Understanding Ladder Logic

What is Ladder Logic

Before the introduction of ladder diagrams, control systems used relay logic. Relay logic is a group of hardware relays, timers, counters and other devices wired together in a series and parallel circuit to provide control. The draw back to relay logic was the change time required if the circuit requirements were to change (lengthy re-wire time).

Ladder logic was invented to bridge the gap of the programmable controller and relay logic. Ladder logic is a graphical representation of relay logic that is operating on a programmble controller (PLC). Using a PLC and ladder logic allows the freedom to make circuit changes only by changing the software; eliminating the costly re-wiring that was once required.

Ladder Logic vs Relay Logic

Ladder Diagram (LD) is a graphical representation of boolean equations, using contacts (inputs) and coils (outputs). The ladder diagram language allows these features to be viewed in a graphical form by placing graphic symbols into the program workspace similar to a Relay Logic electrical diagram. Both ladder diagram and relay logic diagrams are connected on the left and right sides to power rails.

A comparison of a “hard-wired” relay logic system and a “programmable” system using EZ LADDER® as the programming platform will show the similarities which make the programming using EZ LADDER® quick and easy to apply to any application. The diagram belowshows a “hard-wired” relay logic control system. For easy comparison, it is divided into three sections.

1. Input Devices: Includes devices operated manually (ie: pushbuttons) and devices operated automatically
   (ie: limit switches) by the process or machine being controlled.
2. Relay Control Logic: Consists of relays interconnected to energize or de-energize output devices in response
   to status of input devices, and in accordance with the logic designed into the control circuit.
3. Output Devices: Consists of motor starters, solenoids, etc. which would control the machine or process.

In place of “hard-wired” relay circuitry, EZ LADDER® applications are programmed using “relay-type symbology”. This symbology brings ease and familiarity to the programming while adding flexibility.

The simplicity of the EZ LADDER® diagram programming format is shown in below, which illustrates a relay ladder rung as used in relay control systems.

The same circuit is shown as it’s ladder diagram equivalent below.