PLC glossary
Counter
An instruction that remembers how many events occurred. Counting an arrival once is different from adding one on every scan.
- Also called
- event counter
- On site
- A drinks line stops after the third bottle passes the sensor.
- Siemens
- CTU; SCL calls the declared IEC_COUNTER instance's CTU operation.
- Rockwell
- CTU in Ladder; CTUD with an FBD_COUNTER backing tag in ST.
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.