Ladder Logic

Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. The decision to use the relay logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced.

Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure 2.1. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. The contact that closes when the coil is energized is called normally open. The normally closed contacts touch when the input coil is not energized. Relays are normally drawn in schematic form using a circle to represent the input coil. The output contacts are shown with two parallel lines. Normally open contacts are shown as two lines, and will be open (non-conducting) when the input is not energized. Normally closed contacts are shown with two lines with a diagonal line through them. When the input coil is not energized the normally closed contacts will be closed (conducting).

What is a Dedicated PLC?

A dedicated PLC is a microprocessor controlled computer that is designed specifically to perform real-time industrial machine control.
There are 3 general sections to all PLC’s, INPUT, CONTROLLER, and OUTPUT.

INPUT – The input section consists of integrated inputs, or extension inputs that machine input devices will be wired to, like limit
switches, or transistor sensors. When the input voltage reaches its specified level the input becomes active. Once active, the input can be read by controller.
CONTROLLER – The controller is the main unit. Several CPU modules are available depending on the demands of your application.
OUTPUTS – The third section is the outputs. Based on the condition of the inputs, the controller will judge which outputs should turn on, to activate machine devices like lights, buzzers, relays, solenoids, or motors.

The 3 sections are controlled by custom sequencing software called ladder programming, which takes the place of hardwired circuits. The relationship between inputs and outputs is controlled by the logic in your ladder program. Because hard-wired circuits are now replaced by software logic, machine modifications and improvements, are much easier to complete.