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Standardization in Industrial Control Programming

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IEC 61131-3: a standard programming resource

IEC 61131-3 is the first real endeavor to standardize programming languages for industrial automation. With its worldwide support, it is independent of any single company. IEC 61131-3 is the third part of the IEC 61131 family. This consists of: Part 1: General Overview Part 2 Hardware Part 3 Programming Languages Part 4 User Guidelines Part 5 Communication There are many ways to look at part 3 of this standard. Just to name a few: the result of the Task Force 3, Programming Languages, within IEC TC65 SC65B the result of hard work by 7 international companies adding tens of years of experience in the field of industrial automation approx. 200 pages of text, with 60-something tables, including features tables the specification of the syntax and semantics of a unified suite of programming languages, including the overall software model and a structuring language. Another elegant view is by splitting the standard in two parts (see figure 1): 1. Common Elements 2. Programming Languages

Common Elements
Data Typing
Within the common elements, the data types are defined. Data typing prevents errors in an early stage. It is used to define the type of any parameter used. This avoids for instance dividing a Date by an Integer. Common datatypes are Boolean, Integer, Real and Byte and Word, but also Date, Time_of_Day and String. Based on these, one can define own personal data types, known as derived data types. In this way one can define an analog input channel as data type, and re-use this over an over again.

Variables are only assigned to explicit hardware addresses (e.g. input and outputs) in configurations, resources or programs. In this way a high level of hardware independency is created, supporting the reusability of the software. The scopes of the variables are normally limited to the organization unit in which they are declared, e.g. local. This means that their names can be reused in other parts without any conflict, eliminating another source of errors, e.g. the scratchpad. If the variables should have global scope, they have to be declared as such (VAR_GLOBAL). Parameters can be assigned an initial value at start up and cold restart, in order to have the right setting.

Configuration, Resources and Tasks

To understand these better, let us look at the software model, as defined in the standard (see below).
Configuration Resource Resource Task Task Task FB Function Block Program Program FB FB Program Program FB FB Execution control path Access path

The IEC 1131-3 Standard

Common Elements


Programming Languages

Lets look more in detail to these parts:

At the highest level, the entire software required to solve a particular control problem can be formulated as a Configuration. A configuration is specific to a particular type of control system, including the arrangement of the hardware, i.c. processing resources, memory addresses for I/O channels and system capabilities.

Within a configuration one can define one or more Resources. One can look at a resource as a processing facility that is able to execute IEC programs. Within a resource, one or more Tasks can be defined. Tasks control the execution of a set of programs and/or function blocks. These can either be executed periodically or upon the occurrence of a specified trigger, such as the change of a variable. Programs are built from a number of different software elements written in any of the IEC defined languages. Typically, a program consists of a network of Functions and Function Blocks, which are able to exchange data. Function and Function Blocks are the basic building blocks, containing a datastructure and an algorithm. Lets compare this to a conventional PLC: this contains one resource, running one task, controlling one program, running in a closed loop. IEC 61131-3 adds much to this, making it open to the future. A future that includes multi-processing and event driven programs. And this future is not so far: just look at distributed systems or real-time control systems. IEC 61131-3 is suitable for a broad range of applications, without having to learn additional programming languages.

The interfaces of functions and function blocks are described in the same way:
FUNCTION_BLOCK Example VAR_INPUT: X : BOOL; Y : BOOL; END_VAR VAR_OUTPUT Z : BOOL; END_VAR (* statements of functionblock body *) END_FUNCTION_BLOCK

The declarations above describe the interface to a function block with two Boolean input parameters and one Boolean output parameter.

With the above-mentioned basic building blocks, one can say that a program is a network of Functions and Function Blocks. A program can be written in any of the defined programming languages.

Program Organization Units

Within IEC 61131-3, the Programs, Function Blocks and Functions are called Program Organization Units, POUs.

Sequential Function Chart, SFC

Step 1

Transition 1


IEC has defined standard functions and user defined functions. Standard functions are for instance ADD(ition), ABS (absolute), SQRT, SINus and COSinus. User defined functions, once defined, can be used over and over again.

Step 2

Transition 2


Function Blocks, FBs

Function Blocks are the equivalent to Integrated Circuits, ICs, representing a specialized control function. They contain data as well as the algorithm, so they can keep track of the past (which is one of the differences w.r.t. Functions). They have a well-defined interface and hidden internals, like an IC or black box. In this way they give a clear separation between different levels of programmers, or maintenance people. A temperature control loop, or PID, is an excellent example of a Function Block. Once defined, it can be used over and over again, in the same program, different programs, or even different projects. This makes them highly re-usable. Function Blocks can be written in any of the IEC languages, and in most cases even in C. It this way they can be defined by the user. Derived Function Blocks are based on the standard defined FBs, but also completely new, customized FBs are possible within the standard: it just provides the framework.

