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Ladder Diagrams (LD)^ ^ Ladder diagram language (LD) uses a standardizedset of ladder programming symbols to implementcontrol functions.^ ^ This type of programming language is essentially theone that has always been available in PLCs.^ ^ Users familiar with current PLC ladder diagrams canuse the same programming techniques and methodswhen using this language in an IEC 1131-3environment.^ ^ However, interlocking ladder diagram programming ismuch easier to implement in the IEC 1131-3 formatdue to the use of sequential function charts
Ladder Diagram Representationof a PLC Program
Function Block Diagram (FBD)^ ^ In addition to standard and vendor-specified functions, the IEC1131-
allows
users^
to^ “build”
their^
own^ function
blocks
according to control program requirements. This is referred to as encapsulating a block function. The advantage of creating user-defined blocks is that they canbe^ built
using^
other^ function
blocks,
instruction
list,^ or
structured text programming with or without ladder diagraminstructions. This allows great flexibility in function block programming. Encapsulation also lets the user store a newly created block in alibrary and use it as many times as needed in the program, justlike any other function block.
Function Block DiagramExample
Note:^ The
output^
logic^ of^
the
block^ in
Figure
does^
not
incorporate
an^
output^
coil
because^
the^ output
is
represented
by^ the
variable assigned to^ the^
output^ of
the
block.This^ variable
can^
be^ used throughout the program in otherinstructions
and^ as
a^ control output^
through^
the^ address mapping performed
during variable^
declaration.
The^ user may still choose to use an outputcoil^ representation
if^ desired; however, it will only be allowed inthe last (right-most) block.
Example (Contd.)Start/Stop Circuit
Equivalent ladder diagram
Solution^ ^ Note that there are two rungs for the twooutputs and that both the input and outputvariables are specified with the same namesthat they had in the hardwired circuit.^ ^ To implement this simple ladder diagram as afunction block, it must be programmed orstored in an encapsulated block
Final Function Block The inputs to the start/stopblock^ will^
act^ according
to
the^ logic
used^
to^ program the block. If^ the^
driving^
logic^ to
the
start input is ON, then themotor and light will turn ON. If the stop input is ON, thenboth^ the
motor and^ light outputs will be OFF. The two input variables (theSTART^
and^
STOP
commands), as
well^ as
the
two^ output
variables
(the MOTOR^
and^ PILOT_LIGHT signals),
are
Boolean variables
