Historical Context

Ladder Logic was developed to ease the transition from relay-based control panels to PLC systems. Electricians familiar with relay logic could easily understand and troubleshoot ladder programs.

Visual Programming

• Easy to read and understand
• Closely matches electrical schematics
• Excellent for documenting control logic

Contacts (Inputs)

Represent input conditions that can be true or false:

--[ ]--    Normally Open Contact (NO)
--[/]--    Normally Closed Contact (NC)
--[P]--    Positive Edge (Rising Edge)
--[N]--    Negative Edge (Falling Edge)

Coils (Outputs)

Represent outputs that can be energized or de-energized:

--( )--    Output Coil
--(L)--    Latch Coil
--(U)--    Unlatch Coil
--(P)--    Positive Edge Pulse

AND Logic

All contacts in series must be true:

|--[Input1]--[Input2]--[Input3]--( Output )--|

OR Logic

Any contact in parallel branches can be true:

|--[Input1]--+
|            |--( Output )--|
|--[Input2]--+