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C
1.9.2)
Logical sequences
with
UN-commands
A logical sequence
with
UN-commands
effects
a series switcfiing
of contacts.
These contacts
are normally
closed. This corresponds
to an AND gate with
inverted
operands.
PLC-Programme
Gatitmg-logjc
Abbreviated
Truth
table
Contact
symbol
El
E2
Al
example
UN El
"1 " --
&
El
~1I-
0
0
1
t
+
El
UN E2
&
0
1
0
=
Al
El ---iv
E2
1
All
0
0
ti
E2
&
1
1
0
E2 -
41
-Al
Al
It can be deduced
frorn the truth
table that this logic element
is behaving
as a NOR-function.
This also follows
the mathematical
rules governing
Boolean algebra:
-ii
.
E=El
+
E2
Examples:
a)
b)
The lamp L31 should
illuminate
when the contact
S14 is opened.
PLC-Programme:
UN El9
=
A53
This PLC-programme
effects
the following
switching:
+
K17
The relay K17 should
only energise when all three
contacts
S7, S9 and S16 are open.
PLC-Programme:
UN El3
UN E28
UN E63
=
A58
This PLC-programme
effects
the following
switching:
+
24V
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