Programmable Logic Controller(PLC)

What is a PLC?

"Programmable Logic Controller" is the full form of PLC. A PLC is a type of computing device that is specifically made to function dependably in challenging workplaces, like extremely high or low temperatures and damp, dry, or dirty circumstances. PLCs are utilized for automating processes in industries such as the production line of a factory, the processing of ore, or the purification of sewage.

Many functions seen on a house PC are also found on PLCs. They all have retention, inputs and sorties (I/O), a power source, a CPU (central processing unit), and operating software—albeit a distinct computer software.

The two main distinctions are that a PLC is far more adapted to harsh workplace environments and can carry out discontinuous and ongoing operations that a PC can't. A PLC is a type of computer that has been "ruggedized" and is used to control mechanical functions in a factory.

PLCs are essential to the industrial industry because they are a component of more extensive SCADA systems. The procedure's operational specifications may configure a PLC. Because of the shift in manufacturing, programming will be necessary in the manufacturing industry. PLC-based systems of control have been developed to address this issue. Before examining the many uses of PLCs, let's first go over the fundamentals of PLCs.


Dick Morley created PLCs back in 1964. PLC has since changed the production and manufacturing industries. PLCs may perform a broad range of tasks, including analyzing different signals such as analogue timing, counting, computation, and contrasting.

The primary benefit of a PLC over a "hardwired" management device is that once controlled, it can be changed at minimal cost—just the programmer's effort. In essence, you have to remove all of the cables in a programmed system of control that starts over, which is more costly and requires more.


Assume that there's a light switch linked to it. The light generally has two modes of operation: ON and OFF. You have been asked to complete an activity where the light must glow for thirty seconds once you flip the switch ON. We're trapped with this hardwired configuration. Our system must be entirely rewired to incorporate a timer relay. This is quite a bother for such a minor adjustment.

That's when PL controllers come into play; they ensure changes without additional components or wires. Instead, all that needs to be done is a minor code modification to instruct the PLC to activate the illumination only thirty seconds after turning on the switch. Therefore, it is simple to include numerous inputs and outputs when utilizing a PLC.

Programmable Logic Controller(PLC)

This is but a basic illustration of the more extensive and more complex procedures that a PLC is capable of managing. According to the purpose and user requirements, a Programmable Logic Controller (PLC) can be modified.

Working of PLC:

An easy way to understand how a programmable logic controller operates is to think of it as a process of checking or scan cycle.

Programmable Logic Controller(PLC)

The following procedures are part of a PLC scan process:

  • The OS initiates time surveillance and rotating.
  • As it begins to read the information from the input component module, the CPU verifies that each input is operational.
  • The consumer or software program created using relay-ladder logic or another PLC programming language is started to run by the CPU.
  • Subsequently, the central processing unit (CPU) handles all interior analysis and messaging.
  • It puts the information into the result of the module to ensure every output is modified based on the program's outcomes.
  • Insofar as the Programmable Logic Controller is in running mode, this operation keeps going.

Multiple types of PLCs:

Fixed/compact and modular PLC are the two primary forms of PLC.

1. Small PLC:

There could be numerous parts in a single scenario. It has a set quantity of outside I/O adapters and modules for I/O. Consequently, it needs more capacity for growing the components. The producer would determine each input as well as output.

2. PLC Modular:

This kind of PLC is called a Programmable Logic Controller (PLC) because it allows for numerous expansions via "modules." More I/O components are possible. Since one part functions independently of the others, it is more straightforward.

Physical Structure of PLC:

Programmable Logic Controller(PLC)

The structure of a PLC is almost similar to a computer’s architecture.

Depending on the type of production and industry, Programmable Logic Controllers continuously monitor the input values from various input sensing devices (such as accelerometers, weight scales, hardwired signals, etc.) and generate appropriate output. A typical PLC block diagram is composed of the following five parts:

  • Chassis or rack
  • Module for Power Supply
  • CPU, or central processing unit
  • Module of Input and Output
  • Module for Communication Interface
  • Clasp or Structure

The PLC rack, the chassis, is the most crucial component that is the framework of all PLC systems. PLCs come in a variety of sizes and shapes. Greater complexity in control systems necessitates larger PLC racks.


An I/O pin arrangement is fixed in small-sized PLCs. Thus, they have chosen a rack PLC of the modular kind, which

Module for Power Supply:

The entire PLC system receives the necessary electricity from this module. The CPU and I/O module need DC power, which is converted from the available AC power. Generally, a 24V DC supply powers a PLC. PLCs only sometimes use separated power supplies.

RAM and the CPU module:

RAM, ROM, and a central processor are all found in the CPU module. An operating system, drivers, and application software are all stored in ROM memory. Programs and data are stored in RAM. CPU, which has an octal or hexagonal microprocessor, is the brains of the PLC.

It is a microprocessor-based CPU that replaces counters, relays, and timers. A PLC can integrate two different processor types, such as a word processor or a single-bit processor. For logic operations, a single-bit processor is utilized. On the other hand, word processors are used for recording, managing, and processing text and numerical data.

After processing the data it has read from the sensors, the CPU issues commands to the controlling devices. Voltage signals are needed for a DC power source, as was discussed previously. To link connections required by other units, the CPU also has additional electrical components.

Module for Input and Output:

Programmable Logic Controller(PLC)
  • Ever consider how to feel physical attributes such as flow, pressure, temperature, etc.? with a PLC? Naturally, PLCs include a particular module called an input & output module for integrating inputs and outputs.
  • The PLC's input module performs four primary tasks.
  • Process devices send signals to the input module interface, which receives them at 220 V AC.
  • Transforms the input signal into a 5 V DC signal that the PLC may use.
  • The purpose of an isolated block is to keep a PLC from fluctuating.
  • The signal is then routed to the PLC, the output end.
  • The input module is divided into two primary sections: the logical part and the power section. There is electrical isolation between the two portions. The push button is closed at first. Resistors R1 and R2 supply the bridge circuit with a 220 V AC supply.

The AC signal is converted into DC using a bridge rectifier (such as a diode bridge rectifier), and the LED is supplied with a low voltage via a Zener diode. The phototransistor functions in the conduction area when exposed to light from an LED. Lastly, the processor receives a 5V DC supply.

PLC output modules operate similarly to input modules, albeit in reverse. It serves as a processor and output load interface. Thus, the logic session would be the first section, followed by the power section. The following figure illustrates how the output module functions.

Application of PLCs:

  • PLCs can be used for many different purposes, such as:
  • Plants for Process Automation (e.g. mining, oil & gas)
  • Glass Sector
  • Paper Sector
  • Manufacturing of Cement
  • In thermal power plants' boilers

Programming PLCs:

It's crucial to develop and apply concepts for your specific use case while utilizing a PLC. We must first learn more about the intricacies of PLC programming before we can accomplish this.

A PLC program is made up of a series of written or graphical instructions that reflect the logic controlling the process the PLC is operating on. PLC programming languages are categorized into two primary categories, each with numerous subcategories.

Textual Language Guide to Instructions:

  • Organized writing
  • Visual Format
  • Ladder Logic, or Ladder Diagrams (LD)
  • Diagram of a Function Block (FBD)
  • Chart of Sequential Functions (SFC)
  • While many of these PLC programming languages can be used to program a PLC, textual languages (like structured text programming) are usually less preferable to graphical languages (like ladder logic).