Digital Systems: Principles and Design

305Implementation of Combinational Logic by Programmable Logic Devices

sions we need seven. When we compare it with PROM there were 16 AND gates for all the

sixteen minterms. We need 4 ´ 8 ´ 4 PLA in place of 16 ´ 4 PROM. There are (4 + 8) ´ 8 =

96 fusible links compared to 64 fusible links in PROM. Let OR

0

, OR

1

, OR

2

and OR

3

give

the outputs D

0

, D

1

, D

2

and D

3

, respectively. Let us number the fused links of AND

0

as 1, 2,

..., 8 for A,

A

, B,

B

, C,

C

, D, and

D

, and fused links of AND

0

to OR

0

, OR

1

, OR

2

and OR

3

as 9, 10, 11 and 12. Let us number the fused links of AND

1

as 13, 14 ..., 24 for A,

A

, B,

B

,

C,

C

, D,

D

and 4 OR inputs. Similarly, we number