CountersBeginnerIEC 61131-3

CTU Counter in PLC: Rising-Edge Count and Reset

Quick answer
CTU increments its current value once for each FALSE-to-TRUE transition at CU while reset is inactive. Q becomes TRUE when CV reaches the preset comparison condition. Holding CU TRUE does not keep counting. Reset clears CV and Q, but exact types, saturation and overflow behavior differ by vendor.

The CTU (Counter Up) function block increments a counter value (CV) by 1 on each rising edge of the count input (CU). When the counter value reaches or exceeds the preset value (PV), the output (Q) goes TRUE. A reset input (R) sets CV back to 0 and Q to FALSE. CTU counters are fundamental to PLC programming and are used extensively for counting parts, tracking production quantities, sequencing operations, and batch counting.

Reviewed 2026-07-25

Parameters

Implementation note: This page uses a common IEC-style teaching interface. Confirm the actual PV and CV types in your library: current CODESYS Standard documentation, for example, exposes WORD, while other IEC and vendor implementations provide INT, DINT or dedicated counter structures.

Inputs

Name	Type	Description
CU	BOOL	Count up input - increments on rising edge
R	BOOL	Reset input - sets CV to 0
PV	INT	Preset value (target count)

Outputs

Name	Type	Description
Q	BOOL	Counter done - TRUE when CV >= PV
CV	INT	Current counter value

How CTU count-up counter works scan by scan

Read the waveforms and state table together. The diagram shows the sequence; the table states what the program should observe at each execution.

CTU rising-edge count diagram

CV increases only on rising edges of CU. With PV = 3, Q turns on at the third edge and stays on until reset.

Blue is the input, green is the boolean output, and amber is elapsed time or current count. Transitions are shown at task executions; real timing resolution depends on the controller and task.

State table

Moment	Input	Internal state	Output	Engineering meaning
Reset	R/RESET = TRUE	CV = 0	Q = FALSE	Reset has priority and clears the batch state.
First edge	CU changes FALSE → TRUE	CV: 0 → 1	Q = FALSE for PV = 3	One event, not one scan, is counted.
CU held TRUE	CU remains TRUE	CV remains 1	Q unchanged	The internal edge memory prevents scan-by-scan increments.
Preset edge	Third FALSE → TRUE edge	CV reaches 3	Q = TRUE	The target batch count is complete.

Ladder and Structured Text examples

Count a three-part batch

|----[ Part_Sensor ]----[ CTU PartCount ]----|
|                          PV: 3              |
|                          R: Reset_Batch     |
|----[ PartCount.Q ]-------( Batch_Complete )-|

A clean sensor transition increments once. If the sensor chatters, the PLC can see several legitimate edges; use input filtering or debounce instead of adding a second edge detector blindly.

IEC-style Structured Text call

VAR
    PartCount     : CTU;
    PartSensor    : BOOL;
    ResetBatch    : BOOL;
    BatchComplete : BOOL;
END_VAR

PartCount(CU := PartSensor, RESET := ResetBatch, PV := 3);
BatchComplete := PartCount.Q;

Some libraries name the reset input R rather than RESET and use WORD, INT or a typed counter variant for PV/CV. Compile against the installed library instead of copying the signature unchanged.

Vendor compatibility

A shared mnemonic does not guarantee an identical interface or state machine. Confirm the instruction help for the controller, firmware, language and library version used on the project.

Platform	Instruction	What maps cleanly	Caveat to verify
Siemens IEC counters	CTU with IEC counter instance	Positive CU edge increments CV; Q is TRUE when CV ≥ PV	Available counter variants and upper limits depend on CPU/instruction selection. Use one instance at one program location.
Rockwell Logix	CTU with COUNTER structure	False-to-true rung transitions increment ACC and DN indicates ACC ≥ PRE	Logix can continue beyond PRE and exposes overflow behavior; it does not use the IEC PV/CV signature.
CODESYS / IEC libraries	CTU or a typed CTU variant	Rising CU edge increments and RESET clears the value	Current Standard-library documentation can expose WORD for PV/CV, while other implementations use INT. Check the installed version.

