Four kinds of contacts may be used - normally open, normally closed, positive transitional and negative transitional. Coils A coil is either ON or OFF, depending on power flow. When a coil is ON, it either passes power to a discrete output circuit or changes the state of an internal contact. The normal coil turns OFF when power is removed. The memory-retentive coil comes back in the same state when power is restored for one scan.
Shorts Shorts are simply straight-line connections between contacts in a ladder logic network.
A vertical short connects contacts one above the other in a network column. When two contacts are connected by a vertical short, power is passed when one or both contacts receive power. Horizontal shorts are used to expand a rung in a ladder logic network without breaking the power flow. An implementation of the circuit using contacts, coils, and shorts in a ladder logic network is shown below. The ladder logic implementation allows greater flexibility of control and decreased development time, since all the hard-wiring between points of control is done electronically.