Handling S5 Special Function Blocks (OB, SB) in S7

S5 organization blocks (OBs) and step blocks (SBs) have different numbers and functions than their S7 counterparts. The OB concept exists in both systems, but the numbering is completely different. SBs have no S7 equivalent at all. This guide maps every S5 OB to its S7 counterpart and explains how to handle SBs.

S5 Organization Blocks → S7 Mapping#

Cyclic Processing#

Startup / Restart#

Key difference: S5 has separate OBs for different startup types (OB 20/21/22). S7 uses a single OB 100 for all startups. The startup type (warm/cold/complete restart) can be determined from OB 100's start information within the block.

Time-Driven / Cyclic Interrupts#

Key difference: In S5, cyclic interrupt intervals are configured in DB1. In S7, they are configured in the hardware configuration (CPU properties) or directly in OB properties.

Process / Hardware Interrupts#

Error and Diagnostic OBs#

Integral (Built-in) OBs#

OB 251 is not an OB in the S7 sense — it is a built-in function block in S5. In S7, use the PID library functions.

S5 Step Blocks (SB) → S7#

Step blocks (Schrittbausteine, SB) are an S5-specific block type for sequential logic. They have no direct S7 equivalent.

What SBs do: SBs are called from the main program (OB1) and contain logic for a specific "step" in a sequence. The S5 operating system does not provide automatic step management — the programmer must implement the step sequence logic manually using markers and conditional jumps.

S7 options for SB migration:

Recommended approach for step sequences:

// SCL implementation of a step sequence (replacing SB logic)
CASE #Step OF
    0:  // Idle
        IF #Start THEN #Step := 1; END_IF;
    1:  // Step 1: Open valve
        #Valve := TRUE;
        IF #Level_High THEN #Step := 2; END_IF;
    2:  // Step 2: Run mixer
        #Mixer := TRUE;
        #Step_Timer(IN := TRUE, PT := T#30s);
        IF #Step_Timer.Q THEN #Step := 3; END_IF;
    3:  // Step 3: Drain
        #Valve := TRUE; #Mixer := FALSE;
        IF #Level_Low THEN #Step := 0; END_IF;
END_CASE;

The Converter's Limitations with OBs and SBs#

The Siemens S5→S7 converter:

• Converts OB 1 → OB 1 (correct)
• Converts PBs → FCs (correct)
• Converts FBs → FBs (mostly correct)
• Converts SBs → FCs (basic conversion, loses step semantics)
• Does NOT remap OB numbers (OB 21/22 are converted as-is — you must manually place their code into OB 100)
• Does NOT handle OB 31 (watchdog) or OB 251 (PID)

You must manually:

1. Move startup code from converted OB 21/OB 22 into S7 OB 100
2. Move cyclic interrupt code from converted OB 10–18 into S7 OB 30–38
3. Configure interrupt intervals in the S7 hardware configuration (not in DB1)
4. Replace OB 251 PID with PID_Compact or CONT_C
5. Restructure SB logic if step sequences are critical

Frequently Asked Questions#

Are S5 OB numbers the same in S7?

Only OB 1 has the same number and function. All other OBs have different numbers in S7. The converter does not remap them — you must do it manually.

What happens to the code in OB 21 / OB 22 after conversion?

The converter creates blocks with those numbers, but S7 does not use OB 21/22 for startup. You must move the startup logic into S7 OB 100 manually.

Should I use S7-GRAPH for step sequences?

If the original S5 program has clear sequential logic (steps, transitions, parallel branches), S7-GRAPH produces a much cleaner result than converting SBs to FCs. However, GRAPH requires a complete rewrite — it cannot be auto-converted.

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