Intermediate

PLC String Handling in Structured Text: Parsing, Formatting & Communication

Learn to manipulate strings in Structured Text — from basic concatenation to barcode parsing, HMI message formatting, and serial protocol handling.

2026-03-04 · 12 min read

String Handling
Parsing
Serial Communication
HMI
Structured Text

Why String Handling Matters in PLCs

Modern PLCs do far more than control motors and valves. They parse barcode data, format HMI display messages, communicate with serial devices, build log entries, and exchange data with MES/SCADA systems. All of this requires robust string handling.

String Basics in Structured Text

VAR
    // Fixed-length strings
    ProductName : STRING[40] := 'Widget-A';
    Message : STRING[80] := '';
    
    // String length
    NameLen : INT;
END_VARNameLen := LEN(ProductName);  // Returns 8

IEC 61131-3 String Functions

Function	Description	Example
LEN(s)	Length of string	LEN('Hello') → 5
LEFT(s, n)	First n characters	LEFT('Hello', 3) → 'Hel'
RIGHT(s, n)	Last n characters	RIGHT('Hello', 3) → 'llo'
MID(s, n, p)	n chars from position p	MID('Hello', 2, 3) → 'll'
CONCAT(s1, s2)	Join two strings	CONCAT('Hi', ' World') → 'Hi World'
FIND(s1, s2)	Find s2 in s1 (0=not found)	FIND('Hello', 'llo') → 3
INSERT(s1, s2, p)	Insert s2 into s1 at p	INSERT('Helo', 'l', 3) → 'Hello'
DELETE(s, n, p)	Delete n chars from p	DELETE('Hello', 2, 2) → 'Hlo'
REPLACE(s1, s2, n, p)	Replace n chars at p with s2	REPLACE('Hello', 'a', 1, 5) → 'Hella'

Building HMI Display Messages

PROGRAM HMIMessages
VAR
    Temperature : REAL := 72.5;
    BatchCount : INT := 1547;
    MachineName : STRING[20] := 'Press-01';
    StatusLine : STRING[80] := '';
    TempStr : STRING[10];
    CountStr : STRING[10];
END_VAR
// Convert numbers to strings
TempStr := REAL_TO_STRING(Temperature);
CountStr := INT_TO_STRING(BatchCount);// Build display message
StatusLine := CONCAT(MachineName, ' | Temp: ');
StatusLine := CONCAT(StatusLine, TempStr);
StatusLine := CONCAT(StatusLine, '°C | Count: ');
StatusLine := CONCAT(StatusLine, CountStr);
// Result: "Press-01 | Temp: 72.5°C | Count: 1547"

Parsing Barcode Data

Barcodes often encode multiple fields separated by delimiters:

PROGRAM BarcodeParser
VAR
    RawBarcode : STRING[80] := 'PN:12345;LOT:A2024-001;QTY:50;EXP:2026-12';
    
    PartNumber : STRING[20] := '';
    LotCode : STRING[20] := '';
    Quantity : INT := 0;
    ExpiryDate : STRING[20] := '';
    
    SearchPos : INT;
    FieldStart : INT;
    FieldEnd : INT;
    TempStr : STRING[80];
    QtyStr : STRING[10];
END_VAR
// Extract Part Number (after "PN:")
SearchPos := FIND(RawBarcode, 'PN:');
IF SearchPos > 0 THEN
    TempStr := MID(RawBarcode, 80, SearchPos + 3);
    FieldEnd := FIND(TempStr, ';');
    IF FieldEnd > 0 THEN
        PartNumber := LEFT(TempStr, FieldEnd - 1);
    END_IF;
END_IF;
// Extract Lot Code (after "LOT:")
SearchPos := FIND(RawBarcode, 'LOT:');
IF SearchPos > 0 THEN
    TempStr := MID(RawBarcode, 80, SearchPos + 4);
    FieldEnd := FIND(TempStr, ';');
    IF FieldEnd > 0 THEN
        LotCode := LEFT(TempStr, FieldEnd - 1);
    END_IF;
END_IF;// Extract Quantity (after "QTY:")
SearchPos := FIND(RawBarcode, 'QTY:');
IF SearchPos > 0 THEN
    TempStr := MID(RawBarcode, 80, SearchPos + 4);
    FieldEnd := FIND(TempStr, ';');
    IF FieldEnd > 0 THEN
        QtyStr := LEFT(TempStr, FieldEnd - 1);
    ELSE
        QtyStr := TempStr;
    END_IF;
    Quantity := STRING_TO_INT(QtyStr);
END_IF;

