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Ladder Logic Fundamentals and Programming Examples - PLC Tutorial

Logical continuity vs. electrical continuity
I/O Mapping
Mastering examine onon and examine off instructions
The PLC scanning process

PLC Programming Languages

• In the United States, ladder logic is the most popular
  method used to program a PLC
• This course will focus primarily on ladder logic programming
• Other programming methods include:
  … Function block diagrams (FBDs)
  … Structured text (ST)
  … Instruction List (IL)
  … Sequential function charts (SFCs)

Topics 

PLC programming languages
Anatomy of a ladder program
Logic functions

Anatomy of a Ladder Program

• Input instructions are entered on the left
• Output instructions are entered on the right
• The power rails simulate the power supply lines
  L1 and L2 for AC circuits and +24 v and ground for DC circuits

• Most PLCs allow more than one output per rung
• The processor (or “controller”) scans ladder rungs from top-to-bottom and from left-to-right.
  The basic sequence is altered whenever jump or subroutine instructions are executed.

Logic Functions

• PLC programming is a logical procedure
• In a PLC program, “things” (inputs and rungs) are either TRUE or FALSE
  If the proper input conditions are TRUE:The rung becomes TRUE and an output action occurs (for example, a motor turns on)
  If the proper input conditions are not TRUE:The rung becomes FALSE and an output action does not occur
• Ladder logic is based on the following logic functions:
  AND,  OR,  Sometimes called “inclusive OR”, Exclusive OR.

For much more download  here (PLC Downloads -> Usefull PLC PDF's -> Ladder Logic Fundamentals PLC tutorial).

source:http://www2.tech.purdue.edu/Cimt/Courses