the value of the other input variables of
the system.
EXAMPLE 3:
The expression "a" describes the sets of all input events for which the succeeding
a
value of the input variable a is 1, regardless of the value of the other input variables
of the system.
NOTE Used in a transition-condition, this expression could lead to a transient evolution (see 3.12)
4.7 Internal events
Only certain input events could occur from a given situation. The connection between a
situation and input event, which may occur from this situation, is called internal event (see 3.6).
This notion is mainly used by the designer to condition an output allocation to a set of internal
events (see 4.8.2). The description of a set of internal events is realized by one of the following
ways.
4.7.1 Internal events described by the step activation
The step activation, noted graphically (symbol 27), describes the set of internal events each of
which has this step activation as a consequence.
4.7.2 Internal events described by the deactivation of a step
The graphically noted deactivation of a step, (symbol 28), describes the set of the internal
events each of which have this step deactivation as consequence.

60848 © IEC:2002 – 23 –
4.7.3 Internal events described by the clearing of a transition
The graphically noted clearing of a transition (symbol 29), describes the set of internal events
each of which have the clearing of this transition as consequence.
4.8 Output modes
The actions enable links to establish the connection between the evolution of the grafcet chart
and the outputs. Two output modes, continuous mode or stored mode, describe how the
outputs depend on the evolution and on the system inputs.
4.8.1 Continuous mode (assignation on state)
In the continuous mode, the association of an action with a step indicates that an output
variable has a true value if the step is active and if the assignation condition is verified. The
assignation condition is a logical expression of the input variables and/or the internal ones (see
symbol 22). If one of the conditions is not met and provided that no other action relating to the
same output meets the conditions, the output variable concerned takes the false value.
Assignation refers to imposing the value of the output variables (true or false).
The set of the local assignation (relating to the active steps at a given instant) defines the
assignation of all the output variables for this situation.
Assignation rule: For a given situation, the value of the outputs relating to the continuous
actions is assigned:
• to the true value, for each output relating to the actions associated with active steps and for
which the assignation conditions are verified,
• to the false value, for the other outputs (which are not assigned to the true value).
4.8.2 Stored mode (allocation on event)
In the stored mode, the association of an action to internal events is used to indicate that an
output variable takes and maintains the enforced value if one of these events occurs.
Explicit representations are necessary to describe the association of the actions with the
events (activation step, deactivation step, clearing of a transition, etc.).
The value of an output relating to a stored action remains unchanged until a new specified
event modifies its value.
Allocation refers to storing, at a considered moment, a determined value affected to an output
variable.
Allocation rule: The value of an output, relating to a stored action and associated to an event,
is allocated to the indicated value, if the specified internal event occurs; the value of this output
is null at the initialisation.
4.9 Application of the evolution rules
Intuitive interpretation of the evolution, called “step by step”, designates the progressive way
which allows, on the occurrence of an input event and from the preceding situation, to
determine the succeeding situation of this event, by the successive application of the evolution
rules on each transition. The interpretation facility is a device to enable an indirect specification
of the evolution, but the designer shall take care that the clearing of the transitions on this path
does not involve the effective activation of the intermediate situations.

60848 © IEC:2002 – 25 –
4.9.1 Non transient evolution
In general, the evolution is non-transient, which means that the input event only leads to one
evolution stage (the simultaneous clearing of one or more transitions).
11 EXAMPLE: "Non transient evolution"
Preceding situation: step 11 active, a=0 , b=0 and c=0.
(1)
(1)
a
a
Intuitive interpretation of the evolution:
The change in the value “a” involves the clearing of the transition
(1) and the activation of the step 12, the transition (2) can not be
cleared, because b=0, the subsequent situation is therefore: step
(2) b
(2) b
12 active.
Real interpretation of the evolution:
The occurrence of one of the input events such as the value of a
(3) (3)
c c
changes from 0 to 1 leads straight to the subsequent situation: step
12 active.
4.9.2 Transient evolution
In some cases, the application of the evolution rules can lead to successively clearing some
transitions (in several evolution stages) if the transition-conditions associated with the
subsequent transitions are already true, when the first transitions considered are cleared. The
corresponding description, referred to as transient, uses the path taken to indicate how to
move from a preceding situation to a succeeding situation (see 3.9).
The corresponding intermediate steps, referred to as unstable are not activated, but we
consider that they have been "virtually" activated and deactivated along the intuitive evolution
path, as well as for the corresponding transitions which have been "virtually" cleared.
EXAMPLE: "Transient evolution"
Preceding situation: step 11 active, a=0, b=1 and c=0.
