Ladder logic is basically read from the left hand rail to the right hand rail and from the first rung to the last rung (left to right and top to bottom). The rungs contain input symbols that either pass or block the logic flow. The last element of each rung is an output symbol which is the result of the logic expressions contained in that rung.


Contacts & Coils:

Contacts stand between power and Coils, asking questions.  If the answer to the question is true, then power is allowed to flow through the contact, if not the path is blocked.


Contacts placed side by side to form AND statement



Contacts placed one on top of each other to form OR statements.




AND and OR statements may be combined to form complex expressions.






Coils:

They represent relays that are energized when power flows to them. When a coil is energized it causes
 a corresponding output to turn ON by changing the state of the status bit controlling that output