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DL205 User Manual, 4th Edition, Rev. B
5–8
Chapter 5: Standard RLL Instructions - Boolean
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A
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D
Comparative Boolean
Some PLC manufacturers make it really difficult to do a simple comparison of two numbers.
Some of them require you to move the data all over the place before you can actually perform
the comparison. The DL205 Micro PLCs provide Comparative Boolean instructions that
allow you to quickly and easily solve this problem. The Comparative Boolean provides
evaluation of two 4-digit values using boolean contacts. The valid evaluations are: equal to,
not equal to, equal to or greater than, and less than.
In the example when the BCD value in V-memory
location V1400 is equal to the constant value 1234,
Y3 will energize.
Boolean Stack
There are limits to how many elements you can include in a rung. This is because the DL205
CPUs use an 8-level boolean stack to evaluate the various logic elements. The boolean stack is
a temporary storage area that solves the logic for the rung. Each time you enter a STR
instruction, the instruction is placed on the top of the boolean stack. Any other STR
instructions on the boolean stack are pushed down a level. The ANDSTR, and ORSTR
instructions combine levels of the boolean stack when they are encountered. Since the
boolean stack is only eight levels, an error will occur if the CPU encounters a rung that uses
more than the eight levels of the boolean stack.
The following example shows how the boolean stack is used to solve boolean logic.
X1 or (X2 AND X3)
STR X0 STR X1 STR X2
1
STR X0
2
3
4
1 STR X1
2 STR X0
3
4 1
2
3
4
AND X3
1
2
3
4
ORSTR
1
2
STR X0
3
OUT
Y0 X0 X1
X2 X3
X4
X5
STR
OR
AND
ORSTR
ANDSTR
Output
STR
STR
AND
X4 AND {X1 or (X2 AND X3)}
AND
X4
1
2
STR X0
3
NOT X5 OR X4 AND {X1 OR (X2 AND X3)}
ORNOT X5
1
2
3
ANDSTR
XO AND (NOT X5 or X4) AND {X1 or (X2 AND X3)}
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2
3
STR X0
STR X2
STR X1
STR X0
STR X2
STR X1
STR X0
Y3
OUT
V1400 K1234