IEC 61131-3 defines the languages used to program PLCs — ladder diagram (LD), structured text (ST), function block diagram (FBD), instruction list (IL) and sequential function chart (SFC) — giving the controls world a common, portable programming model.
Soft-PLC runtimes like OpenPLC and CODESYS bring these same languages to a Raspberry Pi, so an existing controls engineer can program a Pi with familiar ladder logic rather than learning a scripting language.