Data conversion instructions in PLC Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of INT to REAL conversion

What is the output value of the REAL variable after converting the INT value 123 using the standard INT_TO_REAL instruction?

PLC Programming

INT inputValue := 123;
REAL outputValue;
outputValue := INT_TO_REAL(inputValue);

A 0.123

B 123

C Error: incompatible types

D 123.0

💡 Hint

Remember that INT_TO_REAL converts integer to floating point number preserving the value.

The INT_TO_REAL instruction converts the integer 123 to the floating point number 123.0. It does not change the value, only the data type.

💻 Command Output

intermediate

Result of BYTE to WORD conversion

Given a BYTE variable with value 255, what is the value of a WORD variable after conversion using BYTE_TO_WORD?

PLC Programming

BYTE byteVal := 255;
WORD wordVal;
wordVal := BYTE_TO_WORD(byteVal);

A 255

B 0

C 65535

D Error: data overflow

💡 Hint

BYTE_TO_WORD converts 8-bit to 16-bit preserving the numeric value.

BYTE_TO_WORD converts the 8-bit value 255 to a 16-bit WORD with the same numeric value 255.

📝 Syntax

advanced

Identify the correct syntax for converting REAL to INT

Which of the following code snippets correctly converts a REAL variable to an INT variable using the standard conversion instruction?

A INT output := INT(REAL inputReal);

B INT output := REAL_TO_INT(inputReal);

C INT output := TO_INT(inputReal);

D INT output := CONVERT_INT(inputReal);

💡 Hint

Look for the standard IEC 61131-3 conversion function name.

The correct syntax uses the standard IEC instruction REAL_TO_INT to convert REAL to INT. Other options are invalid or non-standard.

🔧 Debug

advanced

Debugging data conversion overflow error

A program converts a DINT value 70000 to a WORD variable using DINT_TO_WORD. What will happen and why?

PLC Programming

DINT largeValue := 70000;
WORD smallValue;
smallValue := DINT_TO_WORD(largeValue);

A smallValue will be 4464 due to overflow (70000 mod 65536)

B smallValue will be 70000 without error

C Compilation error due to incompatible types

D Runtime error: overflow exception

💡 Hint

WORD can only hold values up to 65535. Larger values wrap around.

Converting 70000 to WORD causes overflow. The value wraps around modulo 65536, resulting in 70000 - 65536 = 4464.

🚀 Application

expert

Combining multiple data conversions in a PLC program

You have a REAL sensor input that must be converted to an INT for processing, then to a BYTE for communication. Which sequence of instructions correctly performs these conversions without data loss or errors?

PLC Programming

REAL sensorInput := 123.45;
INT intValue;
BYTE byteValue;

A

intValue := INT(sensorInput);
byteValue := BYTE(intValue);

B

byteValue := REAL_TO_BYTE(sensorInput);
intValue := BYTE_TO_INT(byteValue);

C

intValue := REAL_TO_INT(sensorInput);
byteValue := INT_TO_BYTE(intValue);

D

intValue := REAL_TO_DINT(sensorInput);
byteValue := DINT_TO_BYTE(intValue);

💡 Hint

Convert from REAL to INT first, then INT to BYTE. Avoid direct REAL to BYTE conversion.

Option C uses standard IEC instructions REAL_TO_INT and INT_TO_BYTE in correct order. Other options use invalid or unsafe conversions.

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