A family of programming languages for programmable logic controllers (PLCs). Of the three graphical and two textual languages, ladder logic (see example below) was the first language and widely used. IEC 61131 was previously known as IEC 1131. See PLC.

   Graphical                   Textual

   ladder logic                structured text
   function block diagram      instruction list
   sequential function chart

   ---[ ]-----|--[ ]--|-----( )---
       a      |   b   |      x
              |       |
              |--[ ]--|
              |   c   |