The Ladder Logic Diagram is an electrical notation and symbology generally used to show how the electromechanical switches and relays are interconnected. The two vertical lines shown here are known as rails; they relate to opposite poles of a power supply, typically 120 volts AC. The M1 START pushbutton input is used in the ladder logic Diagram example to activate the M1 RUN output. A second time, the M1 RUN output is used to latch the M1 RUN output. Because M1 STOP and M1 TOL are wired normally closed (NC) to the PLC inputs, they must be configured in the logic as normally open (NO) symbols.