Understanding Siemens PLC Data Types: A Comprehensive Guide

Key Takeaways: Understanding Siemens PLC Data Types

Key PointDetails
Elementary Data TypesPractical examples of how each data type is used in Siemens PLC programming.
Memory ConsiderationsDiscussion on how data types affect memory usage and performance.
Practical GuideOverview of basic data types like BOOL, BYTE, CHAR, WORD, INT, DINT, and REAL.
Further LearningLinks to tutorials and resources for advanced learning.

Welcome to ControlNexus, your leading provider of Siemens PLCs, HMIs, and Inverters since 2013. Today, we’ll dive into the essential knowledge of data types in Siemens PLC programming, covering both the basics and their applications in real-world scenarios. Understanding these data types is crucial for anyone looking to master PLC programming and optimize their use of industrial control systems.


In the world of automated systems, Programmable Logic Controllers (PLCs) are the backbone of manufacturing and processing industries. Siemens, a renowned leader in the field, offers a range of PLCs that are powerful and versatile. One fundamental aspect of programming these PLCs involves understanding the various data types that are used to store and manipulate data effectively. In this guide, we will explore the basic data types available in Siemens PLCs, how they are used, and why they are critical to successful PLC programming.

Section 1: Basics of Data Types in Siemens PLC

Data types in Siemens PLC programming are much like the different types of variables used in conventional programming languages. They define the kind of data a variable can hold and the operations that can be performed on them. Here’s a closer look at some elementary data types:

BOOL (Boolean)

  • Description: Represents true or false conditions.
  • Usage: Commonly used for flags, conditions, and to control logic flow.
  • Size: 1 bit.


  • Description: A group of 8 bits, used for storing small data or controlling a set of Boolean flags.
  • Usage: Handling inputs and outputs in byte format.
  • Size: 8 bits.


  • Description: Stores a single character or ASCII values.
  • Usage: Used for text displays or simple character data.
  • Size: 8 bits.


  • Description: Larger than a BYTE, used for storing integer numbers.
  • Usage: Used in operations requiring more precision than a BYTE.
  • Size: 16 bits.

INT (Integer)

  • Description: Represents whole numbers.
  • Usage: Common for calculations, counters, and more.
  • Size: 16 bits.

DINT (Double Integer)

  • Description: A double-sized integer that allows for larger values.
  • Usage: Useful for applications requiring large count ranges or calculations.
  • Size: 32 bits.


  • Description: Used for floating-point numbers.
  • Usage: Necessary for precision calculations involving decimals.
  • Size: 32 bits.

Each of these data types plays a crucial role in how information is stored, transferred, and manipulated within a PLC. Whether you are handling inputs from sensors, controlling outputs, or performing intermediate calculations, choosing the right data type is key to efficient programming.

Section 2: Advanced Data Types and Structures

As we delve deeper into Siemens PLC programming, it becomes necessary to understand more complex data structures such as Arrays, Structs, and User Defined Types (UDTs). These advanced data types allow for more organized and scalable code, which is essential in complex industrial applications.


  • Definition: A collection of elements of the same data type.
  • Usage: Ideal for managing lists of items, such as measurements from multiple sensors.


  • Definition: A composite data type that packages related items together.
  • Usage: Useful for grouping different data types that represent a structured data object.

UDTs (User Defined Types)

  • Definition: Custom data types are defined by the user to fit specific needs.
  • Usage: Enhances code reusability and clarity, particularly in large projects.

For a practical guide on how to set up and use these advanced data types in the Siemens TIA Portal, you can refer to our detailed tutorials and resources section.

Section 3: Practical Application and Programming Examples

Moving from theory to practice, let’s explore how to apply these data types in real-world Siemens PLC programming scenarios using the TIA Portal, one of the most advanced and intuitive tools for PLC programming.

Setting Up a Simple PLC Program

  1. Starting a New Project: Open the TIA Portal and create a new project.
  2. Adding a Device: Select the appropriate Siemens PLC model from your hardware catalog. If you’re working with an S7-1200 or S7-1500 PLC, you can find these under respective sections on our website.
  3. Configuring the Device: Setup your PLC’s device configuration to include necessary input and output modules.

Example Scenarios

Monitoring System Status

  • Data Types Used: BOOL for status flags, INT for counting operations.
  • Implementation: Create a system status function that uses BOOL to check operational states (on/off) and INT to count the number of times a certain condition is met.

Managing Analog Inputs and Outputs

  • Data Types Used: REAL for analog values, DINT for extended range of integer values.
  • Implementation: Configure analog input modules to read temperature or pressure, using REAL to handle precise measurements. Use DINT for totalizing or accumulating values that exceed standard integer limits.

Each example includes step-by-step coding snippets that you can replicate and modify according to your needs:

// Example for Monitoring System Status
IF %I0.0 == 1 THEN
   %Q0.0 := TRUE;   // Set output to TRUE if input I0.0 is active
   Counter := Counter + 1; // Increment counter
   %Q0.0 := FALSE;
// Example for Managing Analog Inputs and Outputs
Temp_Input := REAL_TO_INT(%IW64);  // Convert analog input to integer
IF Temp_Input > 100 THEN
   Alarm := TRUE;  // Set alarm if temperature exceeds 100 degrees

These examples illustrate basic implementation, emphasizing how different data types are utilized in programming to achieve functional and reliable automation.

Section 4: Best Practices and Tips

To maximize the efficiency and reliability of your PLC programming, consider the following best practices:

  • Optimize Data Usage: Always choose the smallest data type that can handle the task to conserve memory.
  • Structured Programming: Use STRUCT and UDTs to keep your code organized and maintainable.
  • Error Handling: Implement comprehensive error-checking routines, especially when converting between different data types.

Expert Tip: Always simulate your PLC program in the TIA Portal before deploying it to the actual hardware. This practice helps catch errors and optimize the logic without the risk of affecting the live process.

Section 5: Common Pitfalls and Troubleshooting

Despite careful planning, you might encounter issues in PLC programming. Here are common pitfalls and their solutions:

  • Data Overflow: Be cautious with data types like INT, where exceeding the maximum or minimum value can lead to overflow errors. Use DINT or REAL where higher ranges are expected.
  • Mismatched Data Types: When linking data types across different functions or blocks, ensure they are compatible to prevent logic errors.

For troubleshooting:

  • Use the diagnostic functions within TIA Portal to monitor and test each part of your PLC program.
  • Regularly update your software and hardware tools to ensure compatibility and performance.


Understanding and correctly implementing data types in Siemens PLC programming is key to building robust industrial control systems. By following the guidelines and examples provided, you can enhance your programming skills and ensure your automation projects are successful.

Explore more about Siemens PLCs and their applications by visiting our detailed sections on Siemens PLCs and Siemens HMIs. For further learning, don’t hesitate to contact us or check out our additional resources. Your journey towards mastering Siemens PLC programming starts here!


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