what is parts of a ladder diagram in plc

PLC’s (Programmable Logic Controllers), where first developed in 1968 at General Motors.

At the time, General Motors could spend weeks replacing relay based control systems when making production modifications to cars or when they had to manufacture a different model to demand

General Motors requested a system that could be programmed and maintained by plant engineers which prevent such delays in re-wiring.

The first PLC’s where installed in 1969 and quickly became a success reducing labour costs and where easier for trouble shooting

A Programmable Logic Controller (PLC) is an computerized industrial control system that continuously monitors the state of a production line.

It uses a programmable memory to store instructions and execute simple functions that include On/Off control, timing, counting, sequencing, arithmetic and data handling

Almost any production line, machine function, or process can be greatly enhanced using this type of control system.

One of the main benefits of using PLC’s is their ability to change and replicate the operation/process while collecting and communicating vital information.

  1. Programmable Logic Controllers bring many advantages which include
  2. Eliminated the need for rewiring and adding additional hardware.
  3. Increased the functionality of industrial control
  4. Reduces the cabinet space that housed the control logic.

Examples of input devices in a PLC system

  1. Sensing Devices
  2. Switches/Pushbuttons
  3. Proximity Sensors
  4. Limit Switches
  5. Pressure Switches

Examples of output devices in a PLC system

  1. Valves
  2. Solenoids
  3. Motors
  4. Actuators
  5. Pumps

Table of Contents

Parts of a ladder diagram :

  • In PLC’s, logic circuits can be represented by electrical diagrams known as ladder logic
  • Ladder Logic works on the principle of different switching combinations with the aim of activating a relay coil, when certain condition have been met

There is two types of electrical switches in ladder logic

  •     Normally Open Normally Closed

N.O = Normally Open     N.C = Normally Closed

  • On a ladder diagram, the switches are represented by the following symbols

Normally Open




Normally Closed


N.O = Normally Open     N.C = Normally Closed

  • On a ladder diagram, the switches are represented by the following symbols

Normally Open


Normally Closed



  • In ladder logic a switch which is allowing current to flow is highlighted in green

N.O= Normally Open




N.C = Normally Closed



  • The Physical output of Programmable Logic Controller’s are relay coils

Not Energized





However, not all the outputs on ladder logic are relay coils

  • Other ladder logic outputs are the Processors functions such as Timers, and Counters




  • All programs require an end statement



  • Here is a basic example of a ladder diagram to drive a motor using N.O. and N.C. switches





  • To activate the motor, the N.O start button needs closed to provide a electrical path to the motor via the N.C Stop button





  • Once pressed , the electrical supply has a route to successfully run the motor will be highlighted in Green

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