Philips Semiconductors Programmable Logic Devices
Application Note
AN024
PLS173 as a 10-bit comparator, 74LS460
14
September 1993
DESCRIPTION
The PLS173 is a 24-pin PLA device which
has 10 bidirectional outputs and 12 dedicated
inputs. The output of the device is the sum of
products of the inputs. The polarity of each
output may be individually programmed as
Active-High or Active-Low. A 10-bit
comparator similar to the 74LS460 compares
two 10-bit data inputs to establish if
EQUIVALENCE or NOT EQUIVALENCE
exists. The output has True and Complement
comparison status outputs. The logic diagram
of the comparator is shown in Figure 1.
The truth table is as shown in Table 1 where
vectors a and b are 10-bit inputs to A9 to A0
and B9 to B0. If the input to A9-A0 is bit-to-bit
equivalent to the input to B9-B0, the two input
vectors are considered EQUIVALENT, and
output EQ goes High and NE goes Low. If the
two input vectors are not bit-to-bit equivalent,
then EQ goes Low and NE goes High. The
circuit is implemented with SNAP as shown
in Figure 3.
Table 1.
Function Table
A
9
– A
0
a
b
a
b
B
9
– B
0
a
b
b
a
EQ
H
H
L
L
NE
L
L
H
H
Figure 1. Logical Equivalent Circuit of
10-Bit Comparator
20
19
18
17
16
15
14
13
24
23
12
10
11
9
8
7
6
5
4
3
2
1
22
21
TOP VIEW
GND
A
5
A
4
B
3
A
3
B
2
A
2
B
1
A
1
B
0
A
0
V
CC
B
9
A
9
B
8
A
8
NE
EQ
B
7
A
7
B
6
A
6
B
5
B
4
RESOURCES
This design used 20 product terms in the
PLS173. As shown in Figure 4, expanded
equations, each output needs 20 product
terms. Since the product terms are the
same, each output shares the 20 product
terms with an output polarity fuse determining
the proper output level.
Figure 2. Pin Configuration
I0
I1
I2
I3
I4
I5
I6
I7
I8
GND
B9
B8
B7
B6
B5
B4
B3
B2
B1
V
CC
D Package
A0
B0
A1
B1
A2
B2
A3
B3
A4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
24
23
22
21
20
19
18
17
16
15
B4
A5
GND
B9
A9
B8
A8
NE
EQ
B7
A7
B6
A6
B5
I9
I10
B0
I11