Ladder logic in plain English
Ladder logic is a graphical programming language used to control PLCs. Instead of writing lines of text code, you build logic using symbols that look similar to an electrical relay circuit.
A ladder logic program is made up of horizontal lines called rungs. Each rung checks one or more conditions, then controls an output, memory bit or instruction.
Why do PLCs use ladder logic?
Ladder logic became popular because it made PLC programming easier for electricians, technicians and engineers who already understood relay circuits.
Why is it called ladder logic?
It is called ladder logic because the layout looks like a ladder. There are two vertical rails, with horizontal rungs running between them.
The left side of the rung usually contains conditions, such as inputs, switches, sensors or internal memory bits. The right side usually contains an output or instruction.
Contacts, coils and rungs
You can understand a lot of beginner ladder logic by learning three core building blocks.
Normally open and normally closed
A normally open contact is true when the related signal is on. For example, if a start button input is on, a normally open contact for that input becomes true.
A normally closed contact is true when the related signal is off. These are often used in stop circuits, permissives, interlocks and fault logic.
This can feel confusing because the ladder symbol is not always the same thing as the physical wiring contact. Always check both the real device and the program logic.
Start / stop ladder logic
A classic beginner example is a start/stop circuit. The start pushbutton is used to turn a motor output on. The stop pushbutton is used to turn it off.
A seal-in or latch contact can be used so the output stays on after the start button is released. This is one of the first ladder logic patterns many learners practise.
Series, parallel and latching
Series logic
Behaves like AND logic. All conditions usually need to be true.
Parallel logic
Behaves like OR logic. One path can be enough to make the rung true.
Latching logic
Can hold an output or memory bit on after the original start condition is removed.
Program logic is not field wiring
Ladder logic looks like an electrical circuit, but it is not the same as the physical wiring inside a control panel. It is a program running inside the PLC.
The real buttons, sensors and devices are wired to PLC inputs and outputs. Ladder logic then uses those signals inside the PLC program.
Common beginner mistakes
Common traps include mixing up physical contacts and ladder contacts, forgetting the scan cycle, using output bits without understanding why, and building huge rungs too early.
Keep beginner logic simple. Smaller, clearer networks are easier to test, explain and troubleshoot.