Understanding S5 Data Blocks (DB0, DB1, DX0) in S7

S5 reserves certain data blocks for system functions. DB0 stores the operating system configuration, DB1 contains CPU parameterization (timer behavior, communication settings, interrupt configuration), and DX0 is a special extended data block used on S5-135U/155U systems. None of these have a direct equivalent in S7 — their functions are handled by the S7 hardware configuration. This article explains each one and how to handle them during migration.

DB0 — Operating System Data Block#

DB0 is reserved by the S5 operating system. It stores internal system data such as block headers, address tables, and system configuration. You cannot create, modify, or delete DB0 in STEP 5 — it is managed entirely by the operating system.

S7 equivalent: None. S7 stores system data in the CPU's internal memory, accessible through SDBs (System Data Blocks) that are generated automatically by the hardware configuration in STEP 7 / TIA Portal.

Migration: DB0 is not converted. The Siemens S5→S7 converter ignores it.

DB1 — Parameter Data Block#

DB1 is reserved for parameterizing CPU-internal functions. Its content depends on the CPU model but typically includes:

• Interrupt configuration: Which time-driven and process-driven interrupts are enabled
• Timer behavior: Processing frequency of time-driven interrupts (OB10–OB18)
• Communication settings: SINEC L1 parameters, serial interface configuration
• Interprocessor communication flags: For multi-CPU configurations (S5-135U/155U)
• Remanent marker ranges: Which marker bytes retain their values during power loss
• PID controller parameters: If integral PID functions are used

Example DB1 content (S5-115U CPU 943/944):

S7 equivalent: The functions in DB1 are handled by:

• Hardware configuration (CPU properties in STEP 7 / TIA Portal)
• CPU properties → Cycle / Clock memory (for interrupt OB configuration)
• Communication settings (in the communication processor properties)
• Remanent data (in the DB properties → "Retain" checkbox)

Migration: DB1 is not converted as a data block. Instead, you must manually transfer the relevant settings to the S7 hardware configuration. The Siemens converter does not handle DB1 automatically.

Practical approach:

1. Print the S5 DB1 content (in STEP 5: DB1 → Output → Printer)
2. Identify which settings are active (many DWs may be zero / unused)
3. Configure the equivalent settings in TIA Portal CPU properties

DX0 — Extended System Data Block (S5-135U/155U only)#

DX0 is an extended data block (DX = "Datenbaustein Extended") used on S5-135U and S5-155U systems. It contains system parameterization similar to DB1 but for the extended features of these larger CPUs, including multi-processor coordination, extended interrupt handling, and memory partitioning.

S7 equivalent: None directly. Multi-processor features are handled differently in S7-400 (multi-computing) and do not exist in S7-1500 (single-CPU architecture with distributed I/O).

Migration: DX0 is not converted. The extended features it configures must be re-evaluated for the S7 architecture.

DX Blocks in General (S5-135U/155U)#

The S5-135U/155U supports two types of data blocks: DB (standard) and DX (extended). Both are accessed the same way but in separate address spaces:

A  DB 10       // Open standard data block 10
L  DW 5        // Read from DB10

AX DX 10       // Open extended data block 10
L  DW 5        // Read from DX10 (different block!)

S7 equivalent: S7 has only one type of data block (DB). DX blocks must be renumbered or merged with regular DBs during migration. Common approach: DX n → DB (n + 256) to avoid number conflicts with existing DBs.

User Data Blocks (DB2–DB255)#

User data blocks (DB2 through DB255) are converted by the Siemens S5→S7 converter. The key difference is the DW→DBW addressing change (the ×2 rule documented in our address mapping reference):

How PLCcheck Pro Helps#

PLCcheck Pro identifies all data block references in your S5 code:

• Lists every DB/DX access with the correct S7 address conversion
• Flags DB0 and DB1 references that need manual handling
• Identifies DX blocks that need renumbering
• Generates a complete DB conversion table

Upload your S5 code →

Frequently Asked Questions#

What happens if I try to convert DB1 with the Siemens converter?

The converter may include DB1 in the output but the content will not be meaningful in S7. You must manually transfer the CPU parameterization to the S7 hardware configuration.

Can I use DB0 or DB1 as user data blocks in S7?

Yes. In S7, DB0 and DB1 are not reserved — you can use any DB number from 1 to 65535. However, it is good practice to avoid DB1 for user data to prevent confusion with the S5 convention.

How do I handle DX blocks if I am migrating to S7-1500?

S7-1500 does not have DX blocks. Renumber DX blocks to regular DB numbers (e.g., DX10 → DB266). Update all program references accordingly. PLCcheck Pro can generate a renumbering map.

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