Serial Protocol Message Building

Many industrial devices use ASCII-based serial protocols:

PROGRAM SerialProtocol
VAR
    DeviceAddr : INT := 1;
    Command : STRING[10] := 'RD';
    Register : INT := 4000;
    
    TxMessage : STRING[80] := '';
    Checksum : INT := 0;
    i : INT;
    CharCode : INT;
    AddrStr : STRING[5];
    RegStr : STRING[10];
END_VAR
// Build message: STX + Address + Command + Register + ETX + Checksum
AddrStr := INT_TO_STRING(DeviceAddr);
RegStr := INT_TO_STRING(Register);
TxMessage := '$02';  // STX character
TxMessage := CONCAT(TxMessage, AddrStr);
TxMessage := CONCAT(TxMessage, Command);
TxMessage := CONCAT(TxMessage, RegStr);
TxMessage := CONCAT(TxMessage, '$03');  // ETX character
// Calculate simple checksum (XOR of all bytes)
Checksum := 0;
FOR i := 1 TO LEN(TxMessage) DO
    // XOR each character's ASCII value
    Checksum := Checksum XOR ORD(MID(TxMessage, 1, i));
END_FOR;TxMessage := CONCAT(TxMessage, INT_TO_STRING(Checksum));

CSV Log Entry Builder

PROGRAM CSVLogger
VAR
    Timestamp : STRING[20] := '2026-03-04 14:30:00';
    EventCode : INT := 101;
    EventDesc : STRING[40] := 'Batch Complete';
    Value1 : REAL := 98.7;
    Value2 : REAL := 45.2;
    
    CSVLine : STRING[200] := '';
END_VAR// Build CSV: Timestamp,Code,Description,Value1,Value2
CSVLine := Timestamp;
CSVLine := CONCAT(CSVLine, ',');
CSVLine := CONCAT(CSVLine, INT_TO_STRING(EventCode));
CSVLine := CONCAT(CSVLine, ',');
CSVLine := CONCAT(CSVLine, EventDesc);
CSVLine := CONCAT(CSVLine, ',');
CSVLine := CONCAT(CSVLine, REAL_TO_STRING(Value1));
CSVLine := CONCAT(CSVLine, ',');
CSVLine := CONCAT(CSVLine, REAL_TO_STRING(Value2));
// Result: "2026-03-04 14:30:00,101,Batch Complete,98.7,45.2"

String Comparison and Validation

PROGRAM InputValidation
VAR
    UserInput : STRING[40] := '';
    IsValid : BOOL := FALSE;
    ErrorMsg : STRING[80] := '';
    InputLen : INT;
END_VAR
InputLen := LEN(UserInput);
// Check minimum length
IF InputLen < 3 THEN
    IsValid := FALSE;
    ErrorMsg := 'Input too short (min 3 characters)';
// Check for forbidden characters
ELSIF FIND(UserInput, ';') > 0 OR FIND(UserInput, '$27') > 0 THEN
    IsValid := FALSE;
    ErrorMsg := 'Invalid characters detected';
// Check prefix
ELSIF LEFT(UserInput, 2) <> 'PN' THEN
    IsValid := FALSE;
    ErrorMsg := 'Must start with PN prefix';ELSE
    IsValid := TRUE;
    ErrorMsg := '';
END_IF;

Best Practices for PLC String Handling

Always declare string length — STRING[80] not just STRING to control memory usage

Check lengths before operations — Prevent buffer overflows on concatenation

Use FIND before MID — Validate the delimiter exists before extracting substrings

Handle empty strings — Check LEN(s) > 0 before processing

Avoid strings in fast tasks — String operations are slow; keep them in slower cyclic tasks

Test with edge cases — Empty input, maximum length, missing delimiters

Practice string parsing in our online ST editor and explore the barcode parsing lesson for a real-world walkthrough.