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FX Series Programmable Controllers STL Programming 3
3-2
3.2 How STL Operates
As previousl
y
mentioned, STL is a s
y
stem which
allows the user to write a pro
g
ram which functions
in much the same wa
y
as a flow chart, this can be
seen in the dia
g
ram opposite.
STL derives its stren
g
th b
y
or
g
anizin
g
a lar
g
er
pro
g
ram into smaller more mana
g
eable parts.
Each of these parts can be referred to as either a
state or a step. To help identif
y
the states, each is
g
iven a unique identification number. These
numbers are taken from the state rela
y
devices
(see pa
g
e 4-6 for more details).
3.2.1 Each step is a program
Each state is completel
y
isolated from all other states within the whole pro
g
ram. A
g
ood wa
y
to
envisa
g
e this, is that each state is a separate pro
g
ram and the user puts each of those
pro
g
rams to
g
ether in the order that the
y
require to perform their task. Immediatel
y
this means
that states can be reused man
y
times and in different orders. This saves on pro
g
rammin
g
time
AND cuts down on the number of pro
g
rammin
g
errors encountered.
A Look Inside an STL
On initial inspection the STL pro
g
ram looks as if it is a rather basic flow dia
g
ram. But to find out
what is reall
y
happenin
g
the STL state needs to be put ‘under a microscope’ so to speak.
When a sin
g
le state is examined in more detail, the sub-pro
g
ram can be viewed.
With the exception of the STL instruction, it will be
immediatel
y
seen that the STL sub-pro
g
ram looks
j
ust like ordinar
y
pro
g
rammin
g
.
The STL instruction is shown as a ‘fat’ normall
y
open contact.
All pro
g
rammin
g
after an STL instruction is onl
y
active when the associated state coil is active.
The transition condition is also written usin
g
standard pro
g
rammin
g
.
This idea re-enforces the concept that STL is reall
y
a method of sequencin
g
a series of events or as
mentioned earlier ‘of
j
oinin
g
lots of smaller
pro
g
rams to
g
ether’.
M8002
X0
X1
S 0
S 26
X0
X1
X15
S 22
S 27
T0
T7
S 22
T0
1
2
STL
S 22
SET S 27
T0
Y22
K20
2
1
T0