Migrating S5 Counter Programs (Z/ZV/ZR) to S7

S5 counters use a BCD-encoded value with a range of 0–999. S7 offers both S5-compatible counters (S_CU, S_CD, S_CUD with BCD 0–999) and IEC counters (CTU, CTD, CTUD with INT range). The migration is straightforward for basic counting, but BCD handling and the "U Z n" query behavior require attention. This guide covers all aspects.

S5 Counter Overview#

S5 provides two counter operations and several support operations:

Counter range: 0 to 999 (BCD). The counter cannot go below 0 — ZR at 0 has no effect. The counter cannot exceed 999 — ZV at 999 has no effect.

Edge detection: ZV and ZR trigger on the rising edge of the VKE. If the VKE stays TRUE across multiple scan cycles, the counter increments/decrements only once (not continuously).

S7 Equivalents#

Option A: S5-Compatible Counters (S_CU, S_CD, S_CUD)#

These are S7 STL functions that behave identically to S5 counters. They use BCD format (0–999) and have the same edge behavior.

S7 STL example (S5-compatible):

A   I 0.0          // Count up input
L   C#100          // Preset value = 100
S_CU Z 1           // Count up

Advantage: Direct 1:1 migration. Minimal code changes. Disadvantage: Limited to 0–999 (BCD), not recommended for S7-1500.

Option B: IEC Counters (CTU, CTD, CTUD) — Recommended#

IEC counters use INT (or DINT on S7-1500) instead of BCD. They require an instance data block (like IEC timers).

S7 SCL example (IEC):

#Counter_Instance.CTU(CU := #Part_Sensor,    // Count input (rising edge)
                       R  := #Reset_Button,   // Reset to 0
                       PV := 100);            // Preset value

#Batch_Complete := #Counter_Instance.Q;       // TRUE when CV >= PV
#Current_Count := #Counter_Instance.CV;       // Current value (INT)

Advantage: Larger range, standard IEC behavior, works on all S7 platforms. Disadvantage: Requires instance DB, different interface than S5 counters.

Key Differences to Watch#

1. BCD vs. Integer Format#

S5 counters store and return values in BCD format. When you L Z n in S5, ACCU1 contains BCD (e.g., counter value 123 = hex 0x0123, not 0x007B).

In S7, the L instruction for S5-compatible counters still returns BCD. But IEC counters return plain INT. If your S5 program performs arithmetic on counter values (e.g., L Z 1 / +F / T MW 10), you must handle the BCD-to-INT conversion.

S5 code that needs attention:

L  Z 1            // BCD value in ACCU1
L  KF +10         // INT value in ACCU1
+F                // BCD + INT = WRONG result!

This is a bug in the S5 code (mixing BCD and INT). But it may "work" by coincidence for small values where BCD and binary representations happen to be similar (0–9). During migration, fix these bugs explicitly.

2. Preset Value Format#

S5 uses KZ format for counter presets: L KZ 100 / S Z 1

S7 compatible counters use C# format: L C#100 / S C 1

IEC counters use plain INT: PV := 100

3. The "U Z n" Query#

In S5, U Z n checks if the counter value is greater than 0. It returns TRUE if the count ≥ 1 and FALSE if count = 0.

S7 compatible counters: Same behavior with A C n (EN) / U Z n (DE). IEC counters: Use the Q output — Q is TRUE when CV ≥ PV (different semantics!).

This is a common migration error. S5 "U Z n" means "counter > 0". IEC CTU "Q" means "counter >= preset". These are not the same thing unless the preset is 1.

Fix: If the S5 program uses U Z n to check "counter not zero", use #Counter_Instance.CV > 0 in SCL instead of #Counter_Instance.Q.

Migration Decision#

How PLCcheck Pro Helps#

• Identifies all counter references (Z n, ZV, ZR, S Z, R Z, U Z, L Z)
• Flags programs that mix BCD counter values with INT arithmetic
• Generates the S7-equivalent code for each counter pattern
• Warns about "U Z n" vs. IEC Q semantic differences

Upload your S5 code →

Frequently Asked Questions#

Can I use the same counter numbers in S7?

Yes. S7 supports counter numbers Z 0 to Z 255 (S7-300/400) for S5-compatible counters. IEC counters do not use counter numbers — they use instance DBs.

What if my S5 program uses both ZV and ZR on the same counter?

This is a combined up/down counter. In S7, use S_CUD (compatible) or CTUD (IEC). Both handle simultaneous count-up and count-down inputs.

Is the S5 counter edge detection identical in S7?

Yes. Both S5 counters and S7 S5-compatible counters (S_CU, S_CD) trigger on the rising edge of the VKE. IEC counters also trigger on the rising edge of CU/CD inputs.

Maintained by PLCcheck.ai. Last update: March 2026. Not affiliated with Siemens AG.