Ladder Diagram (LD)

Ladder Diagram (Ladder Logic)

A Ladder Diagram (LD), also known as Ladder Logic, is a graphical programming language used in PLC (Programmable Logic Controller) systems. It was originally developed from relay logic, which used physical devices like switches and mechanical relays to control industrial processes.

In a ladder diagram, the logic is drawn like a ladder — the left rail represents the power or logic “true” line, and the right rail represents the logic “false” or neutral line.

Each rung represents a control circuit, where one side connects the signal (input bit) and the other connects the command or output bit.

Ladder logic is simple to read and widely used in industrial automation, machine control, and process monitoring. It allows engineers to design control systems using familiar relay-based symbols while programming complex automation sequences efficiently.

Instruction	Symbol	Description
Normally Open	[ ]	True when the input signal is ON
Normally Closed	[/]	True when the input signal is OFF
Coil	( )	Energizes the output when logic is true
Timer ON Delay (TON)	TON	When timer completes, the output energizes
Timer OFF Delay (TOF)	TOF	Keeps output energized until timer completes; after completion, the output turns OFF
Counter (CTU / CTD)	CTU / CTD	Counts events — Up Counter or Down Counter
Compare Instruction	> , < , =	Compares values for control decisions
Move Instruction	MOV	Transfers data from one register to another
Math Instructions	ADD, SUB, MUL, DIV	Performs arithmetic operations (Add, Subtract, Multiply, Divide)