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

 a
No.
Graphical form
Function block body
1
Bistable function block (set dominant)
+-----+
| SR |
BOOL---|S1 Q1|---BOOL
BOOL---|R |
+-----+
+-----+
S1----------------| >=1 |---Q1
+---+ | |
R------O| & |----| |
Q1------| | | |
+---+ +-----+
2
Bistable function block (reset dominant)
+-----+
| RS |
BOOL---|S Q1|---BOOL
BOOL---|R1 |
+-----+
+---+
R1----------------O| & |---Q1
+-----+ | |
S-------| >=1 |----| |
Q1------| | | |
+-----+ +---+
NOTE The function block body is specified in the Function Block Diagram (FBD) language
defined in 4.3.
a
The initial state of the output variable Q1 shall be the normal default value of zero for Boolean
variables.
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– 78 –
61131-3 
 IEC:2003(E)
2.5.2.3.2 Edge detection
The graphic representation of standard rising- and falling-edge detecting function blocks shall be as
shown in table 35. The behaviors of these blocks shall be equivalent to the definitions given in this
table. This behavior corresponds to the following rules:
1) The Q output of an R_TRIG function block shall stand at the BOOL#1 value from one execution
of the function block to the next, following the 0 to 1 transition of the CLK input, and shall return
to 0 at the next execution.
2) The Q output of an F_TRIG function block shall stand at the BOOL#1 value from one execution
of the function block to the next, following the 1 to 0 transition of the CLK input, and shall return
to 0 at the next execution.