[ LADDER LOGIC SYMBOLS · REFERENCE ]
How to read ladder logic
Ladder logic looks like an electrical relay diagram turned into a program: two vertical rails, and rungs across them with conditions on the left and an action on the right. It was designed so that electricians could read PLC programs, and it is still the most used PLC language.
- Read
- Top to bottom, each rung left to right
- Conditions
- Contacts, on the left of the rung
- Actions
- Coils and blocks, on the right
- Logic
- In series is AND, in parallel is OR
Read a rung in four steps
- 1. Find the coil on the right: that is what the rung does. An output coil drives an output or sets a flag.
- 2. Read the contacts from left to right: each normally open contact needs its bit on, each normally closed contact needs it off.
- 3. Combine them: contacts in series must all be true (AND); parallel branches need only one (OR).
- 4. Check the tags: the address or tag name above each contact and coil says what it is connected to, an input, an output or an internal bit.
Example: a start-stop rung with seal-in
The classic first rung: a normally open contact for the start button in parallel with a contact of the motor output itself (the seal-in), in series with a contact for the stop button, driving the motor coil.
Pressing start makes the rung true and turns the motor on. The seal-in contact then holds the rung true after start is released. Pressing stop breaks the rung and the motor drops out.
The stop button is usually wired normally closed in the field, so it fails safe if a wire breaks, and programmed with a normally open contact: its input is on when the button is not pressed.
Ladder logic in commissioning
During commissioning the PLC logic is proved against the cause and effect matrix and the control narrative: every input forced or simulated, every interlock and trip shown to act. See the cause and effect matrix and the loop check procedure.
Frequently asked questions
How do you read ladder logic?
Top to bottom, each rung from left to right: the contacts on the left are the conditions, the coil or block on the right is the action. Series contacts are AND, parallel branches are OR.
What is a seal-in contact?
A contact of the output itself, in parallel with the start button, that keeps the rung true after the button is released.
Why is a stop button programmed with a normally open contact?
Because it is wired normally closed in the field for safety, so its input is on while the button is not pressed.
- In production since 2014
- Thousands of daily users
- On MODEC FPSO projects since 2016
The symbols on this page were drawn by Deskely and are free to use. Standards are revised and every project issues its own legend sheets, which take precedence.