PLC Overview

In a customary industrial control system, the control devices like PLC in the field are wired directly to connect each other on how the entire system is to be operated.

The main control is in the human hands which represents a lot of drawbacks such as more wired connections are required which may again prompt to a large number of mechanical faults and difficulties in troubleshooting the errors.

Due to these drawbacks, the PLC-based automation systems were introduced which Focused on controlling the various process control systems with the help of software and Hardware Device.

here we compare the old control system.

Hardware Cost Low Equal Low Can Be High in small quantities Depends on a number of I/O
Troubleshooting and maintainability Low Low Yes Poor good


No No Yes Yes Yes


Arithmetic capability No No Yes Yes Yes
Programming Cost Logic developed By Wiring Logic developed By Wiring High Low Low
Reusable No No Yes Yes Yes
Space Required Largest Large Small Compact Small
Operating Speed Slow Faster Fast Fast Fast

PLC consists of a microprocessor which is programmed using the different type of programming languages.

Like Relay Ladder, Functional Block diagram, structured text, sequential flow chart.

The program is written on a Programming software and is downloaded to the PLC via cable. These loaded programs are stored in non – volatile memory of the PLC.

During the transition of relay control panels to PLC, the hard-wired relay logic was replaced for the program fed by the user.

PLC heart: central processing unit (CPU)

  • This unit is the most part of a unit in the making of a PLC, which is the “brain” of a PLC.
  • In this unit develop a chip of microprocessor – integrated circuit chips which controls the whole operation of the PLC control system.
  • Microprocessors contain an arithmetic unit, control unit and a number of memory units, known as registrars.
  • The main function of the microprocessor is to analyze data coming from field sensors through input modules, make decisions based on the user’s defined control program and return signal back through output modules to the field output devices.
  • Communicating with a programming terminal. The CPU transfers program and data between itself and the programming terminal.
  • Memory Unit Memory is the part or utility that stores information, programs, and data in a PLC. Above figure shows the memory unit in the internal design of PLCs.

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This unit contains two types of PLC Memory:

1)RAM (Random Access Memory)

  • RAM is the memory type of read/write and easy to program and repaired. All users program are stored in this memory.
  • Read indicates that the information stored in the memory can be retrieved or read, while write indicates that the user can program or write information into the memory.
  • The data in the RAM would normally be lost if the power source is removed. This problem is solved by backing up the RAM with the battery

2)ROM (Read Only Memory)ROM is read-only memory types.

  • Read Only indicates that the information stored in memory can be read-only and cannot be changed.
  • Information in ROM is placed there by the manufacturer for the internal use and operation of the PLC.
  • The system program is stored in this memory. This program will not be lost when the power is disconnected.
  • Special equipment is used to delete the program from this memory The memory capacities of PLCs vary.

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PLC display and indicators unit display:

  • Display and Indicators Unit Display and the indicator unit refers to the internal relay PLC status display. This can be seen in the Console programming if the user using the mnemonic code and programming computer screen if the user using software programming methods.

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 PLC Hardware design:

  • Referring to the above figure, the input and output units are the units available in the internal design of PLC.

PLC Input/Output Unit :

  • The Input/Output units are the interfaces between the internal PLC systems and the external processes/ field devices to be monitored and controlled.
  • Input Unit is the unit which input devices (switches, sensors) are connected to it. While the Output unit is a unit which output devices (Lights, motors) are connected to it.
  • The main purpose of the I/O interface is to condition the various signals received from or sent to the external input and output devices.
  • Input modules convert signals from discrete or analog input devices to logic levels acceptable to PLC’s processor.
  • Output modules convert a signal from the processor to levels capable of driving the connected discrete or analog output devices.
  • Small PLC units would have around 40 I/O connections with larger ones having more than 128 with either local or remote connections and extensive upgrade capabilities.

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