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Ladder Diagram - History:

Ladder diagrams were invented before the invention of PLCs. Ladder diagrams are nothing but a graphical diagrams based on the circuit diagrams of relay-based logic hardware.

Ladder Diagram

The above illustration shows a Ladder diagram. Ladder logic can be thought of as a rule-based language, rather than a procedural language. A "rung" in the ladder represents a rule. Normally in a programming language like C or BASIC, the program is executed in order - the command or line of code on top is executed and then the command on the bottom is executed until the end. This is not so in ladder logic. A Ladder logic is executed in parallel - from left to right.

All the input coils will be on the left side, the output coils and commands on the right hand side of the Ladder.

Introduction to PLC Ladder Logic - Video:

PLC Ladder - Presentation: