Function Block Diagram (Fbd) - Siemens SIMATIC S7-1200 Manual

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6.2 Easy-to-use programming languages
To create the logic for complex operations, you can insert branches to create the logic for
parallel circuits. Parallel branches are opened downwards or are connected directly to the
power rail. You terminate the branches upwards.
LAD provides "box" instructions for a variety of functions, such as math, timer, counter, and
move.
STEP 7 does not limit the number of instructions (rows and columns) in a LAD network.
Note
Every LAD network must terminate with a coil or a box instruction.
Consider the following rules when creating a LAD network:
● You cannot create a branch that could result in a power flow in the reverse direction.
● You cannot create a branch that would cause a short circuit.
6.2.2

Function Block Diagram (FBD)

Like LAD, FBD is also a graphical programming language. The representation of the logic is
based on the graphical logic symbols used in Boolean algebra.
Mathematical functions and other complex functions can be represented directly in
conjunction with the logic boxes.
STEP 7 does not limit the number of instructions (rows and columns) in an FBD network.
94
To create the logic for complex operations,
insert parallel branches between the boxes.
Manual, 11/2011, A5E02486774-04
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