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Ladder diagram (LD)
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LD is the best-known PLC programming language
developed for users who understand traditional
relay-based controls. LD continues to be
one of the most popular PLC programming
languages in the United States. Although
it is possible to program all control logic
in LD, supplementing LD with other languages
provides the convenience of using languages
that are best suited for a particular control
task.
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Sequential Function Charts (SFC)
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SFC programming offers a graphical method
of organizing the program. The three main
components of an SFC are steps, actions
and transitions. A step is a grouping of
logic used to carry out a particular control
task. Actions are the aspects of that task.
Transitions are mechanisms used to move
from task to task. Control logic for each
Step, Action and Transition is programmed
in any of the other languages.
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Function Block Diagram (FBD)
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Like SFC, FBD is a graphical language
that allows programming in other languages
(ladder, instruction list, or structured
text) to be nested within the FBD. In FBD,
program elements appear as blocks, which
are "wired" together in a manner
resembling a circuit diagram. FBD is most
useful in those applications involving a
high degree of information/data flow between
control components, such as process control. |
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Structured Text (ST)
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ST is a high-level language that is reminiscent
of Pascal or Basic. ST is ideal for tasks
requiring complex math, algorithms or formulas.
Often ST program lines resemble sentences,
making it comprehensible to the novice users.
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Instruction List (IL)
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IL is a low-level language similar to
Assembly language. While very powerful,
IL is not used very often due to its complexity. |
