Elements of a Control Program

Ladder Programs

In LAD programs, the basic elements of logic are represented with contacts, coils, and boxes. A set of interconnected elements that make a complete circuit is called a network.

A hard-wired input is represented by a symbol called a contact. A normally-open contact enables power flow when closed. A contact can also be normally closed. In this case, power flow occurs when the contact is opened.

A hard-wired output is represented by a symbol called a coil. When a coil has power flow, the output is turned on.

A box is a symbol for a complex operation performed within the PLC. The box simplifies programming of the operation. For example, timers and counters are represented by boxes.

STL Programs

STL program elements are represented by a set of instructions for performing the desired functions. Instead of using the graphic display as shown by ladder programs, the STL program is shown in text format.

FBD Programs

FBD program elements are represented by boxes. AND/OR boxes (gates) are used to manipulate Boolean signals in the same manner as ladder contacts.

The different representations are shown in an example below.