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 components according to IEC 60848, so you can plan your processes precisely and according to standards.

Initial Step

The initial step marks the state in which the control system is immediately after starting the system (or after a reset).

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

Step

A step represents a state in the process. While a step is active, assigned actions are executed. A step is only left when the following transition is fulfilled.

Use ascending numbers when assigning step labels.

Transition

The transition is the switching condition. It contains the condition (to the right of the transition) that must be met for the process to advance from the previous to the next step. A name can optionally be assigned on the left.

Transition Syntax

BG1Single condition (TRUE/FALSE)

BG1 * BG2AND operation

BG1 + BG2OR operation

!BG1Negation (automatically converted to norm-compliant display)

(a + b) * cParentheses

↑BG1Rising edge

↓BG1Falling edge

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

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

X1Step query (step 1 active?)

[C1<5]Analog comparison in square brackets (counter < 5)

1Always true (IEC 60848)

Source Transition

This transition can start a GRAFCET without requiring a preceding step.

Sink Transition

This transition terminates a GRAFCET without a following step.

Continuous Action

This 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

This action is only active as long as the associated step is active AND an additionally specified condition (here SJ2) is met.

Time-Delayed Action

The action is not started immediately when the step is activated, but only after the specified time (left of the condition) has elapsed. To achieve a switch-off delay, a time must be specified to the right of the condition.

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, but is automatically ended after the time has elapsed, even if the step remains active longer.

Stored Action on Activation

The action with an upward arrow means that the variable is written and stored on the rising edge (when the step is activated). The ":=" operator must be used for the assignment.

Action Syntax

MotorVariable active while step active

K1 := 1Set variable to 1

K1 := K1 + 1Increment counter

Stored Action on Deactivation

The action with a downward arrow means that the variable is written and stored on the falling edge (when the step is deactivated). The ":=" operator must be used for the assignment.

Stored Action on Event

This action with a flag writes and stores a variable when the connected step is active AND the event provides a rising or falling edge. Unlike the previous stored actions, this action waits for a defined event to occur.

Comments

To improve the readability of a GRAFCET, comments can be added freely. They must be enclosed in quotation marks.

Feedback

In GRAFCET the standard flow direction is top to bottom. The upward arrow marks a feedback connection, i.e. a jump back to an earlier step.

Jump

Jump back to a previous step. Equivalent to a feedback connection. Here, the process jumps back to step 7.

Alternative Branch

This branching splits the process path into multiple options, with only the required path being executed.

Parallel Branch

This branching splits the process path into multiple paths that are all executed simultaneously.

Macro Step

The macro step combines a sequence of steps and transitions into a single symbol. Unlike the enclosing step, the sequence contained in the macro step must be fully processed.

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

The enclosing step encapsulates one or more sub-GRAFCETs. When the step becomes active, the associated enclosures become active and the contained sub-GRAFCETs are processed. When the enclosing step is no longer active, all sub-GRAFCETs are deactivated.

Enclosure

This frame is used to create the enclosures of an enclosing step. Here the enclosure number is "1" and the enclosure name is "G1".

Forced Control Command

This command forces another GRAFCET immediately into a defined state, overriding its logic. Here, the GRAFCET in enclosure G1 is activated at step 4.

Command Syntax

G1 {}Deactivate all steps in enclosure G1

G1 {4}Activate only step 4 in enclosure G1

G1 {*}Freeze current step in G1 – no switching possible

G1 {INIT}Initialize G1 – step with star on the left becomes active

Hints

●AND / && are automatically converted to *
●OR / || are automatically converted to +
●NOT is automatically converted to !
●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.

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