Iec 61131-3 Second edition 2003-01 Programmable controllers – Part 3: Programming languages

Table 46 - Sequence evolution
No.
Example
Rule
7
|
+-----+
| S30 |
+-----+
|
+ a
|
+----<----+
| |
+-----+ |
| S31 | |
+-----+ |
| |
+ b |
| |
+-----+ |
| S32 | |
+-----+ |
| |
*-----+ |
| | |
+ c + d |
| | |
+-----+ +->-+
| S33 |
+-----+
|
Directional arrows:
When necessary for clarity, the “less than” (<)
character of the character set defined in 2.1.1 can
be used to indicate right-to-left control flow, and
the “greater than” (>) character to represent left-
to-right control flow. When this feature is used,
the corresponding character shall be located
between two “-” characters, that is, in the
character sequence “-<-” or “->-” as shown in the
accompanying example.
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--``````-`-`,,`,,`,`,,`---

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61131-3 
 IEC:2003(E)
– 105 –
a) Transition not
enabled
(X = 0 or 1)
| | | |
+------+ +-----+ +------+ +------+
|STEP10| |STEP9| |STEP13| |STEP22|
| | | | | * | | * |
+------+ +-----+ +------+ +------+
| | | |
+ X ====+========+=========+====
| |
+------+ + X
|STEP11| |
| | ====+====+===+====
+------+ | |
| +------+ +------+
|STEP15| |STEP16|
| | | |
+------+ +------+
| |
b) Transition enabled
but not cleared
(X = 0 )
| | | |
+------+ +-----+ +------+ +------+
|STEP10| |STEP9| |STEP13| |STEP22|
| * | | * | | * | | * |
+------+ +-----+ +------+ +------+
| | | |
+ X ===+========+=========+====
| |
+------+ + X
|STEP11| |
| | ====+====+====+====
+------+ | |
| +------+ +------+
|STEP15| |STEP16|
| | | |
+------+ +------+
| |