Introduction to IEC 61131-3

IEC 61131-3 is the third part of the IEC 61131 standard, focusing specifically on the programming languages and software model for PLCs. This standard promotes a unified approach to PLC programming, allowing engineers to transfer their skills across different PLC brands and platforms. It defines five standard programming languages, data types, and software organization units (POUs), facilitating modular and reusable code.

IEC 61131-3 Programming Languages

IEC 61131-3 defines five standard programming languages for PLCs:

• Ladder Diagram (LD)
• Structured Text (ST)
• Function Block Diagram (FBD)
• Sequential Function Chart (SFC)
• Instruction List (IL)

Each language has its strengths and is suitable for different types of control tasks. Understanding these languages is crucial for effective PLC programming.

Ladder Diagram (LD)

Ladder Diagram is a graphical programming language that resembles electrical relay logic diagrams. It is widely used for discrete control applications and is easy to understand for electricians and technicians familiar with relay logic.

Structured Text (ST)

Structured Text is a high-level textual programming language similar to Pascal. It is well-suited for complex algorithms, data manipulation, and mathematical calculations. ST provides a powerful and flexible way to implement advanced control strategies.

Function Block Diagram (FBD)

Function Block Diagram is a graphical programming language that uses interconnected function blocks to represent control functions. It is ideal for continuous control applications and signal processing.

Sequential Function Chart (SFC)

Sequential Function Chart is a graphical programming language used for designing sequential control systems. It consists of steps, transitions, and actions, allowing programmers to visualize and manage complex sequences of operations.

Instruction List (IL)

Instruction List is a low-level textual programming language similar to assembly language. It provides direct control over the PLC's hardware and is often used for time-critical applications or when memory resources are limited.

Data Types and Variables in IEC 61131-3

IEC 61131-3 defines a set of standard data types for PLC programming, ensuring compatibility and interoperability across different platforms. Common data types include:

• BOOL (Boolean): Represents a binary value (TRUE or FALSE).
• INT (Integer): Represents a whole number.
• REAL (Floating-point number): Represents a real number with decimal precision.
• STRING (Character string): Represents a sequence of characters.
• TIME (Time duration): Represents a time interval.
• DATE (Date): Represents a calendar date.
• TIME_OF_DAY (Time of day): Represents the time of day.
• DATE_AND_TIME (Date and time): Represents a specific date and time.

Variables are used to store and manipulate data within a PLC program. They can be declared as local variables (within a POU) or global variables (accessible from multiple POUs).

Software Organization Units (POUs)

IEC 61131-3 defines three types of Software Organization Units (POUs):

• Functions: POUs that return a single value and do not have internal state.
• Function Blocks: POUs that can have internal state and can be instantiated multiple times.
• Programs: POUs that represent the main control logic of a PLC application.

POUs allow programmers to modularize their code, making it easier to manage, reuse, and test.

Coding Standards and Best Practices

Following coding standards and best practices is crucial for developing robust, maintainable, and efficient PLC programs. Some common coding standards include:

• Use meaningful variable and POU names.
• Add comments to explain complex logic.
• Modularize code into reusable POUs.
• Avoid using global variables excessively.
• Implement error handling and diagnostics.
• Follow a consistent coding style.

Example of Structured Text code with comments:

(* This program controls a motor based on sensor inputs *)
PROGRAM MotorControl
    VAR
        Sensor1 : BOOL; (* Input sensor 1 *)
        Sensor2 : BOOL; (* Input sensor 2 *)
        MotorEnable : BOOL; (* Output to enable the motor *)
    END_VAR

    (* Check if both sensors are active *)
    IF Sensor1 AND Sensor2 THEN
        MotorEnable := TRUE; (* Enable the motor *)
    ELSE
        MotorEnable := FALSE; (* Disable the motor *)
    END_IF
END_PROGRAM
                

IEC 61131-3 Compliance Checklist

Ensuring compliance with IEC 61131-3 standards is vital for creating reliable and portable PLC programs. Here's a checklist to help you verify compliance:

Performance Metrics Analysis

Understanding performance metrics is crucial for optimization and ensuring your PLC code operates efficiently. Key metrics include scan time, memory usage, and CPU load.