Introduction to Ladder Diagram (LD)

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The Ladder diagram (LD) has its origin back to the early sixties of the nineteenth century with the birth of the programmable logic controller (PLC). 
The ladder diagram is set to replace the relay logic. Therefore, ladder diagram was designed to closely resemble the wiring diagram documentation for relay logic.



Above is an example of a ladder logic diagram which illustrates all the components of a ladder diagram. The symbols are laid out in networks in a manner similar to a "rung" of a ladder one uses to climb. The above example has three rungs.

A typical ladder diagram is composed of:
  • Power Rails
  • Link elements and states
  • Contacts
  • Coils
  • Function and Function block

Power Rails

Ladder diagram is delimited on the left by a vertical line known as the left power rail, and on the right by a vertical line known as the right power rail. The right power rail may be explicit or implied.

Link elements and states


Link elements may be horizontal or vertical. Link state is synonymous with the term power flow.
The state of the link element shall be either "ON" or "OFF".
The state of the left rail shall be considered ON at all times.
No state is defined for the right rail.
A horizontal link element is indicated by a horizontal line.
A horizontal link element transmits the state of the element on its immediate left to the element on its immediate right.
The vertical link element shall consist of a vertical line intersecting with one or more horizontal link elements on each side.
The state of the vertical link shall represent the inclusive OR of the ON states of the horizontal links on its left side, that is, the state of the vertical link shall be:
- OFF if the states of all the attached horizontal links to its left are OFF
- ON if the state of one or more of the attached horizontal links to its left is ON.
The state of the vertical link shall be copied to all of the attached horizontal links on its right. The state of the vertical link shall not be copied to any of the attached horizontal links on its left

Contacts

A contact is an element which implies a state to the horizontal link on its right side equal to the Boolean AND of the state of the horizontal link at its left side with a corresponding function of an associated boolean input, output or memory variable.
Standard contacts are shown in below table:

Coils

A coil copies the state of the link on its left to the link on its right without modification, and stores an appropriate function of the state or transition of the left link into the associated Boolean variable. 
Standard coil symbols are given in below table:

Functions and function blocks

Functions and function blocks can be used with the ladder diagram. 
Function and function blocks evaluate inputs and affect outputs according to the associated function.

Order of network evaluation 

Within a ladder diagram, rungs are evaluated from top to bottom order as they appear in the ladder diagram.

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