Glossary · PLC and control programming
Rising edge
Also known as: Positive edge, R_TRIG, Leading edge
German: Steigende Flanke
In PLC programming, a rising edge is the change of a Boolean signal from FALSE to TRUE; IEC 61131-3 provides the standard function block R_TRIG, which outputs TRUE for one execution when it detects a rising edge.
- PLC programming
- Standards
In one sentence
A rising edge is a Boolean signal's change from FALSE to TRUE; R_TRIG detects it and outputs TRUE for one PLC cycle.
Example
Pressing the start button produces a rising edge; R_TRIG turns it into a one-cycle pulse, so holding the button does not restart the sequence repeatedly.
How it applies
- Engineering: Edge detection is needed wherever an action should happen once per event: counting parts, starting a sequence, acknowledging an alarm. Counters, event tasks and many standard blocks react to rising edges internally.
- Commissioning: Common errors are calling one edge detector instance twice per cycle, calling it conditionally or using an undebounced input. Test edge-triggered functions with real signals and at full speed.
- Safety: Requiring a rising edge on a start or reset button prevents a jammed or bridged button from starting the machine. Whether that is sufficient for a given function is decided in the safety design, see Restart prevention.
- Documentation: When the timing of a press matters, describe it: press and release, or press only. Operators notice the difference when a function does not react as expected.
Rising edge vs. falling edge
A rising edge marks the start of a signal (FALSE to TRUE); a Falling edge marks its end (TRUE to FALSE). With normally closed sensors, the physical event and the edge type are inverted.