Introduction to Industrial Automation

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Chapter 7

Basic Programming 

Principles of PLCs

7.1   Introduction to Programming of PLCs

Since a PLC is basically a digital device that monitors, processes, and generates data from an 

industrial process, it requires a software program with instructions to perform the desired func-

tions. This program has two separate parts with two different objectives. The first part is the oper-

ating system that has been described in Chapter 6, which is responsible for all internal functions 

of the PLC, and is developed by the manufacturer of the PLC and cannot be modified in any way 

by the user. The second part of the PLC’s program is related to the logical and other functions 

that the user desires to execute and needs to be written in a language that the PLC understands. 

This second type of programming will be the focus of Chapter 7 and will include the existing pro-

gramming languages, basic issues on the standardization of languages and programming norms, 

individual language instructions, and corresponding applications.

The programming of PLCs has evolved through various stages in parallel to the technological 

(hardware) evolution of these devices. Initially, PLC programming included only digital input and 

output instructions that performed only logical functions corresponding to the case of conven-

tional electromechanical relays. The corresponding language, known as relay ladder logic (RLL), 

was a mixed form language that combined graphic symbols and short-syntax alphanumeric strings. 

The name “ladder” was derived from the fact that a vertical line expressing the “beginning of the 

logic” (or, metaphorically, the high voltage potential) is placed on the left, and the line with the 

“end of the logic” (or, metaphorically, the low voltage potential) is placed on the right. Between 

these two vertical lines, simple logical elements (or combinations of them) form a branch, and 

are called “logical contacts” or “relay logic” as a whole, as shown in Figure 7.1. Between the two 

vertical lines, there is a number of such branches in the form of a ladder. Subsequently, the evolu-

tion in the field of PLCs was massive. Within a few years, PLCs included functions and opera-

tions of timers and counters by introducing corresponding instructions in the ladder language. 

They also introduced the ability to carry out four basic mathematical operations, while nowadays 

PLCs have reached a point that their programs support all mathematical functions, from abso-

lute variable value up to the exponential, logarithmic, and trigonometric functions, for example. 

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  Introduction to Industrial Automation

However, this evolution of timers, counters, and basic arithmetic operations was not enough. The 

world of mechanical automation systems demanded more and more computing power, memory, 

and specific functionalities. It was also looking for the possibility of introducing and using sub-

routines, so that the program had a structure in harmony with the distributed nature of large 

automation systems. With these conditions, PLC manufacturers introduced other programming 

languages in a graphical or text form and at a higher level of programming, while retaining the 

ladder language up to now, mainly because of some key advantages, namely:

 

◾

It is a symbolic or graphical language with a very simple way of representation that closely 

resembles the structure of classic automation circuits, and this is why it is not only familiar 

to all engineers dealing with industrial automation, but to technical electricians as well.

 

◾

It has very short execution times for instructions, as well as short execution time of a pro-

gram branch, so that all branches are considered running almost in parallel mode, just like 

in conventional automation circuits.

 

◾

It provides the ability for online programming (thus allowing the user to make changes to 

the program while the system is running) and the program can be compiled in real time.

At this point it should also be highlighted that ladder language is not suitable for all kinds and 

sizes of industrial applications. If an engineer had to design a large automation system and write 

the required automation program starting from an empty page, then the ladder language would 

not be selected. For this case, the utilization of a flow chart and high-level programming language 

will be needed, such as the sequential function chart (SFC) language and the function block dia-

gram (FBD) that will be described subsequently.

Until about 1990, PLC manufacturers were developing their proprietary programming lan-

guages with complete incompatibility between them. As an example, ladder languages from two 

different manufacturers may have had a similar form, but under no circumstances were they 

compatible. Furthermore, there were cases where two different PLCs of the same company 

were manufactured to use ladder language but with a different notation that did not allow the 

exchange of programs or the direct transfer of a program from one PLC to the other. Under these 

Check for ON

Check for OFF

Start of logic

End of logic

Alphanumeric type

Symbol

Ι = Input

Ο = Output

Logic branch

Energize output if

the IF-THEN rule

is valid 

I1

I2

O1