Siemens Simatic S7 Series Manual page 61

IEC 1131-3 Counters.
In.this.example,.the.storage.location.has.50.spaces..When.
the.accumulated.count.reaches.50,.the.counter.bit.turns.
on,.contact.C48.closes,.and.output.Q0..turns.on..This.could.
trigger.other.logic.in.the.program.to.divert.new.items.to.
another.location.until.such.time.as.an.item.is.removed.from.this.
location.
The.counters.previously.discussed.were.SIMATIC.counters..The.
IEC.3-3.instruction.set.also.includes.three.counters..These.
counters.are.similar.to.the.SIMATIC.counters,.but.there.are.a.
few.differences.
Each.IEC.3-3.counter.has.an.output.(Q).and.cumulative.value.
(CV).in.the.counter.box.
Count Up Counter (CTU).stops.counting.when.CV.equals.the.
preset.value.(PV),.and.turns.on.output.Q.
Except.for.the.CV.and.Q.values,.the.IEC.3-3.Count Down
Counter (CTD).functions.like.the.SIMATIC.version..When.CV.
equals.zero,.it.stops.counting,.and.output.Q.turns.on.
Count Up/Down Counter (CTUD).stops.counting.up.when.CV.
equals.PV.and.turns.on.output.QU..CTUD.stops.counting.down.
when.CV.equals.zero.and.turns.on.output.QD.
IEC 1131-3 Counters
Timer Number (C0 to C255)
Cxxx
CU
CTU
R
PV Q
CV
Count Up Counter
60
Cxxx
CD
CTD
LD
PV Q
CV
Count Down Counter Count Up/Down Counter
Cxxx
CU
CTUD
CD
R
LD
QU
PV
QD
CV