Glossary · PLC and control programming
Falling edge
Also known as: Negative edge, F_TRIG, Trailing edge
German: Fallende Flanke
In PLC programming, a falling edge is the change of a Boolean signal from TRUE to FALSE; IEC 61131-3 provides the standard function block F_TRIG, which outputs TRUE for one execution when it detects a falling edge.
- PLC programming
- Standards
In one sentence
A falling edge is a Boolean signal's change from TRUE to FALSE; F_TRIG detects it and outputs TRUE for one PLC cycle.
Example
When the operator releases the jog button, F_TRIG detects the falling edge and the program stores the current axis position.
How it applies
- Engineering: Edge detection turns a level into an event. A falling edge marks the end of something: a button released, a part leaving a sensor, a signal dropping out. The edge result is TRUE for only one execution of the block, so logic that depends on it must be evaluated in the same cycle.
- Commissioning: Edge blocks store the previous signal state. Calling the same edge block instance twice per cycle, or conditionally, produces missed or double edges.
- Safety: Some protective functions deliberately react to a falling edge, for example a reset that only takes effect when a button is released. Such behavior belongs in the safety-related logic and its validation, see Reset function.
- Documentation: When a function reacts to releasing rather than pressing a control element, say so explicitly in the operating manual. Users expect a machine to react on press.
Falling edge vs. rising edge
A Rising edge is the change from FALSE to TRUE (R_TRIG); a falling edge is the change from TRUE to FALSE (F_TRIG). With inverted sensor logic, the physical event and the edge type can be opposite, so name the physical event in the documentation.