What is GRAFCET? A complete guide to IEC 60848

GRAFCET is a graphical language that describes how a sequential control system behaves: a machine, a production line, a traffic light. A GRAFCET chart is a series of steps (what the system is doing) linked by transitions (the conditions for moving on). It is standardised internationally as IEC 60848 and is the origin of the Sequential Function Chart (SFC) language of PLCs.

This guide explains every element and rule of the standard with diagrams and examples. All the diagrams were drawn with GrafcetMaker, a free GRAFCET editor and simulator that runs in your browser: open the examples and watch them evolve step by step.

What GRAFCET stands for

GRAFCET stands for GRAphe Fonctionnel de Commande Étape-Transition, French for "step-transition functional control chart". It was proposed in 1977 by a working group of AFCET, the French association for cybernetics, together with ADEPA, the French agency for the development of automated production. It grew out of Petri nets and was designed as a clear, technology-independent way to write down what a control system must do, before anyone writes a line of PLC code.

• 1982: French standard NF C03-190.
• 1988: international standard IEC 848, Preparation of function charts for control systems.
• 2002: IEC 60848 edition 2, GRAFCET specification language for sequential function charts.
• 2013: IEC 60848 edition 3, the current one, also published in Europe as EN 60848 (DIN EN 60848 in Germany).

GRAFCET is taught in technical schools, vocational training and engineering degrees, especially in French-, German-, Spanish- and Portuguese-speaking countries. By convention, GRAFCET is the language and a grafcet is one chart written in it.

The elements of a grafcet

• A step is a square with a number. It stands for a stable situation of the system: waiting, filling, drilling. At any moment each step is either active or inactive, and the set of active steps is the situation of the grafcet. The step variable X1 is true while step 1 is active.
• An initial step, drawn as a double square, is active when the system starts. Every grafcet needs at least one.
• A transition is a short horizontal bar across a link. It always sits between steps: two steps are never linked directly, and neither are two transitions.
• The receptivity is the condition written next to a transition, for example start or pressure > 6. The receptivity 1 is always true.
• Directed links connect steps and transitions. They are read from top to bottom; a link that goes upward carries an arrow.
• Actions are rectangles to the right of a step: what the system does while the step is active, for example run the motor.

The five evolution rules

IEC 60848 defines exactly how a grafcet evolves, with five rules.

1. Initial situation: at start, the initial steps are active and all the others are inactive.
2. Clearing a transition: a transition is enabled when all the steps just above it are active. It is cleared (crossed) when it is enabled and its receptivity is true.
3. Evolution of the active steps: clearing a transition activates all the steps just below it and deactivates all the steps just above it.
4. Simultaneous clearing: all the transitions that can be cleared at the same moment are cleared simultaneously.
5. Simultaneous activation and deactivation: a step that is activated and deactivated at the same moment stays active.

When an input changes, the rules are applied again and again, with the same inputs, until no transition can be cleared any more. This is the search for stability. A step that is activated and left again during this search is an unstable step (or transient step): its continuous actions are never executed, but its stored actions are. For example, if the receptivities below steps 1 and 2 are both already true when step 1 becomes active, the grafcet goes straight through step 2 to step 3, and the outputs of step 2 never switch on.

Receptivities: conditions, edges and timers

A receptivity is a logical expression of inputs, internal variables, step variables and time. The textbook notation and the way you type it in GrafcetMaker:

Edges matter when a signal stays true: with ↑start, the cycle starts once per press of the button and not again as long as it is held. The full syntax of the editor is described in Writing receptivities.

Actions

• A continuous action is true as long as its step is active: Lamp is on while step 4 is active.
• A conditional action is a continuous action with an assignment condition written above its box: Horn sounds only while step 4 is active and alarm is true.
• Delayed and time-limited actions are conditional actions whose condition uses a timer: with 3s/X4 the output switches on 3 s after step 4 became active; with /(3s/X4) it is on for the first 3 s only.
• A stored action assigns a value once, and the value stays until another action changes it. It runs when the step is activated (arrow pointing up), when it is deactivated (arrow pointing down) or on an event while the step is active (a flag with the event, here ↑reset). Counters such as C := C + 1 are typical stored actions.

An output driven by continuous actions follows the steps automatically. An output set by a stored action stays set until another stored action resets it, so plan where that happens.

Sequences, alternatives and parallel branches

Choosing between paths: OR divergence

When a step is followed by several transitions, the grafcet takes the path whose receptivity is true: an alternative branch, drawn with a single horizontal line (OR divergence, called a selection of sequence in SFC). The paths join again on a single line (OR convergence).

Make the receptivities exclusive, so that they can never be true together, as a and /a * b here. Otherwise both transitions are cleared at once (rule 4) and both branches become active. GrafcetMaker warns you when it cannot prove that alternative receptivities are exclusive.

Doing things at the same time: AND divergence

When a transition is followed by several steps, they are all activated together: parallel branches, drawn with a double line (AND divergence, a simultaneous sequence in SFC). The branches end on a double line too (AND convergence): the transition below it is enabled only when the last step of every branch is active, so it waits for all branches to finish. This is how a grafcet synchronises tasks.