Step 3

SFC describes graphically the sequential behavior of a control program. It is derived from Petri Nets and IEC 848 Grafcet, with the changes necessary to convert the representation from a documentation standard to a set of execution control elements. SFC structures the internal organization of a program, and helps to decompose a control problem into manageable parts, while maintaining the overview. SFC consists of Steps, linked with Action Blocks and Transitions. Each step represents a particular state of the systems being controlled. A transition is associated with a condition, which, when true, causes the step before the transition to be deactivated, and the next step to be activated. Steps are linked to action blocks, performing a certain control action. Each element can be programmed in any of the IEC languages, including SFC itself. One can use alternative sequences and even parallel sequences, such as commonly required in batch

applications. For instance, one sequence is used for the primary process, and the second for monitoring the overall operating constraints. Because of its general structure, SFC provides also a communication tool, combining people of different backgrounds, departments or countries.

Programming Languages
Within the standard four programming languages are defined. This means that their syntax and semantics have been defined, leaving no room for dialects. Once you have learned them, you can use a wide variety of systems based on this standard. The languages consist of two textual and two graphical versions: Textual: Instruction List, IL Structured Text, ST Graphical: Ladder Diagram, LD Function Block Diagram, FBD
Instruction List (IL)

Structured Text is a very powerful high-level language with its roots in Ada, Pascal and C. It contains all the essential elements of a modern programming language, including selection branches (IF-THEN-ELSE and CASE OF) and iteration loops (FOR, WHILE and REPEAT). These elements can also be nested. It can be used excellently for the definition of complex function blocks, which can be used within any of the other languages. Example in ST:
I:=25; WHILE J<5 DO Z:= F(I+J); END_WHILE IF B_1 THEN %QW100:= INT_TO_BCD(Display) ENDIF CASE TW OF 1,5: TEMP := TEMP_1; 2: TEMP := 40; 4: TEMP := FTMP(TEMP_2); ELSE TEMP := 0; B_ERROR :=1; END_CASE

Structured Text (ST)

Top-down vs. bottom-up

Top Down

FunctionBlock Diagram (FBD)


Ladder Diagram(LD )

Common Elements


Programming Languages
Bottom Up

In the above figure, all four languages describe the same simple program part. The choice of programming language is dependent on: the programmers background the problem at hand the level of describing the problem the structure of the control system the interface to other people / departments All four languages are interlinked: they provide a common suite, with a link to existing experience. In this way they also provide a communication tool, combining people of different backgrounds. Ladder Diagram has its roots in the USA. It is based on the graphical presentation of Relay Ladder Logic. Instruction List is its European counterpart. As textual language, it resembles assembler. Function Block Diagram is very common to the process industry. It expresses the behavior of functions, function blocks and programs as a set of interconnected graphical blocks, like in electronic circuit diagrams. It looks at a system in terms of the flow of signals between processing elements.

Also, the standard allows two ways of developing your program: top down and bottom up. Either you specify your whole application and divide it into sub parts, declare your variables, and so on. Or you start programming your application at the bottom, for instance via derived functions and function blocks. Whichever you choose, the development environment will help you through the whole process.

The overall requirements of IEC 61131-3 are not easy to fulfill. For that reason, the standard allows partial implementations in various aspects. This covers the number of supported languages, functions and function blocks. This leaves freedom at the supplier side, but a user should be well aware of it during his selection process. Also, a new release can have a dramatically higher level of implementation. Many current IEC programming environments offer everything you expect form modern environments: mouse operation, pull down menus, graphical programming screens, support for multiple windows, built in hypertext functions, verification during design.

Please be aware that this is not specified within the standard itself: it is one of the parts where suppliers can differentiate.

The technical implications of the IEC 61131-3 standard are high, leaving enough room for growth and differentiation. This makes this standard suitable to evolve well into the next century. IEC 61131-3 will have a great impact on the whole industrial control industry. It certainly will not restrict itself to the conventional PLC market. Nowadays, one sees it adopted in the motion control market, distributed systems and softlogic / PC based control systems, including SCADA packages. And the areas are still growing. Having a standard over such a broad application area, brings numerous benefits for users / programmers. The benefits for adopting this standard are various, depending on the application areas. Just to name a few for the mindsetting: reduced waste of human resources, in training, debugging, maintenance and consultancy creating a focus to problem solving via a high level of software reusability reduced misunderstanding and errors programming techniques usable in a broad environment: general industrial control combining different components from different programs, projects, locations, companies and/or countries