Ladder Logic Elements and How They Work (GS 3.2)

Ladder logic (LAD) is a graphical language that resembles electrical relay diagrams. When you write a program in LAD, you use graphical components and arrange them to form a network of logic. The following types of elements are available for you to use in creating your program:

Contacts Represent a switch through which power can flow. Power flows through a normally open contact only when the contact is closed (a logical value of one); power flows through a normally closed or negated (NOT) contact only when the contact is open (a logical value of zero).

Coils Represent a relay or output that is energized by power flow.

Boxes Represent a function (for instance, a timer, counter, or mathematical operation) that is executed when power flow reaches the box.

A network is composed of these elements and represents a complete circuit. Power flows from the left power rail (represented in the LAD editor by a vertical line on the left side of the window) through the closed contacts to energize the coils or boxes.