Loops, jumps, sources and sinks

• A loop goes back to an earlier step through an upward link with an arrow: the usual way to repeat a cycle.
• A jump (step skip) is an OR divergence whose one branch leads straight to a later step, for example to skip an optional operation.
• A source transition has no step above it. It is always enabled, so it is cleared whenever its receptivity is true; use an edge such as ↑a.
• A sink transition has no step below it: clearing it only deactivates the steps above it.

Structuring large applications

Real machines are described with several grafcets that work together.

• Partial grafcets each describe one part of the system, for example the production cycle, the modes of operation and safety. They run at the same time and see each other's steps through the step variables Xn. Together they form the global grafcet.
• A macro-step stands for a detailed sequence drawn on its own sheet, its expansion. The expansion has one input step E and one output step S. Clearing the transition above the macro-step activates step E; the transition below the macro-step is enabled when step S is active.
• An enclosing step contains one or more whole grafcets that live only while it is active. When it is activated, the enclosed steps marked with an asterisk become active; when it is deactivated, every enclosed step is deactivated.
• A forcing order, drawn in a double-framed action box, imposes a situation on another grafcet: G2{8} forces only step 8 of G2 to be active, G2{*} freezes G2 in its current situation and G2{} deactivates all its steps. A forced grafcet cannot evolve by itself. Forcing is the standard way to describe emergency stops and initialisation.

GRAFCET and SFC

The Sequential Function Chart (SFC) language of IEC 61131-3 was derived from GRAFCET and looks almost the same, but the two do not have the same purpose.

A usual workflow is to write and check the GRAFCET first, then implement it in SFC, ladder or Structured Text. GrafcetMaker generates Structured Text and PLCopen TC6 XML from a chart as a starting point for that implementation; always review the generated code before use.

Examples

Traffic light with a pedestrian button

Step 0 gives pedestrians the green light for 10 s (10s/X0), then step 1 lets the cars drive. Cars keep the green for 30 s, but a press on the pedestrian button shortens this to 10 s: the press is remembered by a stored action on the event ↑request and forgotten when step 0 becomes active again. Step 2 shows yellow for 3 s.

Drilling station with a part counter

A machining cycle: clamp the part, drill down, retract, unclamp. The clamp is set and reset by stored actions, and step 4 counts the parts. After step 4, an OR divergence with exclusive receptivities goes back to the start, or to maintenance every 100 parts.

The editor has two more examples, a tank filling with numeric inputs and a parallel conveyor with a safety grafcet that forces the line: open them from the start screen of GrafcetMaker.

Draw and simulate GRAFCET online

GrafcetMaker is a free, open-source GRAFCET editor made for automation engineers, teachers and students. It runs entirely in your browser, works offline and needs no account; your charts stay on your computer.

• Draw steps, transitions, actions and branches with the mouse or the keyboard, and keep everything tidy with automatic layout.
• Write receptivities with autocomplete and a variable table, and get clear errors with quick fixes.
• Simulate with the IEC 60848 rules: active steps, timers, edges, unstable steps, forcing and a chronogram of every variable.
• Structure projects with partial grafcets, macro-steps, enclosing steps and forcing orders.
• Export to PDF with a title block, SVG and PNG, or to Structured Text and PLCopen TC6 XML, and share a whole project as a link.

Open the free GRAFCET editor

Frequently asked questions

What is GRAFCET used for?

To specify the behaviour of automated systems with sequences: machines, assembly lines, packaging, batch processes, lifts, traffic lights. The chart is the reference shared by the people who design the machine, program the PLC and test it.

Is GRAFCET the same as SFC?

No. SFC (IEC 61131-3) was derived from GRAFCET and uses the same symbols, but GRAFCET is a specification language with precise evolution rules, and SFC is a PLC programming language whose execution depends on the controller. See GRAFCET and SFC.

What is the difference between a step and a transition?

A step is a situation that lasts (the system is filling the tank); a transition is the instant when the system moves from one situation to the next, when its receptivity becomes true (the tank is full).

What does X1 mean?

X1 is the step variable of step 1: it is true while step 1 is active. It is used in receptivities, in conditions of actions and in other grafcets, for example 5s/X1 or X1 * /X5.

What is an unstable step?

A step that is activated and deactivated again within a single search for stability, because the receptivity below it was already true. Its continuous actions are not executed; its stored actions are.

How do I turn a GRAFCET into a PLC program?

Each step becomes a memory bit, each transition a condition that sets the steps below and resets the steps above, and outputs are written from the step bits. You can do this by hand in ladder or Structured Text, use the SFC language of your PLC, or let GrafcetMaker generate Structured Text or PLCopen XML as a starting point.

Is there free GRAFCET software?

Yes: GrafcetMaker is free and open source (MIT licence). It runs in any modern browser on Windows, macOS, Linux, tablets and phones, and companies can host it on their own network.