Ladder diagram

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Ladder diagram ( LAD abbreviation ) is a method for programming programmable logic controllers (PLC). In English usage it is called a ladder diagram (abbreviation LD ).

It is one in the IEC - Norm EN 61131-3 standard graphical programming language , in particular for logic control is suitable. Its representation is based on circuit diagrams :

Circuit diagram
Corresponding contact plan

If the elements are connected in series, this means an AND link . If they are connected in parallel, this is an OR link . A line through the element means a negation of the element.

Entrances are shown as two vertical parallel lines, while exits are shown as opposing curved lines.


Simple ladder diagram example

In almost all modern LAD languages, however, there are also function blocks available that go far beyond the actual link control.

Example: Maximum search (LAD language STEP 7 )

Ladder diagram, maximum search

If the variable actual value greater than Maxsp (maximum memory), is actual value adopted as the new maximum.

ST is often better suited for more complex program structures, in particular analog value calculations .