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\layout Chapter

Ladder programming 
\layout Section

Introduction
\layout Standard

Ladder logic or the Ladder programming language is a method of drawing electrica
l logic schematics.
 It is now a graphical language very popular for programming Programmable
 Logic Controllers (PLCs).
 It was originally invented to describe logic made from relays.
 The name is based on the observation that programs in this language resemble
 ladders, with two vertical "rails" and a series of "rungs" between them.
 In Germany and elsewhere in Europe, the style is to draw the rails horizontal
 along the top and bottom of the page while the rungs are drawn sequentially
 from left to right.
\layout Standard

A program in ladder logic, also called a ladder diagram, is similar to a
 schematic for a set of relay circuits.
 Ladder logic is useful because a wide variety of engineers and technicians
 can understand and use it without much additional training because of the
 resemblance.
 
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Ladder logic is widely used to program PLCs, where sequential control of
 a process or manufacturing operation is required.
 Ladder logic is useful for simple but critical control systems, or for
 reworking old hardwired relay circuits.
 As programmable logic controllers became more sophisticated it has also
 been used in very complex automation systems.
 
\layout Standard

Ladder logic can be thought of as a rule-based language, rather than a procedura
l language.
 A "rung" in the ladder represents a rule.
 When implemented with relays and other electromechanical devices, the various
 rules "execute" simultaneously and immediately.
 When implemented in a programmable logic controller, the rules are typically
 executed sequentially by software, in a loop.
 By executing the loop fast enough, typically many times per second, the
 effect of simultaneous and immediate execution is obtained.
\layout Section

Example
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The most common components of ladder are contacts (inputs), these usually
 are either NC (normally closed) or NO (normally open), and coils (outputs).
 
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the NO contact 
\begin_inset Graphics
	filename ladder_action_load.png

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the NC contact 
\begin_inset Graphics
	filename ladder_action_loadbar.png

\end_inset 


\layout Itemize

the coil (output) 
\begin_inset Graphics
	filename ladder_action_out.png

\end_inset 


\layout Standard

Of course there are way more components to a full ladder language, but understan
ding these will help grasp the overall concept.
 
\layout Standard

The ladder consists of one or more rungs.
 These rungs are horizontal traces, with components on them (inputs, outputs
 and other), which get evaluated left to right.
\layout Standard

This example is the simplest rung: 
\begin_inset Graphics
	filename example_link_contact_coil.png

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\layout Standard

The input on the left, a normal open contact is connected to the output
 on the right Q0.
 Now imagine a voltage gets applied to the leftmost end, as soon as the
 B0 turns true (e.g.
 the input is activated, or the user pushed the NO contact), the voltage
 reaches the right part Q0.
 As a consequence, the Q0 output will turn from 0 to 1.
\the_end