Intuitive interpretation of the evolution:
(1)
(1)
a
a The change in the value “a” involves the clearing of the transition
(1) and the virtual activation of the step 12, then the transition (2) is
12 virtually cleared, because b=1, leading to the succeeding situation:
step 13 active.
(2) b
(2) b
Real interpretation of the evolution:
The occurrence of one of the input events, such as the value of a
change from 0 to 1, leads to the succeeding situation: step 13
active.
(3) (3)
c c
4.9.3 Consequence of a transient evolution on the assignations
The assignation of an output value by a continuous action associated with a step, which is an
unstable step in the case of a transient evolution, is not effective, since the step is not really
activated (see 4.8.1).
60848 © IEC:2002 – 27 –
EXAMPLE: “Continuous action associated with an
11 11
unstable step”
Preceding situation: step 11 active, a=0, b=1 and c=0.
(1) (1)
a a
The occurrence of one of the input events such as the
value of "a" changes from 0 to 1, leads straight to the
12 12
B B
subsequent situation: step 13 active.
The preceding situation (step 11 active) and the
b (2) b
(2)
succeeding situation (step 13 active) assign the value 0
to the output variable B. The unstable step 12 being not
13 13
really activated, the assignation of B to the value 1 is
not effective on the transient evolution.
(3) (3)
c c
4.9.4 Consequence of a transient evolution on the allocations
The allocation to a determinate value of an output by a stored action (symbol 26) associated to
a step, which is an unstable step in the case of a transient evolution, is effective since this
allocation is associated to the events releasing this evolution (see 4.8.2).
EXAMPLE 1: “Stored action associated with the
11 11
activation of an unstable step”
Preceding situation: step 11 active, a=0, b=1 and c=0.
(1) (1)
a a
The occurrence of one of the input events such as the
value of "a" changes from 0 to 1, leads straight to the
12 12
B := 1
B := 1
subsequent situation: step 13 active.
The allocation of the value 1 to the output variable B is
(2) b (2) b
realized on the occurrence of one of the input events
having the real or the virtual activation of the step 12 as
13 13
consequence.
(3) (3)
c c
EXAMPLE 2: “Stored action associated with the
11 11
deactivation of an unstable step”
Preceding situation: step 11 active, a=0, b=1 and c=0.
(1) (1)
a a
The occurrence of one of the input events such as the
value of "a" changes from 0 to 1, leads straight to the
12 12
B := 0
B := 0
subsequent situation: step 13 active.
The allocation of the value 0 to the output variable B is
b b
(2) (2)
realized on the occurrence of one of the input events
having the real or the virtual deactivation of the step 12
13 13
as consequence.
(3) (3)
c c
60848 © IEC:2002 – 29 –
4.10 Comparison between the two output modes
The choice of the output mode depends on the practice and methodology used. However the
designers attention is drawn to the important differences between the two modes.
4.10.1 Determination of the value of the outputs
• In continuous mode, all the outputs are assigned according to the situation, to the true
value for the outputs explicitly indicated in the actions associated to the active steps, and to
the false value for the other ones which are implicitly set by omission (see assignation rule,
4.8.1).
• In the stored mode, only the considered outputs are modified according to the indicated
value, the other stored values of the outputs remain unchanged (see allocation rule, 4.8.2).
4.10.2 Analysis of the value of the outputs for a grafcet-chart at a defined instant
• In the continuous mode, the knowledge of the situation and the value of the inputs is
sufficient to determine the value of the outputs (see 4.8.1).
• In the stored mode, the knowledge of the situation and the value of the inputs is not
sufficient, the preceding evolutions shall also be known to determine the value of the
outputs (see 4.8.2).
4.10.3 Actions relative to transient evolution
• In the continuous mode, the actions associated with an unstable step are not taken into
consideration because this step is not activated (see 4.9.1).
• In the stored mode, the actions associated with events and in relation with a transient
evolution are taken into consideration because the triggered events releasing this evolution
occur (see 4.9.2).
4.10.4 Possible conflict on the value of the outputs
• In the continuous mode, the assignation principles ensure every assignation conflict on the
particular output to be avoided.
• In the stored mode, the allocation rules do not allow the possible assignation conflicts on a
same output to be avoided. The designer shall ensure that two contradictory allocations can
not occur simultaneously.
NOTE 1 Both output modes can be used in one specification in GRAFCET, but the value of an output variable is
determined either by assignation or by allocation. The specification of an allocation to an output variable (stored
mode), excludes this output variable of any assignation (continuous mode).
