Siemens SIMATIC S7-200 System Manual page 70

S7-200 Programmable Controller System Manual
Features of the LAD Editor
The LAD editor displays the program as a graphical representation similar to electrical wiring
diagrams. Ladder programs allow the program to emulate the flow of electric current from a power
source through a series of logical input conditions that in turn enable logical output conditions. A
LAD program includes a left power rail that is energized. Contacts that are closed allow energy to
flow through them to the next element, and contacts that are open block that energy flow.
The logic is separated into networks.
The program is executed one network at
a time, from left to right and then top to
bottom as dictated by the program.
Figure 5-3 shows an example of a LAD
program. The various instructions are
represented by graphic symbols and
include three basic forms.
Contacts represent logic input conditions
such as switches, buttons, or internal
conditions.
Coils usually represent logic output
results such as lamps, motor starters,
interposing relays or internal output interposing relays, or internal output
conditions.
Boxes represent additional instructions, such as timers, counters, or math instructions.
Consider these main points when you select the LAD editor:
Ladder logic is easy for beginning programmers to use.
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Graphical representation is easy to understand and is popular around the world.
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The LAD editor can be used with both the SIMATIC and IEC 1131--3 instruction sets.
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You can always use the STL editor to display a program created with the SIMATIC LAD
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editor.
Features of the FBD Editor
The FBD editor displays the program as a graphical representation that resembles common logic
gate diagrams. There are no contacts and coils as found in the LAD editor, but there are
equivalent instructions that appear as box instructions.
Figure 5-4 shows an example of an FBD
program.
FBD does not use the concept of left and
right power rails; therefore, the term
"power flow" is used to express the
analogous concept of control flow analogous concept of control flow
through the FBD logic blocks.
The logic "1" path through FBD elements is called power flow. The origin of a power flow input and
the destination of a power flow output can be assigned directly to an operand.
The program logic is derived from the connections between these box instructions. That is, the
output from one instruction (such as an AND box) can be used to enable another instruction (such
as a timer) to create the necessary control logic. This connection concept allows you to solve a
wide variety of logic problems.
Consider these main points when you select the FBD editor:
The graphical logic gate style of representation is good for following program flow.
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The FBD editor can be used with both the SIMATIC and IEC 1131--3 instruction sets.
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You can always use the STL editor to display a program created with the SIMATIC FBD
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editor.
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Figure 5-3 Sample LAD Program
Figure 5-4 Sample FBD Program