LD

Ladder Diagram

A programming aid used for programming PLC's (Programmable Logic Controllers). A Boolean expression can be represented by ladder logic, a set of conditionally open and closed switches connected in series and parallel. A Ladder Diagram is an extension of this, in which symbols represent the various inputs and outputs of the plant equipment to be controlled. Part of the IEC 1131 standard. See FBD, IL, SFC and ST.