Ladder logic is a graphical language for controlling machines with a programmable logic controller (PLC). It uses relay-style symbols so you can follow a control decision from input conditions to an output.

In PLC ladder logic, the controller repeatedly reads field inputs, evaluates instructions, and updates outputs. Learning to follow that sequence makes a basic ladder logic diagram predictable rather than a collection of unfamiliar symbols.

Ladder logic in the PLC scan cycle

A ladder diagram is arranged between two vertical lines called rails. The left rail represents the beginning of logical power flow, and the right rail represents the destination. Horizontal lines between them are rungs. Each rung contains instructions that determine whether an output instruction becomes true.

The PLC does not physically send power through the drawing as a relay panel would. Instead, it evaluates each instruction as a Boolean condition. A rung is true when at least one complete path from the left rail to the output is true. A rung is false when every possible path is blocked.

The controller generally evaluates rungs from top to bottom and instructions from left to right. The exact scan details vary by PLC, but this ordering is the essential model for following control logic.

Contacts, coils, rails, and rungs in a ladder logic diagram

A contact tests a Boolean value, usually an input, internal bit, timer, counter, or output status.

• Normally open (NO) contact: This instruction is true when its referenced bit is on. It passes logic when the bit equals 1.
• Normally closed (NC) contact: This instruction is true when its referenced bit is off. It passes logic when the bit equals 0.
• Output coil: This instruction writes the rung result to an output or internal bit. A true coil turns its assigned bit on; a false coil turns it off.
• Rail: A vertical boundary that frames the logical path.
• Rung: A horizontal line containing the conditions and result for one control decision.

“Normally open” and “normally closed” describe the instruction’s logic behavior or the associated device’s unactuated design. The drawn contact does not prove the live field device’s current state. Check the referenced input or bit to know whether the instruction is currently true.

How ladder logic programming follows input, logic, and output stages

Ladder logic programming follows a repeating PLC scan with three practical stages:

1. Input read: The PLC samples connected input devices, such as push buttons, switches, and sensors, and stores their current states in an input image or memory area.
2. Logic execution: The PLC evaluates the program, including ladder rungs, using the stored input states and current internal values. It calculates whether each contact path and output coil is true.
3. Output update: The PLC transfers calculated output states to physical outputs, energizing or de-energizing devices such as contactors, solenoid valves, and indicator lamps.

Because outputs are commonly updated after logic execution, a change at a push button normally affects the controlled device during the scan in which the PLC reads that change and completes the program. The next scan then uses the updated output or internal status where applicable.

How to read ladder logic in a simple start-stop circuit

Consider a motor-control rung with this arrangement:

Left rail → NC Stop → (NO Start in parallel with NO Motor Auxiliary) → Motor coil → Right rail

The parallel branch is a seal-in, or holding, circuit. The motor’s auxiliary status keeps the motor command true after the Start button is released.

1. At rest: The Stop button is not pressed, so its physical input is on when the button uses normally closed wiring. The NC Stop instruction is therefore true. Start is not pressed, so the NO Start instruction is false. The motor auxiliary bit is also false, leaving both parallel paths open. The rung is false, and the Motor coil is off.
2. When Start is pressed: The input read stage records Start as on. During logic execution, the NC Stop instruction remains true and the NO Start instruction becomes true. A complete path now reaches the Motor coil, so the coil becomes true. During output update, the motor output energizes.
3. After Start is released: The Start input returns off, making its NO instruction false. The energized motor’s auxiliary bit becomes on during the relevant scan, so the parallel auxiliary contact becomes true. The rung remains true and the motor stays energized.
4. When Stop is pressed: The Stop input changes off. The NC Stop instruction becomes false, breaking the rung before either start path can reach the coil. The Motor coil becomes false, and the output update de-energizes the motor. On a later scan, the auxiliary bit also returns off.

This left-to-right trace is the core method for how to read ladder logic: identify each referenced bit, determine whether each instruction is true, follow every complete path, and then check which output coil receives the rung result.