Siemens STEP 7 Data Types: TIA Portal Complete Reference

STEP 7 Data Type Architecture Overview

STEP 7 — Siemens' integrated engineering framework inside TIA Portal — organizes every tag, constant, and block parameter under a typed system. Three families are defined for S7-300/400, S7-1200, and S7-1500 controllers:

• Elementary data types provided by the system (binary, integer, floating-point, time, character)
• Complex data types constructed by combining elementary types (DTL, STRING, ARRAY, STRUCT, UDT)
• Parameter types used to formalize FB/FC interfaces (TIMER, COUNTER, BLOCK_FB, POINTER, ANY, VARIANT)

Each instruction in STL, LAD, or FBD operates on objects of a specific size: bit-logic instructions work on BOOL, move/load/transfer instructions operate on bytes, words, and double words, and math instructions process integers or REAL/LREAL. Choosing the correct type up front prevents silent truncation, sign extension errors, and IEC 61131-3 check warnings at compile time.

The complete type catalog for STEP 7 V14.0 and later is documented in the Siemens entry Overview of the valid data types — STEP 7 Professional V14.0.

Bit, Byte, Word, and Double Word Foundations

Every elementary type resolves to one of four storage widths. The naming follows IEC 61131-3 conventions adopted by STEP 7:

Elementary Data Types — Binary Numbers

Binary types are unsigned bit-aggregates used for status flags, masks, and I/O mapping:

Bit-logic instructions in LAD/FBD operate exclusively on BOOL. To work with multi-bit fields use AW (AND word), OW (OR word), XOW (exclusive OR word), AD, OD, XOD for double words.

Elementary Data Types — Signed and Unsigned Integers

STEP 7 distinguishes signed (S prefix) and unsigned (U prefix) integers. The byte width defines the numerical range:

Integer arithmetic uses two's-complement representation. When migrating S7-300 STL code that uses accumulators to TIA Portal SCL, replace ITD (Integer to Double Integer) conversions with explicit INT_TO_DINT casts to avoid sign extension surprises. For analog scaling, prefer DINT_TO_REAL / REAL_TO_DINT over implicit LAD/MOVE boxes — implicit MOVE between incompatible widths generates a compile warning in TIA Portal V17+.

Elementary Data Types — Floating-Point Numbers

STEP 7 supports IEEE 754 single and double precision:

Elementary Data Types — Time, Date, and TOD

Time-domain types separate short-duration timers from absolute clocks:

TOD (Time of Day) Bit Structure and Millisecond Encoding

Unlike DTL, the legacy TOD type is a flat 32-bit unsigned integer. Its semantic value is the number of milliseconds elapsed since midnight (00:00:00.000). The maximum representable time is therefore:

TOD_max = 24 × 60 × 60 × 1000 − 1 = 86 399 999 ms

The exact bit layout is:

The S7-1500 high-resolution sibling LTOD stores nanoseconds since midnight as a 64-bit unsigned integer and supports up to nine fractional digits in the literal (LTOD#23:59:59.999999999).

Unpacking TOD into Hours, Minutes, Seconds, Milliseconds

Once a TOD value (DWORD, ms since midnight) is in an MD register or DINT tag, splitting it into H / M / S / ms fields uses a cascade of integer divisions and modulo operations. The pure STL snippet from the engineering record multiplies each component into milliseconds and accumulates; the cleaner SCL pattern uses DIV and MOD:

STL pattern (multiplicative)

      L     L#16              // hours = 16
      L     L#60
      *D                        // hours × 60 = minutes-base
      L     L#60
      *D                        // hours × 3600 = seconds-base
      L     L#1000
      *D                        // hours × 3 600 000 ms
      T     #hd1                // intermediate accumulator

      L     L#45               // minutes = 45
      L     L#60
      *D                        // minutes × 60
      L     L#1000
      *D                        // minutes × 60 000 ms
      L     #hd1
      +D
      T     MD 504              // total ms since midnight = 60 600 000

This STL block reconstructs the millisecond count from components. For the inverse operation — given a DWORD of milliseconds, extract H / M / S / ms — use the SCL implementation:

