Relay Ladder Diagram is the primary programming language for programmable logic controllers (PLCs). Ladder logic programming is a graphical representation of a program that is meant to look like relay logic. This convention dates back to the early days of PLCs, when it was expected that electricians and technicians would be trained in relay logic and troubleshoot these new devices. This Relay Ladder Diagram depicts the similarities and differences between the two types of programs. The relay logic diagram illustrates electrically connected switches to coils-solenoids, pilot lights, and other components. In the ladder diagram, contacts represent switches or any input, and the coil symbol represents an output. A rung is a line with input or multiple inputs and an output.