It was about 120 years ago when Mark Twain used the phrase “more than one way to skin a cat.” In the world of PLC programming, that cliché is still applicable today. Thanks to the International Electrotechnical Commission (IEC), five standard programming languages have emerged as the most common, used for both process and discrete programmable controllers. The IEC is an organization that prepares and publishes international standards for all electrical, electronic and related technologies, including controllers. With its IEC61131-3 publication, the organization identifies these five programming languages and their common abbreviations as: Ladder Diagram (LD), Instruction List (IL), Function Block Diagram (FBD), Structured Text (ST) and Sequential Function Chart (SFC).
Bosch Rexroth Corporation (2009) Understanding the IEC61131-3 Programming Languages