Introduction to Industrial Automation

7.2   The IEC 61131 Standard

The standards that are generally created by international associations and committees play an 

important role in the development of all industrial technologies. The standards generally help 

in achieving compatibility, transparency, and interoperability across different products; increase 

users’ confidence in their products; and help in developing tools and methodologies under a com-

mon umbrella of technological specifications. Thus, the IEC came to address the chaotic situation 

in the incompatibility of programming languages for PLCs with the establishment of the open 

standard IEC 61131, the first part of which was issued in 1992 and referred to PLCs in general. 

The third part of this standard, the so-called IEC 61131-3, was issued in 2003 (second edition) 

and referred to the programming languages of PLCs. Nowadays, all major PLC manufacturers 

have accepted this standard, and their products are developed accordingly. It must be noted that 

the IEC 61131-3 standard, by definition, is not an additional programming language; instead, it 

supports modern software engineering methods in order to lead PLC operation in improved lan-

guages and programs, creation of usable and interoperable code, and in easy debugging.

The IEC 61131-3 standard incorporates five different approaches to programming control systems, 

and especially for the case of PLCs. Each approach is based on a simple language that targets a par-

ticular type of automation problem or application. These languages can be easily combined to generate 

a mixed code, or used in other types of control systems than PLCs, such as smart sensors, program-

mable drivers, process controllers, SCADA software, and so on. The main objective of the standard is 

to normalize the existing programming languages, rather than to prevent the possible development of 

new languages. Every PLC manufacturer is free to develop either extensions of a language that complies 

with the standard, or a new language from scratch if it is deemed necessary. On the other hand, the IEC 

61131-3 standard has introduced function blocks that can be programmed in a non-standard language, 

such as C

++

. The five embedded languages for the PLCs are the following ones:

  1. Ladder Diagram, LD, or LAD—Graphic Language.

    A program written in a Ladder Diagram language is similar to the classic wired automa-

tion circuits. It uses logical symbols such as:

X

X

 =  Open logical contact if the variable X is in a logic “0” (e.g., digital input, output, 

logic coil, etc.)

X

X

 = Closed logical contact if the variable X is in a logic “0”.

 =  Internal logic coil or digital output at a logic of “0” or “1” if the logic just before in 

the branch (see Figure 7.1) is “0” or “1”, respectively.

    The horizontal branches between the two vertical lines at the beginning of the logic and 

the end of the logic, presented in Figure 7.1, include logical elements such as the above or 

combinations of them which are called “rungs” or “networks” which make up the “ladder”. 

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  Introduction to Industrial Automation

Each branch ends on the right side in a single logic coil or output. Some manufacturers allow 

the parallelism of two or more logical coils or outputs in the same branch. In addition to the 

above simple elements, the LAD language has block symbols 

ff

 for all the basic functions 

that are needed in an automation system, such as timers, counters, arithmetic operations, 

etc.

  2. Function Block Diagram (FBD)—Graphic Language.

    Because there is not a generally established translation of the FBD language, it is possible 

to refer to this with the terms “logic components language”, “logic diagram language”, 

“logic gates language”, and other similar terms. Literally, the function block diagram lan-

guage contains symbol blocks of functional operations, which implement various functions 

from the simple AND function from Boolean algebra up to PID control. Every block has 

the name of the function that it implements and accepts on the left side the inputs and on 

the right side the output, which carries the result of the function as presented in Figure 7.2. 

In fact, both the inputs and the outputs contain an additional set of assistive functions, for 

example the output ENO of the block ADD_I for adding integers, that inform us if the 

result is out of limits.

    The input and output variables of the PLC are connected to the inputs and outputs of 

the functional blocks respectively by interconnection lines or are simply declared before 

them. The output of a block can be connected to the input of another functional block. 

Any interconnection line is directed in the sense that data are transferred from left to right, 

while both ends of the interconnection line should be of the same type. This means that the 

output of the logical block AND in Figure 7.2, which is a discrete variable (0 or 1), cannot 

be connected to the input 1 of the block ADD_I, which should be an integer variable.

  3. Instruction List (IL)—Text-Based Language.

    The instruction list is a low-level language that looks like the assembly language, but it 

is in a higher level than the LAD and FBD, in the case that the classification criterion is 

the translation of a program from one language to another. Due to the fact that the pro-

grams from other languages can always be translated into the corresponding IL language 

(the opposite is not true), it is considered as the main PLC language. IL is also known as 

the statement list (STL) or Boolean language. Because the most basic IL commands are 

just the basic functions of Boolean algebra (such as AND, OR, AND NOT, OR NOT, etc.) 

abbreviated, the last name will be adopted subsequently as the more descriptive one.

    An instruction in the IL language consists of two parts: the operation of the instruction 

and the name of the operand for which the function will be implemented, as shown in Figure 

7.3. The name of the utilized variable is directly related to the addressing system adopted by 

the PLC manufacturer referred to in Section 6.8.3. At the application level, both parts of the 

instruction are abbreviated as far as possible, as presented in Figure 7.3. The programming 

AND

X

Y

Z

AND

X

Y

Z

ADD_ I

IN1

IN2

EN

OUT

ENO

Result of

addition

Enable

output

EN = enable input

AND

X

Y

Z

ADD_ I

IN1

IN2

EN

OUT

ENO