S5→S7 Counter Formats: BCD vs. INT — Conversion Reference

S5 and S7 legacy counters store their values in BCD (Binary-Coded Decimal) format. IEC counters (CTU, CTD, CTUD) store values as INT. Confusing these formats is one of the most common migration errors — the program compiles without error but produces wrong results for counter values above 9.

How S5/S7 Legacy Counters Store Values#

When you load a counter value in STL/AWL (L Z n or L C n), the CPU places the value in Accumulator 1 in two formats simultaneously:

• Bits 0–11: Binary integer (BI) representation of the counter value
• Bits 0–11 (separate read): BCD representation when accessed via CV_BCD output

In KOP/FUP, counters expose two outputs:

• CV — Counter value as binary integer (hexadecimal display)
• CV_BCD — Counter value in BCD format

BCD Format Structure (16-bit Counter Word)#

Bit:  15 14 13 12 | 11 10 9 8 | 7 6 5 4 | 3 2 1 0
       0  0  0  0 | Hundreds  | Tens    | Ones
                   |  (BCD)   | (BCD)   | (BCD)

Range: 0 to 999 (three BCD digits, each 0–9)

Example: Counter value 247

• BCD: 0000 0010 0100 0111 = 0x0247
• Binary: 0000 0000 1111 0111 = 0x00F7 = 247 decimal

For values 0–9, BCD and binary are identical. At 10 and above, they differ:

The trap: If you load a BCD counter value and perform integer arithmetic on it without conversion, the result is wrong for any value ≥ 10. BCD 0x0010 is not decimal 16 — it represents the decimal value 10. But the CPU treats 0x0010 as integer 16 if you use integer arithmetic.

Conversion Instructions (STL/AWL)#

BCD → Integer#

Example:

L  Z 1         // Load counter 1 (BCD in ACCU 1)
BTI            // Convert BCD to INT
T  MW 20       // Store as integer — now safe for arithmetic

Integer → BCD#

Example:

L  MW 20       // Load integer value
ITB            // Convert INT to BCD
T  MW 30       // Store as BCD — for BCD display or legacy interface

Overflow: If the value exceeds the BCD range (>999 for BTI/ITB, >9999999 for BTD/DTB), the OV and OS status bits are set.

SCL Conversion Functions#

In SCL, use the built-in conversion functions:

// BCD Word to INT
intValue := BCD_TO_INT(bcdWord);

// INT to BCD Word
bcdWord := INT_TO_BCD(intValue);

IEC Counters: No BCD — Always INT#

IEC counters (CTU, CTD, CTUD) on S7-1200/1500 store the counter value as INT (or DINT on S7-1500). There is no BCD representation. The CV output is a plain integer.

Migration Checklist: Counter Format#

When migrating counter code from S5/S7 legacy to IEC:

1. Remove BTI/ITB instructions — IEC counters output INT directly, no BCD conversion needed
2. Adjust comparison logic — S5 U Z n tests CV > 0; IEC CTU.Q tests CV ≥ PV (see Counter Migration Guide)
3. Check arithmetic — if the old program performed arithmetic on counter values without prior BTI conversion, it may have been silently wrong for values ≥ 10. Verify the intended behavior.
4. Update HMI/SCADA interfaces — if external systems read counter values via communication, change the expected data type from BCD to INT

Frequently Asked Questions#

Why does my counter show wrong values only above 9?

You are likely performing integer arithmetic on a BCD value without conversion. BCD and binary are identical for 0–9, so the error only appears at 10+. Insert a BTI instruction after loading the counter.

Can I still use legacy counters (S_CU, S_CD) on S7-1500?

Yes, for compatibility. But they run in emulation mode. Siemens recommends IEC counters (CTU, CTD, CTUD) for all new development.

Convert Your Counter Code Automatically#

PLCcheck Pro detects legacy counter access patterns, identifies missing BCD conversions, and generates correct IEC counter equivalents.

Upload code for counter analysis → | Counter Migration Guide →

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