A digitally operating electronic apparatus which uses a programming memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through digital or analog modules, various types of machines or process.In this session we are gonna discuss about basics of programming logic devices (PLC)

 Historical Background of programming logic devices (PLC)

Developed to replace relays in the late 1960s, their primary goal was to eliminate the high costs associated with inflexible, relay-controlled systems

  • The controller had to be designed in modular form, so that sub-assemblies could be removed easily for replacement or repair.
  • The control system needed the capability to pass data collection to a central system.
  • The system had to be reusable.

The method used to program the controller had to be simple, so that it could be easily understood by plant personnel

Major Components of a Common PLC


Provides the voltage needed to run the primary PLC components


Provides signal conversion and isolation between the internal logic-level signals inside the PLC and the field’s high level signal


Provides intelligence to command and govern the activities of the entire PLC systems.


Used to enter the desired program that will determine the sequence of operation and control of process equipment  or driven machine.

I/O Module

  • The I/O interface section of a PLC connects it to external field devices.
  • 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 converts signals from discrete or analog input devices to logic levels acceptable to PLC’s processor.

Output modules converts signal from the processor to levels capable of driving the connected discrete or analog output devices


Analog Input

An analog input is an input signal that has a continuous signal. Typical inputs may vary from 0 to 20mA, 4 to 20mA or 0 to10V. Below, a level transmitter monitors the level of  liquid in the tank. Depending on the level Tx, the signal to the PLC can either increase or decrease as the level increases  or decreases.


Digital Output

A discrete output is either  in an ON or OFF condition. Solenoids,  contractors coils, lamps are example of devices connected to the  Discrete or digital outputs. Below, the lamp can be turned ON or OFF by the PLC output it is connected to.


Analog Output

An analog output is an output signal that has a continuous signal. Typical outputs may vary from 0 to 20mA, 4 to 20mA or 0 to10V.


The processor module contains the PLC’s microprocessor, its supporting circuitry, and its memory system.

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 devices. Field sensors: switches, flow, level, pressure, temp. Transmitters, etc. Field output devices: motors, valves, solenoids, lamps, or audible devices.

The memory system in the processor module has two parts: a system memory and an application memory.

Areas of Application

  • Manufacturing / Machining
  • Food / Beverage
  • Metals
  • Power
  • Mining
  • Petrochemical / Chemical


to know more about  BASIC CONTROL PRINCIPLES

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