The Iec 61131-3 Standard; Software Structure - Mitsubishi MELSEC System Q Manual

The IEC 61131-3 Standard

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-
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
b Header
b 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
POU Pool Task 1
POU 1
POU 1
POU 2
POU 3
POU 3
POU 4
POU 4
POU 5
Task 2
POU 6
POU 6
POU 7
POU 7
POU 8
An Introduction to Programming
POUs are stored in a so called POU Pool.
Program POUs are grouped together in
Task.
Individual Tasks are grouped together to
form the actual PLC program.
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