I've been using this since 1975 when I started working as an electrical draftsman. Ladder diagrams are used to describe the logic of electrical control systems. There are differences in the way ladder logic was implemented in computerized form as compared to hard wired so I will be talking about the old way first. The basic component of the control system is the control relay which is a solenoid that operates a number of switches or contacts. The contacts come normally open and normally closed, normal being when the relay is not energized.
Relays come in various breeds like time delay and latching types.
Other components of the control system are the field devices such as push buttons, limit switches, lights, and controlled devices like motor starters and solenoid operated valves. As I said, ladder diagrams show the logic of the controls but they are not used to build the system, a wiring diagram is used for that. 1 Standard motor control circuit. When viewing the pictorial version of the controls as in Figure 1, one can see that the devices on a rung of the ladder are in series reading horizontally and in parallel reading vertically. Control voltage is supplied to the vertical rails, L1 being hot and L2 being common or ground.