IEC 61131-3

 IEC 61131-3 is the third part (of 10) of the open international standard IEC 61131 for programmable logic controllers, and was first published in December 1993 by the IEC. The current (third) edition was published in February 2013.
Part 3 of "IEC 61131" deals with basic software architecture and programming languages of the control program within PLC. It defines three graphical and two textual programming language standards:
Variables.
Variable attributes: RETAIN, CONSTANT, AT