Ladder logic basics

Ladder logic is the most widely used PLC programming language. It looks like an electrical relay diagram: power flows from the left rail, through contacts, to a coil on the right. If you can read a relay circuit, you can read ladder.

Rails and rungs

A ladder program has two vertical rails and horizontal rungs between them. Each rung is one line of logic: conditions (contacts) on the left, an action (coil or instruction) on the right. The PLC solves rungs from top to bottom on every scan.

Contacts

Important: the contact symbol describes the program, not the field device. A normally closed stop button that is not pressed sends a 1 to the PLC, so in the program you use a normally open contact for it. This confuses almost every beginner.

AND and OR

Contacts in series are AND: all must be closed. Contacts in parallel (a branch) are OR: any one closes the path.

|--[ Guard_closed ]--[ Start ]--------( Motor )--|     AND
|--[ PB_1 ]--+-------------------------( Lamp  )--|     OR
|--[ PB_2 ]--+

The start/stop seal-in circuit

A momentary start button must keep a motor running after it is released. The output's own contact is placed in parallel with the start button, sealing the rung in:

|--+--[ Start ]--+--[ Stop ]--[ Overload ]--( Motor )--|
|  +--[ Motor ]--+

Press Start: Motor turns on, and its contact holds the rung on. Press Stop (the NC button opens, the input goes 0): the rung breaks and Motor drops out. Because Stop is in series before the coil, stop always wins.

Set and reset (latch / unlatch)

SET (OTL, S) turns a bit on and leaves it on; RESET (OTU, R) turns it off. They are handy for sequences and alarms, but every SET needs a matching RESET somewhere, and you should not mix OUT and SET/RESET on the same address.

Rules that avoid bugs

• One coil per address. A double coil means only the last rung wins.
• Every rung ends in an output instruction.
• Don't connect a coil straight to the left rail; use an always-ON system bit if you need it.
• Comment every address and rung.

Timers and counters

For delays and counting, see PLC timers and counters.

The PLC Sim Lab Playground has a grid ladder editor (with F-key shortcuts like real software) where you can build and run every circuit on this page.