Glossary · Automation fundamentals, platforms and components
Transition (SFC)
Also known as: Transition condition, Step enabling condition
German: Transition
In IEC 61131-3 sequential function charts, a transition is the element between steps with an associated Boolean transition condition. It is cleared (fires) when all preceding steps are active and the condition is TRUE, deactivating the preceding steps and activating the following ones.
- Automation components
In one sentence
In an SFC, a transition links steps; it fires when the preceding steps are active and its condition is TRUE, activating the next steps.
Example
The transition from "fill tank" to "heat" fires when the level switch reports the tank full and the inlet valve is confirmed closed.
How it applies
- Engineering: Transition conditions should be explicit and testable, often combining sensor feedback with interlocks. Step monitoring with a Timeout detects transitions that never fire.
- Service: When a sequence stops, the unfulfilled transition condition tells service staff which signal is missing. Displaying it on the HMI shortens troubleshooting.
- Documentation: Functional specifications list the transition conditions for each step. Use the same signal names as in the program and the I/O list.
SFC transition vs. state transition
An SFC transition is a language element of IEC 61131-3 with a Boolean condition between steps. A State transition in systems engineering is the change from one state of a state model to another, triggered by an event or condition. The concepts are similar, but the SFC transition has precise firing rules defined by the standard.