[PDF] LADDER LOGIC - Sharif University of Technology • Ladder diagrams (sometimes called "ladder logic") are a type of electrical notation and symbology frequently used to illustrate how electromechanical switches and relays are interconnected.[PDF] Chapter 2: Basic Ladder Logic Programming Ladder Logic Evaluation. For most PLC's, the ladder scan starts at the top of the ladder and proceeds to the bottom of the ladder, examining each rung from left to right. Once a rung is examined, it is not examined again until the next scan. The rungs are not examined in reverse order.