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                          Understanding the IEC61131-3 
                          Programming Languages
                          It was about 120 years ago when 
                          Mark Twain used the phrase “more                                                                       Choosing Your Language
                          than one way to skin a cat.” In 
                          the world of PLC programming,                                                                          The International Electrotechnical 
                          that cliché is still applicable today.                                                                 Commission (IEC) identifies five 
                          Thanks to the International                                                                            standard programming languages 
                          Electrotechnical Commission                                                                            as the most common for both 
                          (IEC), five standard programming                                                                       process and discrete programmable 
                          languages have emerged as the most                                                                     controllers: Ladder Diagram (LD), 
                          common, used for both process and                                                                      Function Block Diagram (FBD), 
                          discrete programmable controllers.                                                                     Sequential Function Chart (SFC), 
                          The IEC is an organization                                                                             Instruction List (IL), and Structured 
                          that prepares and publishes                                                                            Text (ST)
                          international standards for all 
                          electrical, electronic and related                                                                     With the different programming 
                          technologies, including controllers.                                                                   languages available, it’s important to 
                          With its IEC61131-3 publication,                                                                       consider a few factors before deciding 
                          the organization identifies these                The complexity of the application, the                which to use for your application:
                          five programming languages and                   capabilities of the PLC/PAC and the ability           •	Ease of maintenance by the final 
                                                                           to transfer the program code are among 
                          their common abbreviations as:                   the key factors to consider with selecting             user: SFC
                          Ladder Diagram (LD), Instruction                 controller programming language.                      •	Universal language acceptance: LD
                          List (IL), Function Block Diagram                                                                      •	Acceptance in Europe: IL or ST
                          (FBD), Structured Text (ST) and                  applications, and the multiplying                     •	PLC speed of execution: IL or ST
                          Sequential Function Chart (SFC).                 methods for implementing control                      •	Applications mainly using digital 
                                                                           functions. But many controls                           I/O and basic processing: LD  
                          Long dismissed as just being                     engineers may be wondering                             or FBD
                          a European phenomenon, the                       about the characteristics of each                     •	Ease of changing code: LD
                          IEC’s programmable controller                    programming language. When                            •	Ease of use by newer engineers: ST
                          languages are gaining traction                   should one be used over another?                      •	Ease of implementing complex 
                          in the United States. The IEC                    What are the benefits and                              mathematical operations: ST
                          developed these programming                      disadvantages of each? This article                   •	Applications with repeating 
                          standards in response to the                     will provide a brief overview and                      processes or processes requiring 
                          growing number of automation                     comparison of each of the five                         interlocks and concurrent 
                          vendors, the growing complexity of               main PLC programming languages.                        operations: SFC
            Ladder Diagram (LD)
            This programming language, 
            invented in the U.S. decades ago, 
            is probably the most widely used. 
            Invented to replace hardwired relay 
            control systems, Ladder Diagram 
            programming is a mainstay in the 
            U.S. today, used in probably 95 
            percent of all applications. Visually, 
            this language resembles a series 
            of control circuits, with a series of 
            inputs needing to be “made” or 
            “true” in order to activate one or 
            more outputs. 
            Ladder Diagram language has 
            experienced such widespread             This language resembles a series of control circuits, with a series of inputs needing to be 
            adoption that almost every              “made” or “true” in order to activate one or more outputs.
            programmer in any country or 
            industry can read and write this 
            language. Because it resembles the      fires an actuator to push the box        be daunting — picture a ladder 
            familiar electric circuit format,       to another conveyor. Digital inputs      rung with an output used in several 
            even a non-programmer with an           are checking for various conditions,     phases of a process with many input 
            electrical background can follow        and a basic program is analyzing         conditions attempting to control 
            the program for purposes of             the inputs and firing digital outputs    exactly when that output needs to 
            troubleshooting a problem. It’s also    in response. There may be timers         turn on.
            easy to start writing a program         in the program, or some basic 
            in Ladder Diagram. With just a          comparisons or math, but there are       Function Block Diagram
            basic outline of input and output       no complex functions involved.           Although Ladder Diagramming 
            signals, one can sit down are start                                              may be the most widespread 
            churning out code. Most of the          As the complexity of PLC                 language, a survey conducted by 
            other IEC languages require more        functionality has grown, however,        Control Engineering magazine 
            preparation, such as flowcharting       Ladder Diagram language has              several months ago highlighted 
            all the potential process flows.        been challenged to meet these            growth in the use of programming 
            Finally, most implementations           advances and still maintain the          languages other than ladder. 
            of Ladder Diagram allow a               paradigm of easy visualization and       Function Block Diagram 
            program to be organized into            understanding. Functions such            programming is an example. 
            folders or subprograms that             as PID, trigonometry and data            Even though the adoption rate for 
            are downloaded to the PLC,              analysis are commonly required           this language has recently slowed 
            allowing for easy segmentation.         in many control applications, but        relative to other languages such as 
                                                    difficult to implement. Another          Structured Text, Function Block 
            Ladder Diagram programming is           challenge is that as program size        Programming is probably the 
            ideal for a simple material handling    grows, the ladder can become very        second most widely used language.
