S5 to S7 Data Block Address Mapping: DW → DBW (×2 Rule)

This is the most error-prone address conversion in S5→S7 migration. S5 addresses data block words by word number (DW 0, DW 1, DW 2). S7 addresses by byte offset (DBW 0, DBW 2, DBW 4). Since each word is 2 bytes, the conversion rule is: S7 byte address = S5 word number × 2.

The ×2 Rule#

S7 DBW address = S5 DW number × 2

Word Conversion Table#

Double Word (DD → DBD)#

Note: DD in S5 accesses 4 bytes starting at word n. DD 0 = DW 0 + DW 1. In S7, DBD 0 = DBW 0 + DBW 2 = bytes 0–3. Same data, different numbering.

Byte Access (DL/DR → DBB)#

S5 uses DL (left byte = high byte) and DR (right byte = low byte) within a word:

Bit Access (D x.y → DBX)#

S5 bit access within data blocks uses D word.bit:

Bit mapping formula:

• S5 bit 0–7 (low byte) → S7 DBX (word×2 + 1).bit
• S5 bit 8–15 (high byte) → S7 DBX (word×2).(bit - 8)

DB Access Syntax#

S7 advantage: In S7, you can access DB10.DBW10 directly without opening the DB first. In S5, A DB 10 must precede data access.

This Is the #1 Migration Error#

Forgetting the ×2 conversion causes the program to read/write wrong data. The program compiles without errors — the addresses are syntactically valid. But the data is shifted by half, causing unpredictable behavior.

Example of the error:

• S5 code reads DW 5 (timer setpoint = 3000 ms)
• Incorrect migration: L DBW 5 → reads the wrong memory location
• Correct migration: L DBW 10 → reads the correct setpoint

Use our free S5→S7 Address Converter to verify any conversion.

Part of the S5→S7 Address Mapping Reference. Maintained by PLCcheck.ai.