Programmable Logic Controller (PLC): Advantage, PLC Architecture and more

Programmable Logic Controller (PLC): Advantage, PLC Architecture and more

Programmable Logic Controller (PLC)

The programmable logic controller is a dedicated computer used to control machines and processes in an industrial area. These are one of the most advanced and simplest kinds of control systems that are replacing hard-wired logic relays at a very large scale. It shares common terms with typical computer systems like CPU, memory, software, and communications.  Unlike a normal PC, the PLC is designed to survive in a rugged industrial environment and design to be very flexible in how to interact with inputs and outputs in the real world.

The major components of PLC work can be divided into three core areas.

The power supply and racks

The central processing unit (CPU)

The input/output (I/O) section

Shapes and size of PLC varies based on requirement and number of input and outputs. They can be so small as to fit in your pocket whereas other involves controls systems require large PLC racks.  Smaller PLCs are typically designed with a fixed number of I/O points.  Whereas modular rack-based systems can accept many different types of I/O modules that simply slide into the rack and plugin.

Programmable Logic Controller (PLC): Advantage, PLC Architecture and more

Fig. PLC setup


PLCs are being widely used these days

These are user-friendly and easy to operate.

They eliminate the need for hard-wired relay logic.

PLCs are fast.

These are suitable for automation in industries.

Its input/output modules can be extended depending upon the requirements

PLC Architecture:

Programmable Logic Controller (PLC): Advantage, PLC Architecture and more

A basic PLC system consists of the following sections:

Input/ Output Section: The input module consists of devices like switches, sensors, and many other real-world input sources. The input from the sources is linked to the PLC through the input connector rails. The output module can be a motor or any kind of load whose functioning is controlled by changing the input signals.

CPU or Central Processing Unit: Is the brain of the PLC. It can be an advanced microprocessor. It carries out all the processing based on the input data in order to control the output signals based on the control program.


Programming Device: It is the platform where the program or the control logic is written based on user requirements.

Power Supply and The Rack: It generally used to power input and output devices.

The rack is the component that contains everything together.  Depending on the needs of the control system it can be of different sizes to hold more modules.  The rack contains a backplane at the rear which allows the cards to interconnect with the CPU.  The power supply plugs into the rack and supplies a regulated DC power to other modules that are plugged into the rack.  Generally, power supplies work by using 120 VAC or 24 VDC sources.

Programmable Logic Controller (PLC): Advantage, PLC Architecture and more

Fig. Rack

Memory: The memory is divided into two parts- The first is data memory and the second is program memory. The control logic is stored in the user memory or the program memory from where CPU fetches the program instructions. The input and output signals and timer and counter signals are stored in the input and output external image memory respectively.

Working of a PLC:

The input module converts the real-time analog signals to suitable digital signals and these signals are applied to the PLC through the connector rails.

These input signals are stored in the PLC external image memory locations.

The program instructions are written into the programming device through symbols or through mnemonics and is stored in the user memory.

The CPU fetches these instructions from the user memory and executes them by manipulating, computing, processing them to control the output devices.

The execution results are stored in the external image memory which controls the output drives.

The CPU also performs internal programming functioning like set and reset the timer, checking the user memory.

Program a PLC

The basic functioning of the PLC depends on the control logic or the programming technique used. Programming can be complete using flowcharts or using ladder logic or using simple statement logics or mnemonics.

 let us understand how we can actually write a program in PLC.

Prepare a flowchart. A flowchart is the symbolic representation of the instructions. It is the most basic and simplest form of writing control logic which involves only logical decisions.

Write the Boolean expression for each logic. Boolean algebra usually involves logical operations like AND, OR, NOT, NAND and NOR. The different symbols are:

(+)   OR operator
(. )     AND operator
(! )    NOT operator.

Write the ladder logic program. It is the most essential part of PLC programming. few symbols and terminologies of the ladder logic program discussed.

Rung: One step in the ladder is called a rung. In simpler words, the basic statement or one control logic is called a Rung.
Y- Normal Output signals
M – Motor symbol
T – Timer
C – Counter

Symbols used:

Programmable Logic Controller (PLC): Advantage, PLC Architecture and more

Basic Logic Functions using Ladder Logic

Writing Mnemonics: Mnemonics are instructions written in symbolic form. They are also known as Opcode and are used in handheld programming devices.

Different Symbols are as given below:

Ldi – Load Inverse
Ld- Load
AND- And the logic
OR- Or logic
ANI – NAND logic
ORI- NOR logic
Out – Output

Dewakar Pandey

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