            application, for example, where         difficult to read and interpret, 
            a sensor detects the presence of        unless it’s extensively documented.      In many ways, this graphical 
            a box, other sensors check for          Finally, implementing full               language resembles a wiring 
            obstructions, and then an output        processes in Ladder Diagram can          diagram even more so than 
            Ladder code. With Function 
            Block Diagram, the blocks are 
            “wired” together into a sequence 
            that’s easy to follow. It uses the 
            same instructions as Ladder, but 
            visually is more understandable to 
            a viewer who is not versed in relay 
            logic. The major advantage is that 
            programs written in Function Block 
            tend to be easy to follow — just 
            follow the path! This language 
            is ideal for simpler programs 
            consisting of digital inputs, such 
            as photoelectric sensors, and 
            outputs such as valve manifolds, 
            and could be appropriate in any 
            application where Ladder is ideal.      With Function Block Diagram, the blocks are “wired” together into a sequence that’s easy 
                                                    to follow. It uses the same instructions as Ladder, but is visually more understandable to a 
            However, this language is not ideal     viewer who is not versed in relay logic.
            for large programs using special I/O 
            and functions. The large amount of      turned off, and the next one in the      necessary conditions are met to 
            screen space required by this style     sequence is active. The transition       allow the program to advance to  
            of programming can quickly make         step also has code to check that the     the next step.
            a program unwieldy if it reaches 
            any substantial size. Also, writing a 
            program in Function Block requires 
            more preparation upfront to 
            understand the program and how it 
            will flow before any code is written, 
            since it can be more difficult to 
            make corrections later.
            Sequential Function Chart 
            Sequential Function Chart (SFC) 
            programming resembles the 
            computer flowcharts that many 
            will remember drawing up in 
            their college days. An initial step 
            “action box” (the starting point 
            of a flowchart) is followed by a 
            series of transitions and additional 
            action steps. The concept of SFC 
            is simple: an action box, with code 
            inside written in any language of 
            the programmer’s choice, is active      In this example, SFC programming includes a flowchart on one side and two small 
            until the transition step below it      programs to the right. In an SFC program, the flowchart boxes (called actions) and the  
                                                    little horizontal lines with names (called transitions) actually have small programs running 
            activates. The current action box is    inside them.
           For appropriate applications which 
           have a repeatable multi-step process 
           or series of repeatable processes, this 
           form of programming is the easiest 
           to implement. An example would 
           be a pick and place application, 
           where product is constantly picked 
           up from one area, moved through 
           a specific path, and placed in 
           another area. While exceptions 
           exist, since there is typically only 
           one active piece of code and 
           one transition to be concerned 
           with, condition checking and the 
           control of the process should be 
           achievable without large rungs. 
           The language is also very friendly 
           to maintenance engineers because 
           the visual nature of the program 
           plus code segmentation makes it 
           easy to troubleshoot. For example, 
           if the mechanism in a pick-and-
           place application has moved to          Instruction List consists of many lines of code, with each line representing exactly  
           the product but not picked it, the      one operation.
           troubleshooter could bring up the 
           program and look at the transition      List, Function Block and Ladder         programming will see similarities 
           condition between the “move             programs can easily be converted        with Instruction List programming. 
           to product” box and the “pick           into each other, allowing a piece       This language consists of many lines 
           product” box to see what is holding     of code to be displayed in the          of code, with each line representing 
           up the process.                         way most comfortable to the             exactly one operation. Thus, it is 
                                                   user. Structured Text can also be       very step-by-step in layout and 
           On the downside, this style of          converted into any of these three       format, which makes the entry of 
           programming is not suitable for         languages, but SFC stands alone.        a series of simple mathematical 
           every application, as the structure     It cannot be converted. Therefore,      functions easy. In addition, if the 
           that is forced on a program could       you may want to consider this           programmer uses only the IEC-
           add unneeded complexity. A large        language only for end users who         defined instructions, a program 
           amount of time must be spent up         are comfortable with the language       written in this language can be 
           front preparing and planning before     and are unlikely to display it in a     moved easily between hardware 
           any programming is attempted            different format, or for applications   platforms. These advantages make 
           or else the functions charts could      where the hardware has the speed        this language very popular in 
           become unwieldy and difficult           and memory necessary to store and       Europe, a fact that is surprising 
           to follow. The overhead required        execute an SFC program.                 to many U. S. programmers who 
           for this type of program causes it                                              prefer the ease of maintenance 
           to execute slower than the other        Instruction List                        in the graphical languages, and 
           languages. A final consideration        Anyone who has experience               place a lower premium in the 
           is the inability to convert to          programming microprocessors or          transferability of programs.
           other languages. Instruction            experience with Assembler language