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What is Programmable Logic Controller?

A Programmable Logic Controller is a programmable control system with functions to the control logic, timing, sequencing,   counting capabilities, and arithmetic data manipulation.

PLC Operation:

  • Continuous ladder diagram are scanned
  • Consists of three main steps
  • Checks input status
  • Executes the program
  • Updates output status

Programming basics:

  • NO contact
  • NO output coil
  • NC contact
  • NC output coil
  • AND gate
  • OR gate

Building of PLC program:

  • It will start at relay logic and convert to ladder logic at the end
  • Need to remove or replace some components
  • Overload device is removed
  • All components in relay logic are not addressed in ladder logic
  • Motor relay is not used in ladder logic as in relay logic
  • Jog function is added
  • Usually jog function is added for easy troubleshooting purpose
  • Now there are two ways to run a motor – press start, or press and hold jog button
  • Status indicators should be added
  • Green color is for output on
  • Red color is for output off
  • Now relay logic is converted to ladder logic
  • Double pole pushbutton is switched to single pole by jog
  • PLC checks state of motor output instead of motor relays


  • Approximately 80% of wires are reduced
  • Detection of error is fast and easy
  • Compact
  • Moving parts are less
  • Less maintenance cost


  • Costly setup
  • Skilled operators are required
  • Programming experience is required

Large amount of connecting wires, difficulty in replacements and finding errors has been reduced with the invention of PLC and nowadays every company is grasping PLC for their advantages.

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