String OperationsIntermediateIEC 61131-3

LEFT (Left String Extract)

Extracts a specified number of characters from the beginning of a string.

The LEFT function extracts a specified number of characters from the beginning (left side) of a string. It is used for parsing fixed-format data, extracting prefixes, and processing communication messages where specific fields are at known positions from the start of the string.

Parameters

Inputs

Name	Type	Description
IN	STRING	Source string
L	INT	Number of characters to extract from left

Outputs

Name	Type	Description
OUT	STRING	Extracted substring

Structured Text Example

// Extract device code from 'DEV001-MOTOR-A'
Device_Code := LEFT(IN := Device_ID, L := 6);  // Result: 'DEV001'

// Extract date from formatted string
Date_Part := LEFT(IN := DateTime_Str, L := 10);  // '2025-01-15'

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Platform-Specific Implementation

Siemens (TIA Portal)

Use LEFT instruction in TIA Portal.

Allen-Bradley (Studio 5000)

Use MID instruction with starting position 1.

CODESYS

Use LEFT() function in ST.

Common Applications

Fixed-format data parsing
Device code extraction
Protocol header parsing
Barcode prefix extraction

Frequently Asked Questions

What is LEFT (Left String Extract) in PLC programming?

Extracts a specified number of characters from the beginning of a string. The LEFT function extracts a specified number of characters from the beginning (left side) of a string. It is used for parsing fixed-format data, extracting prefixes, and processing communication messages where specific fields are at known positions from the start of the string.

What are common applications of LEFT (Left String Extract)?

LEFT (Left String Extract) is commonly used for: Fixed-format data parsing, Device code extraction, Protocol header parsing, Barcode prefix extraction.

How do I use LEFT (Left String Extract) in different PLC platforms?

Siemens: Use LEFT instruction in TIA Portal. Allen-Bradley: Use MID instruction with starting position 1. CODESYS: Use LEFT() function in ST.