SCL (preferred) for decomposing TOD

// Input:  tod_ms : DWORD   (ms since midnight, 0 .. 86_399_999)
// Outputs: hours, minutes, seconds, millis
hours    := tod_ms / 3600000;
minutes  := (tod_ms MOD 3600000) / 60000;
seconds  := (tod_ms MOD 60000) / 1000;
millis   := tod_ms MOD 1000;

For an S7-1500 with the higher-resolution LTOD, the same logic scales to nanoseconds:

// Input: lTod_ns : LINT    (ns since midnight, 0 .. 86_399_999_999_999)
hours   := lTod_ns / 3600000000000;
minutes := (lTod_ns MOD 3600000000000) / 60000000000;
seconds := (lTod_ns MOD 60000000000)    / 1000000000;
ms      := (lTod_ns MOD 1000000000)     / 1000000;
us      :=  lTod_ns MOD 1000000;

Complex Data Types

Complex types group elementary types. They are mandatory for structured DBs, recipe handling, and HMI faceplate data exchange:

The S7-1200/1500 quota — up to 65 534 PLC data types per CPU, each holding up to 252 components — is documented in Basics of PLC data types — STEP 7 Professional V13.0. Plan UDT libraries accordingly when migrating from S7-300, where each UDT was limited to 252 bytes of declared structure size.

Parameter Types for FB / FC Interfaces

Parameter types exist only in formal-parameter declarations of FBs and FCs. They let a block accept generic objects:

Number Notation and Constant Formats

STEP 7 accepts multiple literal formats. Choosing the right notation prevents accidental truncation:

Underscores are permitted inside numeric literals (W#16#1234_5678) and are ignored by the compiler — use them to group high-bit fields. Decimal constants with no explicit prefix default to the smallest signed integer that fits; large constants above 2 147 483 647 must be qualified explicitly as L# for 64-bit literal or as UDINT#/ULINT# unsigned literal.

Symbolic Addressing and PLC Data Types (UDT)

Symbolic addressing binds a name to a tag plus its type. TIA Portal enforces that every symbolic variable in a global DB, instance DB, or FB static section is assigned exactly one elementary or complex type. Three practical guidelines apply:

1. Allocate UDTs for repeated structures. A UDT ("PLC data type") named, for example, typeAxis with members setpoint (LREAL), actual (LREAL), enable (BOOL), status (WORD) can then be instanced as axis1, axis2, … in a global DB. Any change to typeAxis propagates automatically.
2. Prefer LREAL over REAL on S7-1500 when scaling analog values — even modest filter chains accumulate error past the 7-digit boundary of REAL.
3. Bind HMI tags to symbolic DB members. HMI faceplates read only the symbols exported by the PLC, and using symbolic addressing means a DB renumbering never breaks the HMI connection.

Verification, Diagnostics, and Cross-References

Use the following checklist to confirm that a STEP 7 program is type-correct before commissioning:

• Compile project — zero warnings under "Type conversion / Possible loss of information".
• Open the watch table, load the binary TOD MD504, and verify the unpack yields the expected H/M/S/ms values. Discrepancy of exactly ±3600 000 ms points to the hours field; ±60 000 ms to minutes.
• Read OB1 stack information when the CPU enters STOP with SF: confirm no REAL divide-by-zero (event ID 16#2520 area pointer fault) or BCD-conversion error.
• Cross-reference (Show usage) every UDT instance in the project; any consumer expecting a deprecated member must be migrated.
• Confirm the S7-1500 CPU firmware supports the requested types: LTOD, DTL, and WSTRING require FW ≥ V2.0 (LTOD) / V4.4 (DTL, WSTRING) on the S7-1200 side.

The authoritative reference for the type catalog remains the Siemens Support entry Overview of the valid data types — STEP 7 Professional V14.0, supplemented by Basics of PLC data types — STEP 7 Professional V13.0 for UDT limits, and the product page STEP 7 — TIA Portal for licensing and version compatibility.

Frequently Asked Questions