To understand how to draw ladder logic diagrams the seven basic parts of a ladder diagram are detailed below…..

👉 Rails – There are two rails in a ladder diagram which are drawn as vertical lines running down the far most ends. Here power flow goes from the left hand side to the right hand side.

👉 Rungs – The rungs are drawn as horizontal lines and connect the rails to the logic expressions. Each rung is numbered in ascending sequential order.

👉 Inputs – The inputs are external control actions such as a push button being pressed or a limit switch being triggered. The inputs are actually hardwired to the PLC terminals and represented in the ladder diagram by a normally open (NO) or normally closed (NC) contact symbol.
👉Outputs – The outputs are external devices that being are turned on and off such as an electric motor or a solenoid valve. The outputs are also hardwired to the PLC terminals and are represented in the ladder diagram by a relay coil symbol.
👉 Logic Expressions – The logic expressions are used in combination with the inputs and outputs to formulate the desired control operations.
👉Address Notation & Tag Names – The address notation describes the input, output and logic expression memory addressing structure of the PLC. The tag names are the descriptions allocated to the addresses.
👉 Comments – These are an extremely important part of a ladder diagram. Comments are displayed at the start of each rung and are used to describe the logical expressions and control operations that the rung, or groups of rungs, are executing. Understanding ladder diagrams is made a lot easier by using comments.



 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