Introduction to Industrial Automation

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Chapter 6

Basic Operating 

Principles of PLCs

6.1   Introduction to PLCs

Programmable logic controllers (PLCs) initially appeared in the industry during the 1960s and 

had a completely different form than those implemented today, since they were built out of logical 

components that only replaced the operation of the auxiliary relays. Even primitive PLCs were 

very reliable for a long time when compared to classical electromechanical relays, demanding 

much less space in the overall automation. Subsequently, their evolution passed through multiple 

stages, while the most important ones were inclusion of digital components for timing, synchroni-

zation and counting, and use of microprocessors. The microprocessors had already started to be a 

fundamental part of the personal computers (PCs). Nowadays, PLCs can be either simple or com-

plex, come in a variety of sizes, and are equipped with a wide variety of extensions and interfaces 

that fulfill all the type of needs found at factory level, including the need to communicate with 

other devices and computers. It should also be mentioned that there are multiple programming 

languages for tuning the behavior of PLCs so that they can match the different programming 

skills of the end users. All these issues will be analytically covered in this chapter.

Every PLC, independently of its type and size, can be characterized as a digital device with a 

microcontroller and a programmable memory that can store and execute user instructions express-

ing Boolean logic, sequential logic, timing, counting, and mathematical processing, in order to 

control the operation of a complex machine or an overall industrial process through the utilization 

of digital and/or analog inputs and outputs (I/Os).

PLCs have the basic structure of a personal computer, with two significant differences. The 

first is related to the available hardware for the I/Os of the PLC, while the second is related to the 

microcontroller operation manner and its interaction with the rest of the electronic components 

of the PLC. A PC’s main objective is to communicate with the end user for the successful execu-

tion of various arithmetic and algebraic calculations, graphical editing and representation, com-

munication tasks, etc. Thus, in these cases, the end user provides the corresponding commands 

through a proper interface, such as a keyboard or mouse, while the outcome of these actions is 

either displayed on the monitor of the PC or printed. The PLC’s main task is to communicate with 

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  Introduction to Industrial Automation

the industrial environment and, more specifically, with either the input devices that are provid-

ing the sensorial measurements or with the actuators that interact with the process. For example, 

typical input devices are sensors, buttons, and switches, while typical output devices are power 

relays, valve coils, and indicating lights. Since these devices are operating at a different power level 

than the one that PLCs are usually operating at, it is necessary for PLCs to have the proper I/O 

hardware to adjust and adapt the power levels accordingly. In Figure 6.1 the basic parts of a PLC 

are presented: the CPU, the I/O modules, the RAM, and the power supply.

A programming device is a peripheral device that is used only for the programming stage of 

PLCs, and is not necessary for its operation, therefore it is removed afterwards. In some specific 

types of small PLCs, the programming device is embedded in its main body. In general, the 

programming device may be either a specially manufactured digital device (usually portable and 

specific to a PLC) or a classical PC equipped with the software that the PLC’s manufacturer is 

developing for PLC programming. Before proceeding in analyzing the operation and the interac-

tion of the PLC components, it is very important to define which hardware devices and tasks of 

the classical industrial automation the PLC is replacing. As has been mentioned in Chapters 2 

through 4, a classical industrial automation system needs the following:

  1.  Auxiliary devices (such as time relays, hour meters, counters, auxiliary relays, etc.) that constitute 

the basic electrical components of the automation and are mounted in an electrical enclosure.

  2. Design of the overall automation electrical circuit that has to achieve the desired operation 

of the controlled process.

  3. Wiring that is needed inside the electrical enclosure for connecting the auxiliary devices 

between them and also with the I/O devices existing in the enclosure.

  4. Wiring that is needed for connecting the electrical enclosure with the I/O devices as a 

whole, existing far from the enclosure. Input devices may be photoelectric switches, proxim-

ity switches, selector switches, etc., while output devices may be motors, electrovalve coils, 

other actuators, indication lights, etc.

As indicated in Figure 6.2, the first three cases are now embedded in the operation and pro-

gramming of the PLC, while the last case remains the same, as in classical industrial automations. 

Power

supply

CPU

Memory

Digital

output

module

Digital

input

module

5 V

5 V

220 V

220 V

Programming

device

CPU = central processing unit

Industrial environment

complex machine

or process

PLC