Data conversion instructions in PLC Programming

Introduction

Data conversion instructions help change data from one type to another so the PLC can understand and use it correctly.

When you read a sensor value as a raw number but need it as a floating-point number for calculations.

When you receive data as a string but want to convert it to an integer to compare or process.

When you want to display a number as text on an HMI screen.

When combining different data types in a calculation and need them to match.

When storing data in a different format for communication with other devices.

Syntax

PLC Programming

CONVERT_TYPE(Source, Destination)

Example: INT_TO_REAL(MyInt, MyReal)

Source is the original data you want to convert.

Destination is where the converted data will be stored.

Examples

Converts an integer value MyInt to a real (floating-point) value MyReal.

PLC Programming

INT_TO_REAL(MyInt, MyReal)

Converts a real number MyReal to an integer MyInt by removing the decimal part.

PLC Programming

REAL_TO_INT(MyReal, MyInt)

Converts a string containing digits to an integer.

PLC Programming

STRING_TO_INT(MyString, MyInt)

Converts an integer to a string for display or communication.

PLC Programming

INT_TO_STRING(MyInt, MyString)

Sample Program

This program converts an integer to a real number and a string, then prints all values to show the conversion results.

PLC Programming

PROGRAM DataConversionExample
VAR
  MyInt : INT := 123;
  MyReal : REAL;
  MyString : STRING(10);
END_VAR

// Convert integer to real
INT_TO_REAL(MyInt, MyReal);

// Convert integer to string
INT_TO_STRING(MyInt, MyString);

// Output results
// (Assuming a debug or print function available)
Print('Integer value: ' + TO_STRING(MyInt));
Print('Converted to real: ' + TO_STRING(MyReal));
Print('Converted to string: ' + MyString);
END_PROGRAM

Important Notes

Always ensure the source data is valid for the target type to avoid errors.

Some conversions may lose information, like converting real to int removes decimals.

Check your PLC's documentation for exact conversion instruction names and usage.

Summary

Data conversion instructions change data types so PLCs can process them correctly.

Use them when combining or displaying different data types.

Be careful with conversions that can lose data, like real to int.

Practice

1. What is the main purpose of data conversion instructions in PLC programming?

easy

A. To create new variables automatically

B. To speed up the PLC scan time

C. To change data types so PLCs can process them correctly

D. To reset the PLC program

Solution

1. Step 1: Understand data conversion instructions

   Data conversion instructions change one data type to another so the PLC can handle the data properly.

2. Step 2: Identify the main purpose

   The main goal is to ensure data compatibility during processing or display.

3. Final Answer:

   To change data types so PLCs can process them correctly -> Option C

4. Quick Check:

   Data conversion = change data types [OK]

Hint: Data conversion = changing data types for compatibility [OK]

Common Mistakes:

• Thinking it speeds up the PLC scan
• Confusing with variable creation
• Assuming it resets the program

2. Which of the following is the correct syntax to convert an integer to a real number in a typical PLC program?

easy

A. INT_TO_REAL(value)

B. REAL(value)

C. TO_REAL(value)

D. CONVERT_REAL(value)

Solution

1. Step 1: Recall standard PLC conversion functions

   Most PLCs use functions like INT_TO_REAL to convert integers to real numbers.

2. Step 2: Match syntax options

   INT_TO_REAL(value) is the standard and correct syntax for this conversion.

3. Final Answer:

   INT_TO_REAL(value) -> Option A

4. Quick Check:

   Integer to real = INT_TO_REAL() [OK]

Hint: Look for function names starting with source type + _TO_ + target type [OK]

Common Mistakes:

• Using generic names like REAL(value)
• Assuming TO_REAL is standard
• Using CONVERT_REAL which is not typical

3. Given the following PLC code snippet:

INT_VAR := 1234;
REAL_VAR := INT_TO_REAL(INT_VAR);
INT_CONV := REAL_TO_INT(REAL_VAR);

What is the value of INT_CONV after execution?

medium

A. 1235

B. 1234

C. 1233

D. 0

Solution

1. Step 1: Convert INT_VAR to REAL_VAR

   INT_VAR is 1234, converted to real: REAL_VAR = 1234.0 (no change in value, just type).

2. Step 2: Convert REAL_VAR back to INT_CONV

   REAL_TO_INT(1234.0) converts back to integer 1234 without loss.

3. Final Answer:

   1234 -> Option B

4. Quick Check:

   Conversion back and forth keeps value same [OK]

Hint: Converting int to real and back keeps original integer value [OK]

Common Mistakes:

• Assuming rounding changes value
• Thinking conversion truncates incorrectly
• Confusing data types with values

4. Identify the error in this PLC code snippet:

REAL_VAR := 12.7;
INT_VAR := REAL_TO_INT(REAL_VAR);
BOOL_VAR := INT_TO_BOOL(REAL_VAR);

medium

A. REAL_VAR should be integer type

B. REAL_TO_INT cannot convert real to int

C. Missing semicolon after first line

D. INT_TO_BOOL cannot convert REAL directly

Solution

1. Step 1: Check each conversion

   REAL_TO_INT(REAL_VAR) is valid to convert real to int.

2. Step 2: Check INT_TO_BOOL(REAL_VAR)

   INT_TO_BOOL expects an integer input, but REAL_VAR is real type, causing an error.

3. Final Answer:

   INT_TO_BOOL cannot convert REAL directly -> Option D

4. Quick Check:

   Conversion functions require correct input types [OK]

Hint: Check input type matches conversion function parameter [OK]

Common Mistakes:

• Assuming all conversions accept any type
• Ignoring type mismatch errors
• Thinking missing semicolon causes conversion error

5. You have a PLC program that reads a sensor value as a 16-bit integer but needs to display it as a floating-point number with two decimal places. Which sequence of data conversion instructions is best to achieve this?

hard

A. INT_TO_REAL(sensor_value) then multiply by 0.01

B. REAL_TO_INT(sensor_value) then divide by 100

C. INT_TO_BOOL(sensor_value) then convert to REAL

D. BOOL_TO_INT(sensor_value) then multiply by 100

Solution

1. Step 1: Convert integer sensor value to real

   Use INT_TO_REAL to convert the 16-bit integer to a floating-point number.

2. Step 2: Adjust scale for decimal places

   Multiply the real number by 0.01 to shift decimal two places for display.

3. Final Answer:

   INT_TO_REAL(sensor_value) then multiply by 0.01 -> Option A

4. Quick Check:

   Convert int to real, then scale for decimals [OK]

Hint: Convert int to real first, then scale for decimals [OK]

Common Mistakes:

• Trying to convert real to int first
• Using boolean conversions incorrectly
• Multiplying before converting data type