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.
Steps
Initial Step
The initial step marks the state in which the control system is immediately after starting the system (or after a reset). Each GRAFCET must have at least one initial step.
Step
A step represents a stable state in the process. While a step is active, assigned actions can be executed. A step is only left when the following transition is fulfilled.
Enclosing Step
The enclosing step encapsulates a complete sub-GRAFCET. The diagonal corner lines indicate that this step contains its own sequence structure. It is connected like a normal step, but internally encapsulates a complex sub-process. The variant with a double frame is the enclosing initial step.
Macro Step
The macro step combines a sequence of steps and transitions into a single symbol. The horizontal lines at the top and bottom edges indicate that the step refers to a separate, detailed sub-GRAFCET (macro expansion). It is connected like a normal step.
Transitions & Flow
Transition
The transition is the "gatekeeper logic". It contains the condition that must be met for the process to jump from the previous to the next step.
Loop
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.
Actions
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.
Time-Delayed Action
The action is not started immediately when the step is activated, but only after the specified time has elapsed.
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 Actions
Stored actions write a variable on specific edges: The symbol with an up arrow means the variable is written on the rising edge (when the step is activated). The symbol with a down arrow means the variable is written on the falling edge (when the step is left).
Event-Driven Stored Action
This action is triggered by a specific event while the associated step is active. Unlike a normal stored action (which executes immediately when the step is activated), this action waits for a defined condition to occur.
Action with Assignment Condition
This action is only active while the associated step is active AND an additional specified condition is true. If the condition becomes false, the action pauses until the condition becomes true again (as long as the step is still active).
More Elements
Forced Command
The forced command (Forçage) forces the setting or resetting of steps in another sub-GRAFCET. The double frame identifies this special action type. It is connected to a step like a normal action.
Comment Box
The comment box is used for visual grouping and labeling of GRAFCET elements. The text in the top gap serves as a label or reference. The size of the box can be freely adjusted.
Syntax Reference
Transition Syntax
Syntax for transition conditions (transitions between steps).
BG1Single condition (TRUE/FALSE)BG1 * BG2AND operationBG1 + BG2OR operation!BG1Negation(a + b) * cParentheses↑BG1Rising edge↓BG1Falling edge5s/X1On-delay (5s after step 1)sensor/3sOff-delay (3s after falling edge)X1Step query (step 1 active?)C1<5Comparison (counter < 5)1Always true (IEC 60848)Action Syntax
Syntax for action labels (assignments or variable names).
MotorVariable active while step activeK1 := 1Set variable to 1K1 := K1 + 1Increment counterHints
- ●Text in quotes ("...") is interpreted as a comment.
- ●AND / && are automatically converted to *
- ●OR / || are automatically converted to +
- ●NOT is automatically converted to !
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