LD, SFC, FBD, CFC, and ST are the IEC 61131-3 Programming Languages. Because of its graphical nature, the ladder diagram is still widely used. Sequential Function Chart is a visual language that is great for expressing state machines and appeals to engineers' graphical nature. The organization identifies these five programming languages and their common abbreviations in its IEC61131-3 publication as Ladder Diagram (LD), Instruction List (IL), Function Block Diagram (FBD), Structured Text (ST), and Sequential Function Chart (SFC). PLCs are classified into three different types based on their output: relay output, transistor output, and Triac output.
