Essentially the program was hard-wired into the physical relays themselves, and this approach is still preferred for some special applications. So, before the days of PLCs, all control systems consisted of large racks of relays all wired to turn each other off and on according to the design of the system. This offers a middle ground between diagrams using full-grammar language (which is easy for humans to read but difficult to translate for the controller), and diagrams using machine code, which is easy for the controller to parse, but can be nearly incomprehensible to humans.Ī relay is simply a switch that can be electrically actuated. The simple visual format helps programmers parse the logic sequence at a glance, and easily catch errors before they are implemented. Ladder logic is easy for both a human and a machine controller to process. The popularity and longevity of ladder logic notation can be traced to its ease of use. You will find schematic diagrams for relay racks written out using ladder logic notation to this day. Engineers needed a way to represent the complicated arrangements and relationships in a relay control system, so the ladder logic representation was developed to make things clear. Relays were used, and still are, to control various systems and these control systems are often very complicated. You don’t get anything too fancy with ladder logic rather, the graphical program is meant to represent a facsimile of a wiring diagram for traditional relay logic. This can be confusing to programmers who come from a background in traditional languages, such as C or C++. Unlike other computer languages, ladder logic is graphical, meaning that the program is literally drawn out rather than typed. The most important thing about learning to program PLCs is understanding ladder logic.

Allen Bradley > Wrapping Your Head Around Ladder Logic

Allen bradley ladder logic program