Which programming language is used to program a PLC and is described as ladder-like with rails and rungs?

• A. Ladder Logic
• B. Structured Text
• C. Function Block Diagram
• D. Instruction List

Answer: (A)

The concept being tested is how PLC programming languages map to the way control circuits are drawn. The description of a ladder-like structure with rails and rungs points to Ladder Logic. In Ladder Logic, you see a vertical pair of rails representing power, and horizontal rungs that contain symbols for inputs (like normally open or normally closed contacts) and outputs (coils). When the conditions on a rung are satisfied, the corresponding output coil is energized, effectively closing a circuit to drive machinery. This graphical style mirrors traditional relay-based control diagrams, which makes it intuitive for technicians and easy to translate from hard-wired designs to software.

Other languages in IEC 61131-3 are not drawn as ladders: Structured Text is a high-level, text-based language similar to Pascal or C; Function Block Diagram uses blocks and connections to represent data flow between functions; Instruction List is a low-level, assembly-like textual language. Their forms don’t resemble the ladder with rails and rungs, which is why Ladder Logic is the one described.