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STL Programming 3
3-1
3. STL Programming
This chapter differs from the rest of the contents in this manual as it has been written with a
trainin
g
aspect in mind. STL/SFC pro
g
rammin
g
, althou
g
h havin
g
been available for man
y
y
ears, is still misunderstood and misrepresented. We at Mitsubishi would like to take this
opportunit
y
to tr
y
to correct this oversi
g
ht as we see STL/SFC pro
g
rammin
g
becomin
g
as
important as ladder st
y
le pro
g
rammin
g
.
3.1 What is STL, SFC And IEC1131 Part 3?
The followin
g
explanation is ver
y
brief but is desi
g
ned to quickl
y
outline the differences and
similarities between STL, SFC and IEC1131 part 3.
In recent
y
ears Sequential Function Chart (or SFC) st
y
le pro
g
rammin
g
(includin
g
other similar
st
y
les such as Grafcet and Funktionplan) have become ver
y
popular throu
g
h out Europe and
have prompted the creation of IEC1131 part 3.
The IEC1131 SFC standard has been desi
g
ned to become an interchan
g
eable pro
g
rammin
g
lan
g
ua
g
e. The idea bein
g
that a pro
g
ram written to IEC1131 SFC standards on one
manufacturers PLC can be easil
y
transferred (converted) for use on a second manufacturers
PLC.
STL pro
g
rammin
g
is one of the basic pro
g
rammin
g
instructions included in all FX PLC famil
y
members. The abbreviation STL actuall
y
means STep Ladder pro
g
rammin
g
.
STL pro
g
rammin
g
is a ver
y
simple concept to understand
y
et can provide the user with one of
the most powerful pro
g
rammin
g
techniques possible. The ke
y
to STL lies in its abilit
y
to allow
the pro
g
rammer to create an operational pro
g
ram which ‘flows’ and works in almost exactl
y
the
same manner as SFC. This is not a coincidence as this pro
g
rammin
g
technique has been
developed deliberatel
y
to achieve an eas
y
to pro
g
ram and monitor s
y
stem.
One of the ke
y
differences to Mitsubishi’s STL pro
g
rammin
g
s
y
stem is that it can be entered
into a PLC in 3 formats. These are:
Ι
) Instruction - a word/mnemonic entr
y
s
y
stem
ΙΙ
) Ladder - a
g
raphical pro
g
ram construction method usin
g
a rela
y
lo
g
ic s
y
mbols
ΙΙΙ
) SFC - a flow chart st
y
le of STL pro
g
ram entr
y
(similar to SFC)
Examples of these pro
g
rammin
g
methods can be seen on pa
g
e 2-1.
General note:
• IEC1131-3: 03.1993 Pro
g
rammable controllers; part 3: pro
g
rammin
g
lan
g
ua
g
es.
The above standard is technicall
y
identical to the ‘Euro-Norm’
EN61131-3: 07.1993
FX Series Programmable Controllers