YASKAWA SIEMENS CNC Series, Maintenance Manual, Serviceman Handbook

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8.2 Program structure
8-3
8.2 Program structure
A STEP7 structured program consists of blocks, each of which in turn consists of networks (see
the figure below). Each network can be expressed either in LAD, FBD, or STL. Thus a block
can consist of mixed LAD and STL networks, or mixed FBD and STL networks. LAD and
FBD networks, however, cannot be mixed in a block.
While most instructions can be written in any of LAD, FBD, and STL, the LOOP, variable-
address, and some other STEP7 instructions can be written only in STL. In such a case, LAD
and STL networks need to be mixed.
I 0.0
I 0.2
I 0.1
Q 1.0
O I 2.0
O I 2.1
= Q 3.0
I 4.2
M5.1
M 5.0
Network 1
Network 2
Network 3
M5.0 I 6.2 Q 5.0
Network 4
( )
( )
( )
I
I 4.0
( )
( )