Failure modes and edge cases to test

Sensor bounce creates three rising edges

CTU counts three events. Edge-triggered does not mean debounced.

CU is TRUE while reset is removed

First-scan/prescan behavior is platform specific; require a clean FALSE state before the next intended edge.

CV reaches its numeric upper limit

Some implementations saturate and others expose overflow/rollover behavior. Test and alarm before the limit.

The same counter tag is used in two instructions

Both calls modify shared edge/state memory and can miscount. Use one owning instruction instance.

Evidence and review status

The edge-counting and reset behavior were checked against current CODESYS documentation. Parameter names and numeric types differ between vendor libraries and standard editions.

Technical review date: 2026-07-25

Critical behavior

CTU increments once on a false-to-true transition at CU, not once for every scan that CU remains true.
Reset has priority according to the implementation and sets the current value back to its initial state.
Q becomes true when the current value reaches the preset comparison condition.
Input bounce or repeated transitions can create extra counts even though the block is edge-triggered.

Verification checklist

1Apply one clean rising edge and confirm CV increases by exactly one.
2Hold CU true for several scans and confirm CV does not keep increasing.
3Test reset while CU is false and while CU is true.
4Confirm PV and CV types, upper limit and overflow behavior in the target library.

Worked test

Count a five-part batch

Set PV = 5, drive CU from a debounced part sensor and use ResetBatch for R or RESET.

1Reset the counter and confirm CV = 0 and Q = false.
2Apply four separated false-to-true sensor transitions.
3Apply the fifth transition.
4Hold the sensor true for two more scans, then reset.

Expected result: CV reaches 4 after four edges, then 5 and Q true after the fifth. Holding the input true does not add counts; reset returns CV and Q to their initial state.

Technical sources

The target controller's version-specific instruction help remains authoritative.

CTU Standard function block
CODESYS Development
IEC-style CTU_INT declaration example
PLCopen — Illustrates a typed CTU implementation and saturation logic
Count Up (CTU)
Rockwell Automation Studio 5000 — Logix COUNTER structure and overflow behavior
CTU: Count up
Siemens STEP 7 V21 documentation — Positive-edge counting, reset and preset comparison

Platform-Specific Implementation

Siemens (TIA Portal)

Use CTU instruction in TIA Portal. Counter data type is IEC_Counter. Also available as legacy S7 counter (C#).

Allen-Bradley (Studio 5000)

Use CTU instruction in Studio 5000. Counter data type is COUNTER. Access .DN (done), .ACC (accumulated), .PRE (preset).

CODESYS

Check the installed Standard library version before declaring the instance. Current CODESYS 3.5.22.0 documentation exposes RESET, PV and CV with WORD for CTU; other typed counter blocks and library versions use different integer types.

Common Applications

Part counting on conveyors
Batch quantity tracking
Production monitoring
Event counting
Sequence step counting

Common Mistakes to Avoid

Not debouncing the count input (double counting)
Forgetting to reset after batch complete
Using wrong data type for PV (INT vs DINT for large counts)

Frequently Asked Questions

What is CTU (Counter Up) in PLC programming?

Counts up on each rising edge; output activates when count reaches preset. The CTU (Counter Up) function block increments a counter value (CV) by 1 on each rising edge of the count input (CU). When the counter value reaches or exceeds the preset value (PV), the output (Q) goes TRUE.

What are common applications of CTU (Counter Up)?

CTU (Counter Up) is commonly used for: Part counting on conveyors, Batch quantity tracking, Production monitoring, Event counting, Sequence step counting.

How do I use CTU (Counter Up) in different PLC platforms?

Siemens: Use CTU instruction in TIA Portal. Counter data type is IEC_Counter. Also available as legacy S7 counter (C#). Allen-Bradley: Use CTU instruction in Studio 5000. Counter data type is COUNTER. Access .DN (done), .ACC (accumulated), .PRE (preset). CODESYS: Check the installed Standard library version before declaring the instance. Current CODESYS 3.5.22.0 documentation exposes RESET, PV and CV with WORD for CTU; other typed counter blocks and library versions use different integer types.