NOTE 2 Clause 5 gives the graphic symbols which enable the stored actions (indicated by explicit representation
according to the set of specified events) to be distinguished from the continuous ones (indicated by absence of any
representation).
NOTE 3 In the frequent case of the specification of control system behaviour, the current industrial practice forces
the employment of the continuous mode for all the outputs to the actuators, and the stored mode for describing
internal control tasks. These tasks, such as the incrementation of a counter, or the modification of the value for a
numerical register, refer to internal variables, which are not necessarily Boolean ones. The internal tasks
associated with the stored actions, as well as the calculation of expressions associated with transition-conditions,
are not described in the present standard, but are associated by the use of the logical description of the grafcet
evolutions. The designers should take care to make their descriptions consistent and clear.

60848 © IEC:2002 – 31 –
5 Graphical representation of the elements
The elements of GRAFCET have their own symbolic representation which when correctly
associated, enable clear and synthetic function-charts to be implemented.
NOTE 1 Only the global representation of the symbols is imposed; dimensions and details (thickness of lines, font
of characters, etc.) are left up to the users.
NOTE 2 The stippled representation indicates the context of the symbol.
Table 1 – Steps
No. Symbol Description
Step: At a given moment, a step is either active or inactive. The set of active steps defines the
situation of the given system at the considered instant.
[1]
*
The height-width ratio of the rectangle is arbitrary, although a square is recommended.
For the purposes of identification, the steps shall have a label, for example, alphanumerical.
The label assigned to the step shall replace the asterisk at the upper half of the general symbol.
EXAMPLE 1: “ Step 2 “
EXAMPLE 2: “ Step 3 represented in its active state “
NOTE It may be useful to indicate which steps are active at a given instant by marking these
steps with a dot. This dot is not part of the step symbol and is only used for explanatory
purposes.
Step variable: The active or inactive state of the step may be represented by the logical values
X
*
[2]
"1" or "0" respectively of a boolean variable X , in which the asterisk shall be replaced by the
* *
label of the relevant step.
EXAMPLE: “ Step variable of the step 8 ” X8
Initial step: This symbol means that this step participates in the initial situation.
NOTE 1 The rules of the symbol 1 apply.
[3]
*
NOTE 2 An initial step could be “ unstable”, see 4.9.2.
EXAMPLE: “ Initial step 12 “
Enclosing step: This symbol indicates that this step contains other steps referred to as
enclosed steps.
[4]
*
NOTE 1 The rules of the symbol 1 apply.
NOTE 2 The properties and the examples of the use of the enclosing step are given in 7.3.
Initial enclosing step: This symbol means that this enclosing step participates in the initial
situation.
[5]
* NOTE An initial enclosing step contains at least one enclosed initial step.
Macro-step: Unique representation of a detailed part of the function-chart referred to as the
expansion of the macro-step.
[6]
M
*
NOTE The properties and the examples of the use of the macro-step are given in 7.4.

60848 © IEC:2002 – 33 –
Table 2 – Transitions
No. Symbol Description
Transition from one step to another: A transition is represented by a line
perpendicular to the link joining two steps.
NOTE 1 The transition is enabled when the immediate preceding step is active
[7]
(see the evolution rule No. 2, 4.5.2).
NOTE 2 Only one transition is ever possible between two steps (see 4.4).
NOTE 3 It is possible, for graphical representation reasons, to place transitions
on horizontal directed links (see annex B: figure B5, partial grafcet G1).
Transition designation:
The transition may have a designation, generally placed to the left, which should
not be mistaken for the associated transition-condition.
[8]
An alphanumerical label for the transition shall replace the asterisk.
( )
*
Synchronization preceding and/or succeeding a transition:
When several steps are connected to the same transition, the directed links from
and/or to these steps are grouped, to succeed or precede the synchronization
[9]
symbol represented by two parallel horizontal lines.
NOTE The reference for the synchronization symbol is 9.2.2.5 of ISO 5807.
EXAMPLE 1: Transition from one step
(12) to several (13, 23, 33).
The transition (8) is enabled when the
(8)
step 12 is active.
13 23 33
EXAMPLE 2: Transition from several
18 34 45
steps (18, 34, 45) to one step (12).
The transition (6) is only enabled when
all preceding steps are active.
(6)
EXAMPLE 3: Transition from several
14 28 35
steps (14, 28, 35) to several steps (15,
29, 36, 46).
The transition (14) is only enabled when
all preceding steps are active.
(14)
15 29 46
60848 © IEC:2002 – 35 –
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