PLC glossary
Falling edge
The moment a signal changes from TRUE to FALSE. A signal that was already FALSE has not just fallen.
- Also called
- negative edge, TRUE-to-FALSE
- On site
- A bottle clearing the sensor creates one falling edge.
- Siemens
- F_TRIG in SCL; N contact or N_TRIG in LAD.
- Rockwell
- OSF in Ladder; this academy's ST exercise compares the current and previous input explicitly.
Taught in: Count events, not scans
Practise it on a 3D rig
- Count the arrivalsCount each bottle once. Stop at three.
- Count with a one-shotOne bottle covers the sensor for many scans. Count it once.
- Count bottles as they leaveCount each bottle as it clears the sensor, not as it arrives.
- It never reaches threeSomething keeps clearing the count. Find it.
- Review the AI: One bottle, one countCheck a bottle that stays under the sensor.