GRAFCET basics

GRAFCET (IEC 60848) describes the behaviour of a sequential system — a machine, a process, a traffic light — as a series of steps linked by transitions. It is the specification language behind the SFC language of PLCs (IEC 61131-3).

Steps

A step is a stable situation of the system: waiting, filling, drilling. It is drawn as a square with its number. At any moment a step is either active or inactive; the set of active steps is the situation of the grafcet.

• An initial step (double square) is active when the system starts. A grafcet needs at least one.
• The variable X3 is true while step 3 is active. You can use it in receptivities and conditions.

Transitions and receptivities

A transition (short horizontal bar) links steps. Its receptivity, written next to it, is the condition to move on: an input (start), a combination (start * /door), an edge (↑b), a timer (5s/X2) or a comparison (level >= 80). The receptivity 1 is always true.

• A transition is enabled when all the steps just above it are active.
• An enabled transition is cleared (crossed) as soon as its receptivity is true.

Actions

Rectangles to the right of a step are its actions: what the system does while the step is active, e.g. run the pump. A continuous action names an output that is true while the step is active. Stored actions change a value once, when a step is activated or deactivated or when an event occurs. See Actions.

The five evolution rules

These rules, from IEC 60848, decide how a grafcet evolves. The simulator applies them exactly.

1. Initial situation: at start, the initial steps are active.
2. Clearing condition: a transition is cleared when it is enabled and its receptivity is true.
3. Clearing: clearing a transition activates all the steps just below it and deactivates all the steps just above it.
4. Simultaneous clearing: all transitions that can be cleared at the same time are cleared at the same time.
5. Activated and deactivated at once: a step that is activated and deactivated at the same time stays active.

After an input change, the rules are applied again and again until no transition can be cleared: this is the search for stability. Steps that are activated and left again during this search are unstable (transient): their continuous actions are not executed, but their stored actions are.

Choosing between paths: OR

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). The paths join again on a single line (OR convergence).

Make the receptivities exclusive (they can never be true together), here a and /a * b. Otherwise both paths would be taken at once. GrafcetMaker warns you when it cannot prove that alternative receptivities are exclusive, and offers to give one branch priority.

Doing things at the same time: AND

When a transition is followed by several steps, they are all activated together: parallel branches, drawn with a double line (AND divergence). 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.

Loops

A link going upward, drawn with an arrow, returns to an earlier step: the usual way to repeat a cycle. Press L on a step, then click the step to go back to.

Bigger applications

• A project can contain several partial grafcets that run together, e.g. one for the cycle and one for safety. They see each other's steps through Xn.
• A macro-step stands for a detailed sequence drawn on its own sheet, its expansion.
• An enclosing step contains a whole grafcet that runs only while it is active.
• A forcing order imposes a situation on another grafcet, e.g. an emergency stop.

See Macro-steps, enclosing steps and forcing.

Learn more

• Writing receptivities — the full expression syntax.
• Glossary — every standard term in one place.
• The exact interpretation of each rule in GrafcetMaker is documented in docs/semantics.md of the source repository.