CTU — Count Up

CTU

Counter

counter

(CU)

Preset

?

(DN)

Accum

?

Increments .ACC by 1 on each rising edge of the rung. Sets .DN when .ACC reaches .PRE; keeps counting past the preset.

Operands

Name	Type
counter	COUNTER

How It Works

Event	.ACC	.CU	.DN
Rising edge of rung	+1	1	1 when .ACC ≥ .PRE
Rung stays true	unchanged	1	unchanged
Rung false	unchanged	0	unchanged
RES on the tag	0	0	0

CTU counts on the transition from false to true. Holding the input true does not keep counting.

A rung that is already true when the program starts does not count either. The counter only counts a change it sees while the program is running — the rung has to go false, then true. (Prescan sets this up before the first scan.)

Example — Parts Counter

Count boxes passing a sensor. Signal "full" at 10 boxes.

Tag setup: Boxes is a COUNTER with .PRE = 10.

XIC(BoxSensor)CTU(Boxes);XIC(Boxes.DN)OTE(BinFull);XIC(ResetBtn)RES(Boxes)

Practice

Try CTU in the Count Up exercise — count input pulses and act when the target is reached.

Common Mistakes

• Adding ONS before a CTU — unnecessary. CTU already counts on the rising edge.
• Expecting .DN to block further counting — it does not. .ACC keeps climbing. Add a RES or an interlock if you need to stop.

Related

• Counters overview — members, units, shared behaviour
• CTD — count-down counterpart
• RES — clears the accumulator