Ladder diagram programming, or LD, traces its history back some 100 years to relay ladder logic (RLL), which was created to describe systems of electrical components such as relays, timers and motors. In the early days of automation, when PLCs were replacing relays and timers, it made perfect sense to create a programming language familiar to the user base and similar to the tool it was replacing. Unfortunately, as controllers became more capable and evolved past relays and timers, the original LD language was pressed into services it was never intended for and was poorly suited.




IEC 61131-3: Programming Industrial Automation ...