SCL Counter Functions: CTU, CTD, CTUD — Complete Reference

The IEC 61131-3 standard defines three counter function blocks: CTU (count up), CTD (count down), and CTUD (count up and down). In Siemens SCL (TIA Portal), these replace the legacy S5/S7 counter functions (ZV, ZR, S_CU, S_CD) with a cleaner, more portable interface.

CTU — Count Up#

CTU increments a counter value on each rising edge of the CU input. When the current value (CV) reaches or exceeds the preset value (PV), the output Q becomes TRUE.

Parameters:

SCL Example:

VAR
    myCounter : CTU;
END_VAR

// Call the counter
myCounter(CU := "Count_Sensor",
          R  := "Reset_Button",
          PV := 100);

// Use the outputs
"Batch_Complete" := myCounter.Q;     // TRUE when 100 parts counted
"Parts_Count"    := myCounter.CV;    // Current count value

Behavior:

• Each rising edge on CU increments CV by 1
• When R = TRUE, CV is set to 0 and Q = FALSE
• Q becomes TRUE when CV ≥ PV and remains TRUE until reset
• CV continues counting beyond PV (no automatic stop at PV)
• CV range: 0 to 32767 (INT). For larger ranges, use data type DINT (S7-1500 only)

CTD — Count Down#

CTD decrements a counter value on each rising edge of the CD input. When CV reaches 0, the output Q becomes TRUE.

Parameters:

SCL Example:

VAR
    myDownCounter : CTD;
END_VAR

myDownCounter(CD := "Part_Ejected",
              LD := "Load_Batch",
              PV := 50);

"Batch_Empty" := myDownCounter.Q;       // TRUE when all 50 parts ejected
"Remaining"   := myDownCounter.CV;      // Remaining count

Behavior:

• LD loads PV into CV (like "preset" in legacy counters)
• Each rising edge on CD decrements CV by 1
• Q becomes TRUE when CV ≤ 0
• CV can go negative (no automatic stop at 0)

CTUD — Count Up and Down#

CTUD combines both count directions in one function block. It has separate up and down inputs, separate done outputs for each direction, and both reset and load functions.

Parameters:

SCL Example:

VAR
    myBiCounter : CTUD;
END_VAR

myBiCounter(CU := "Part_In",
            CD := "Part_Out",
            R  := "Reset_Count",
            LD := FALSE,
            PV := 200);

"Buffer_Full"  := myBiCounter.QU;    // TRUE when 200 parts in buffer
"Buffer_Empty" := myBiCounter.QD;    // TRUE when buffer empty
"Buffer_Level" := myBiCounter.CV;    // Current buffer level

Migration from S5 ZV/ZR to IEC Counters#

Critical difference: In S5, U Z n tests whether the counter is greater than zero (CV > 0). In IEC CTU, Q tests whether CV has reached the preset (CV ≥ PV). These are fundamentally different conditions. When migrating, you must adjust the logic accordingly.

For the detailed migration guide, see Migrating S5 Counter Programs to S7.

Availability by Platform#

Convert Your Counter Logic Automatically#

PLCcheck Pro analyzes your S5/S7 counter code and generates IEC-compliant SCL equivalents with correct preset mapping and condition adjustments.

Upload code for counter conversion → | AWL to SCL Guide →

Part of the SCL Reference. Maintained by PLCcheck.ai. Not affiliated with Siemens AG.