What are GRAFCET symbols?

GRAFCET symbols are standardized graphical elements per IEC 60848 that describe control sequences. They include initial steps, steps, transitions, actions, and AND/OR branches.

Which symbols does GRAFCET include?

GRAFCET includes: initial step (double border), step, transition (horizontal bar), actions (rectangle attached to step), AND divergence/convergence (double horizontal line), OR divergence/convergence (single line), enclosing steps, and loops.

How are GRAFCET transition conditions written?

Transition conditions can be combined with AND (*), OR (+), and NOT (!). Time conditions are written as "5s/Sensor" (on-delay) or "Sensor/3s" (off-delay). Comparisons use square brackets: [C1=8], [Temp>20].

What is the difference between AND and OR divergence in GRAFCET?

With AND divergence (simultaneous branch), all branches are activated simultaneously. With OR divergence (alternative branch), exactly one branch is executed — the one whose transition condition is satisfied first.

GRAFCET Symbols and Rules

Here you will learn everything about the standardized GRAFCET symbols according to IEC 60848, so you can plan your processes precisely and according to standards.

This page was translated from German with the help of AI. In case of discrepancies, the German version prevails.

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Initial Step

State of the control system immediately after start-up, shown with a double frame. Numbering is arbitrary, but “0” is common.

A GRAFCET typically starts with step "0", but any number is valid.

Step

Represents a state in the process. While a step is active, its associated actions are executed. Steps are numbered in ascending order.

Use ascending numbers when assigning step labels.

Transition

The transition condition is placed to the right of the transition. An optional transition name goes on the left. The preceding step is only deactivated once the transition condition is fulfilled.

Transition Syntax

BG1Single condition (TRUE/FALSE)

BG1 · BG2AND operation (entered as *)

BG1 + BG2OR operation

BG1Negation (entered as !BG1)

(a + b) · cParentheses

↑BG1Rising edge

↓BG1Falling edge

5s/X1On-delay (5 s after step 1)

sensor/3sOff-delay (3 s after falling edge)

X1Step query (is step 1 active?)

[C1<5]Analog comparison in square brackets

1Always true

Per IEC 60848, wrap analog / comparison expressions in square brackets: [C1=8], [Temp>20], [Temp>20]*S1. The bracket content is treated as a boolean value.

Source Transition

Starts a GRAFCET without a preceding step.

Sink Transition

Deactivates the preceding step without a following step.

Continuous Action

The action is active as long as the associated step is active. As soon as the step is left, the action ends immediately.

Continuous Action with Assignment Condition

The action is only active while the step is active and the additionally specified condition (SJ2) is fulfilled.

Delayed Action

The action only starts after the time specified to the left of the condition has elapsed and always ends with its step.

Note: Time-delayed actions end with their step, even if a switch-off delay would still apply.

Time-Limited Action

The action starts immediately with the step and ends automatically after the specified time, even if the step remains active.

Stored Action on Activation

The variable is written when the step is activated and remains stored. The assignment uses “:=”.

Action Syntax

MotorVariable active while the step is active

K1 := 1Set variable to 1

K1 := K1 + 1Increment counter

Stored Action on Deactivation

The variable is written when the step is deactivated and remains stored.

Stored Action on Event

The variable is written and stored as soon as the step is active and the event (here BG2) delivers a (here rising) edge.

Comments

Comments improve the readability of a GRAFCET. They must be enclosed in quotation marks.

Feedback

The default flow direction in GRAFCET is from top to bottom. An upward arrow marks the feedback to an earlier step.

Jump

Jumps back to a previous step and is equivalent to a feedback connection. In this example, the jump leads to step 7.

Alternative Branch

Splits the sequence into alternative paths. The transition conditions must be mutually exclusive. As a result, only one path is executed.

Parallel Branch

Splits the sequence into several partial sequences that run simultaneously. The sequence only continues once all partial sequences are completed.

Macro-Step

Combines a sequence of steps and transitions. This sequence must be processed completely. The sequence starts with an E and ends with an S. The macro-step is labeled with an M.

Macro step numbers are prefixed with "M", here "M1". The entry step of the sub-GRAFCET uses "E" (here E1); the exit step uses "S" (here S1).

Enclosing Step

Encapsulates partial GRAFCETs. Activating the step activates its enclosures, deactivating it deactivates them. The step marked with an asterisk starts the partial GRAFCET.

Enclosure

Frame for the partial GRAFCETs of an enclosing step. The number (here 1) and name (here G1) are placed on the left of the frame. An initial step is also permitted here.

Forcing Order

Forces another GRAFCET immediately into a defined state and overrides its logic. In this example, step 4 in enclosure G1 is activated.

Forcing Orders

G1 {}Deactivate all steps in G1

G1 {4}Activate only step 4 in G1

G1 {*}Freeze the current step, no further switching

G1 {INIT}Initialize G1, the initial step becomes active

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