Comparing Large Arrays of Double Integer (DINT) Values in SIMATIC PCS 7

To compare a large number of Double Integer (DINT) values (e.g., more than 20 variables) against constants or setpoints in SIMATIC PCS 7, the most efficient approach is to implement a custom SCL function block utilizing an array loop ('FOR i := 1 TO 20 DO...'), or use standard library comparator blocks ('CMP_DI' or APL 'CompIn02') in CFC. While cascading individual comparator blocks in CFC is viable for smaller groups, a compact SCL block dramatically reduces CFC chart clutter, minimizes memory consumption, and executes in a fraction of the CPU scan time.

Executive Summary

Industrial automation applications—such as recipe batch code validation, multi-lane barcode verification, part sorting, and state matrix matching—frequently require comparing dozens of 32-bit integer (DINT) values against configured target thresholds. Placing 20 to 50 separate comparison blocks on a CFC chart results in bloated, unreadable charts, high engineering maintenance overhead, and inefficient CPU execution order. This guide contrasts standard CFC comparator blocks (CMP_DI, CompIn02) with an optimized SCL multi-value comparison block.

Architecture and Fundamental Mechanics

Comparison Architecture: CFC Cascading vs SCL Array Looping:

Method A: Cascading Comparator Blocks in CFC (CMP_DI / CompIn02):
Standard Library Block CMP_DI (FB40) or APL Block CompIn02 (FB1818) compares two DINT values (IN1 and IN2).
Comparing 20 values requires 20 separate block instances, cluttering the CFC sheet and consuming run sequence scheduling slots.
Method B: Optimized SCL Array Block (CMP_DINT_ARRAY):
Encapsulates the 20 comparisons within a single compact Function Block (FB).
Declares an input array: Values : ARRAY[1..20] OF DINT; and target constants Targets : ARRAY[1..20] OF DINT;.
An internal FOR loop evaluates comparisons in nanoseconds during a single block invocation.
Provides composite outputs: AllEqual : BOOL;, AnyEqual : BOOL;, MatchMask : DWORD;.

Step-by-Step Implementation Procedure

Step-by-Step Implementation of an SCL Multi-DINT Comparator Block:

Create SCL Source in SIMATIC Manager:
- In the S7 Program container, open the Sources folder -> Insert New Object -> SCL Source File.
- Name it CMP_DINT_20.
Program the SCL Comparison Logic:
```scl
FUNCTION_BLOCK FB2020
TITLE = 'Compare_20_DINT_Values'
{ S7_m_c := 'false' }
VAR_INPUT
Values : ARRAY[1..20] OF DINT;
Constants : ARRAY[1..20] OF DINT;
END_VAR
VAR_OUTPUT
AllMatch : BOOL;
AnyMatch : BOOL;
MatchCount : INT;
MatchBits : DWORD;
END_VAR
VAR_TEMP
i : INT;
cnt : INT;
bits : DWORD;
END_VAR

cnt := 0;
bits := 16#0;
FOR i := 1 TO 20 DO
IF Values[i] = Constants[i] THEN
cnt := cnt + 1;
bits := bits OR SHL(IN := DWORD#16#1, N := (i - 1));
END_IF;
END_FOR;
MatchCount := cnt;
MatchBits := bits;
AllMatch := (cnt = 20);
AnyMatch := (cnt > 0);
END_FUNCTION_BLOCK
`` 3. Compile SCL Source into Blocks Folder: - In the SCL editor, click **File -> Compile** (or pressCtrl+F9). - Verify compilation completes with zero errors, generatingFB2020in the Blocks container. 4. Insert into CFC Chart: - DragFB2020` into the cyclic interrupt chart (OB35).
- Connect process variables to array elements; connect outputs to downstream interlocking logic.

Verification

To verify multi-value comparison operation:
1. In CFC test mode, force Values[1] := 100 and Constants[1] := 100; verify MatchCount increments to 1 and AnyMatch asserts TRUE.
2. Force all 20 values to match their respective constants; verify MatchCount = 20 and AllMatch asserts TRUE.
3. Change one value; confirm AllMatch drops instantly to FALSE.

Limitations and Operational Precautions

Operational precautions:
- Array loops in S7-400 CPUs must be bounded; never use unbounded WHILE loops in SCL, which can trigger CPU watchdog timeouts (OB80).
- If using standard CMP_DI blocks instead of SCL, organize them inside a nested CFC chart (Chart-in-Chart) to maintain plant hierarchy readability.
- Ensure that DINT sign boundaries are observed (-2,147,483,648 to +2,147,483,647).

Engineer FAQ

What standard library block compares two DINT numbers?

The CMP_DI (FB40) block in the standard library or CompIn02 (FB1818) in the APL.

Why is an SCL block preferred over 20 separate CMP_DI blocks?

An SCL block reduces 20 blocks down to a single compact call, eliminating chart clutter and reducing memory overhead.

Can CFC connect arrays directly?

CFC supports connecting to individual array elements (e.g., Block.Values[1]).

Does an SCL comparison block consume an AS Process Object?

If S7_m_c := 'false', the block is treated as an elementary calculation block and consumes zero Process Objects.

What is the data range of a DINT in Siemens S7?

A 32-bit signed double integer ranging from -2,147,483,648 to +2,147,483,647.

Technical References

Related Tags

DINT Comparison
SCL Programming
CMP_DI
CompIn02
Array Looping

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Comparing Large Arrays of Double Integer (DINT) Values in SIMATIC PCS 7