4.6 The IEC 61131-3 Standard
IEC 61131-3 is the international standard for PLC programs, defined by the International Elec
-
tromechanical Commission (IEC). The IEC 61131-3 not only covers PLC programming lan
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guages but also offers guidelines for PLC applications.With the software GX IEC Developer
PLCs can be programmed according to the IEC 61131-3 Standard.
In this Beginner's Manual only the terms which are necessary for understanding the program
examples are explained. For further information about GX IEC Developer please refer to the
Beginner's Manual (art. no. 043596) or the Reference Manual (art. no. 043597) for this soft
-
ware. During programming you can also use the Help function of GX IEC Developer.
4.6.1 Software Structure
Program Organisation Unit (POU)
In IEC 61131-3 a PLC program is divided into individual program modules called Program
Organisation Units (POUs). A POU is the smallest independent element of a sequence
program.
Every POU consists of
쎲
Header
쎲
and Body
In the
Header
the variables which are used in this POU are declared.
The
Body
is the part of the project where the program is edited.Several languages are avail
-
able for editing a program.
4–10 MITSUBISHI ELECTRIC
The IEC 61131-3 Standard An Introduction to Programming
POUsarestoredinasocalledPOUPool.
Program POUs are grouped together in
Task.
Individual Tasks are grouped together to
form the actual PLC program.
POU Pool
POU 1
POU 2
POU 3
POU 4
POU 5
POU 6
POU 7
POU 8
Task 1
Task 2
POU 1
POU 3
POU 4
POU 6
POU 7
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