Function Blocks

PLC Counter Programming: CTU, CTD & CTUD Examples in Structured Text

Learn how to use CTU, CTD, and CTUD counters in Structured Text with practical industrial examples — from part counting to batch control.

2026-02-20 · 10 min read

CTU
CTD
CTUD
Counters
IEC 61131-3
Structured Text

Understanding PLC Counters

Counters are fundamental building blocks in PLC programming. The IEC 61131-3 standard defines three counter function blocks that every automation engineer must know:

Counter	Full Name	Purpose
CTU	Count Up	Increments on each rising edge
CTD	Count Down	Decrements on each rising edge
CTUD	Count Up/Down	Combines both directions

CTU — Count Up Counter

The CTU counter increments its CV (Current Value) each time the CU input sees a rising edge. When CV reaches the preset PV, the output Q goes TRUE.

PROGRAM PartCounter
VAR
    PartSensor : BOOL := FALSE;
    ResetButton : BOOL := FALSE;
    Counter : CTU;
    PartsCount : INT := 0;
    BatchComplete : BOOL := FALSE;
END_VAR
Counter(CU := PartSensor, RESET := ResetButton, PV := 50);
PartsCount := Counter.CV;
BatchComplete := Counter.Q;// When 50 parts counted, signal batch complete
IF BatchComplete THEN
    // Trigger conveyor stop, alert operator
END_IF;

Key Points for CTU

CU: Count input — increments on rising edge only

RESET: Sets CV back to 0

PV: Preset value (target count)

Q: TRUE when CV >= PV

CV: Current count value

CTD — Count Down Counter

The CTD counter decrements from a loaded value. It's ideal for tracking remaining items or countdown sequences.

PROGRAM MaterialTracker
VAR
    ItemRemoved : BOOL := FALSE;
    LoadParts : BOOL := FALSE;
    DownCounter : CTD;
    Remaining : INT := 0;
    BinEmpty : BOOL := FALSE;
END_VAR
DownCounter(CD := ItemRemoved, LOAD := LoadParts, PV := 100);
Remaining := DownCounter.CV;
BinEmpty := DownCounter.Q;IF BinEmpty THEN
    // Request material refill
END_IF;

Key Points for CTD

CD: Count down input — decrements on rising edge

LOAD: Loads PV into CV (resets to full)

Q: TRUE when CV <= 0

Useful for inventory tracking, magazine feeders, and remaining-cycle counters

CTUD — Count Up/Down Counter

The CTUD combines both directions, essential for tracking bidirectional processes like reversible conveyors or buffer zones.

PROGRAM BufferZoneTracker
VAR
    ItemEnters : BOOL := FALSE;
    ItemExits : BOOL := FALSE;
    ResetCount : BOOL := FALSE;
    LoadCount : BOOL := FALSE;
    Buffer : CTUD;
    ItemsInZone : INT := 0;
    ZoneFull : BOOL := FALSE;
    ZoneEmpty : BOOL := FALSE;
END_VAR
Buffer(
    CU := ItemEnters,
    CD := ItemExits,
    RESET := ResetCount,
    LOAD := LoadCount,
    PV := 20
);
ItemsInZone := Buffer.CV;
ZoneFull := Buffer.QU;   // TRUE when CV >= PV
ZoneEmpty := Buffer.QD;  // TRUE when CV <= 0IF ZoneFull THEN
    // Stop infeed conveyor
ELSIF ZoneEmpty THEN
    // Signal upstream to send more
END_IF;

Real-World Example: Production Batch Counter with Reject Tracking

Here's a more complete example combining multiple counters:

PROGRAM BatchProduction
VAR
    GoodPartSensor : BOOL := FALSE;
    RejectSensor : BOOL := FALSE;
    NewBatchCmd : BOOL := FALSE;
    
    GoodCounter : CTU;
    RejectCounter : CTU;
    
    GoodParts : INT := 0;
    RejectParts : INT := 0;
    TotalParts : INT := 0;
    RejectRate : REAL := 0.0;
    BatchDone : BOOL := FALSE;
END_VAR
GoodCounter(CU := GoodPartSensor, RESET := NewBatchCmd, PV := 1000);
RejectCounter(CU := RejectSensor, RESET := NewBatchCmd, PV := 100);
GoodParts := GoodCounter.CV;
RejectParts := RejectCounter.CV;
TotalParts := GoodParts + RejectParts;
BatchDone := GoodCounter.Q;
IF TotalParts > 0 THEN
    RejectRate := INT_TO_REAL(RejectParts) / INT_TO_REAL(TotalParts) * 100.0;
END_IF;// Alarm if reject rate exceeds 5%
IF RejectRate > 5.0 THEN
    // Trigger quality alert
END_IF;

Common Mistakes with PLC Counters

Forgetting edge detection — Counters count on rising edges. If your input stays TRUE, it only counts once.

Not resetting — Always provide a reset mechanism, especially for CTU counters in continuous production.

Overflow — INT counters max out at 32767. For large counts, use DINT or LINT.

Using counters as timers — Counters count events, not time. Use TON/TOF for time-based logic.

Practice in Our Simulator

Try building a part counting system with our online PLC simulator. Start with a simple CTU, then add reject tracking with a second counter.