Ladder logic

How it is used

The most widely used PLC language, especially in North America and for machine control. Its official IEC 61131-3 name is Ladder Diagram (LD). Engineers say 'in ladder' and talk about rungs, contacts, coils and latches; people with an electrical background find it easy to read.

Example sentences

1. The interlock is on rung 12 of the ladder logic; the door switch is in series with the start button.

2. Write the sequence in ladder logic so the maintenance team can follow it.

Common collocations

• in ladder
• ladder rung
• ladder diagram
• write ladder logic
• contacts and coils

Related terms

• PLCShort for programmable logic controller: a rugged industrial computer that reads inputs from sensors and switches, runs a control program and drives outputs such as motors, valves and lamps.
• function blockA reusable piece of PLC code with its own inputs, outputs and memory, used to control one type of device, such as a motor, valve or conveyor, many times in a program.
• structured textA text-based PLC programming language defined in IEC 61131-3 that looks similar to Pascal, with IF, CASE and FOR statements. It suits calculations, data handling and complex logic.
• relayAn electrically operated switch: a small current through its coil opens or closes contacts that switch another, often larger or separate, circuit.
• scan cycleThe repeating loop a PLC runs: it reads all inputs, executes the program from top to bottom, then updates the outputs. The time one loop takes is the scan time.

More terms: PLCs and Industrial Control

• HMI
• I/O
• analog signal
• tag