7 Programming
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NX-series Safety Control Unit User’s Manual (Z930)
In POUs that are written in the FBD language, networks are executed in order from top to bottom. Pro-
cessing ends when the network at the very bottom of the program is executed. Elements in the same
network are executed from top to bottom for FUN and FB inputs and left to right for blocks that are con-
nected in series. In the following example, execution is in the following order: Block 1, Block 2, Block 3,
and then Block 4.
However, if there is a Return instruction in the middle of a program and the execution condition is met,
the POU is ended and a return is made to the source of the call. No processes after the Return instruc-
tion are executed.
Safety programs that are written in the FBD language are generally executed from top to bottom, but
you can use the Jump instruction to change the execution order.
For example, when the value of variable a changes to TRUE in the following example, execution will
move to the network labeled “Exception.”
You cannot jump to a network that is above the current network.
Execution Order of Safety Programs Written in the FBD Language
Execution Control
Block1 Block4
Block3Block2
Execution jumps here.
a Exception
Exception:
FB1
FB2
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