---
title: "PROM vs PLA; functions using PAL"
subject: "Digital Logic"
chapter: 4
key: "semester_3/digital_logic:prom-pla-pal-functions"
source: https://www.bctengineer.com/q/semester_3/digital_logic/prom-pla-pal-functions
site: BCT Engineering
---

# PROM vs PLA; functions using PAL

Digital Logic · Chapter 4: Data Processing Circuits

- Asked in: 2078 Kartik, 2+2×2 marks
- Video: [Programmable Array Logic (PAL)](https://www.youtube.com/watch?v=qlq4NHk5Y_w) — Neso Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/digital_logic/prom-pla-pal-functions
- Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-4

## Question

Differentiate between PROM and PLA. Implement the following boolean functions using PAL. a) A(x,y,z) = Σ(2,4,5,7) b) B(x,y,z) = Σ(0,2,6)

## Short answer

PROM has a fixed AND array (full decoder) and programmable OR; PLA has both arrays programmable. For PAL, A(x,y,z)=Σ(2,4,5,7) simplifies to xy'+xz+x'yz', B=Σ(0,2,6) to x'z'+yz'. Program these product terms in the AND array; fixed ORs combine them.

## Answer

#### PROM vs PLA

| Point | PROM | PLA |
|---|---|---|
| AND array | Fixed (full decoder, all $2^n$ minterms) | Programmable |
| OR array | Programmable | Programmable |
| Functions in form | Sum of minterms (no simplification) | Simplified SOP, shared terms |
| Size for many inputs | Grows as $2^n$, wasteful | Only needed product terms |
| Typical use | Look-up tables, code converters | Multi-output logic with common terms |

#### Functions using PAL

In a PAL the AND array is programmable and the OR array is fixed, so each function is first simplified to SOP.

**a) $A(x,y,z) = \Sigma(2,4,5,7)$**

```text
       yz=00 01 11 10
x=0      0   0  0  1
x=1      1   1  1  0
```

- $m_4, m_5$ give $xy'$; $m_5, m_7$ give $xz$; $m_2$ alone gives $x'yz'$.

$$
A = xy' + xz + x'yz'
$$

**b) $B(x,y,z) = \Sigma(0,2,6)$**

```text
       yz=00 01 11 10
x=0      1   0  0  1
x=1      0   0  0  1
```

- $m_0, m_2$ give $x'z'$; $m_2, m_6$ give $yz'$.

$$
B = x'z' + yz'
$$

**PAL programming table** (each OR gate has 3 AND inputs; 1 = true, 0 = complement, – = not connected):

| Product term | x | y | z | Output |
|---|---|---|---|---|
| 1: $xy'$ | 1 | 0 | – | A |
| 2: $xz$ | 1 | – | 1 | A |
| 3: $x'yz'$ | 0 | 1 | 0 | A |
| 4: $x'z'$ | 0 | – | 0 | B |
| 5: $yz'$ | – | 1 | 0 | B |
| 6: (unused) | – | – | – | B |

```text
x y z --[prog. AND]-- P1,P2,P3 --[fixed OR]--> A
               \----- P4,P5,P6 --[fixed OR]--> B
```

The unused term 6 has all its fuses intact (both $x$ and $x'$ connected) so it gives 0 and does not affect B.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which statement correctly differentiates PROM and PLA?
   A. PROM has a programmable AND array; PLA has a fixed AND array
   B. Both have fixed OR arrays
   C. PROM has a fixed AND array (all minterms); PLA has a programmable AND array
   D. Both have fixed AND arrays
   Answer: C. PROM's AND array is a full decoder (fixed) with programmable OR; a PLA programs both. (Key point: PROM vs PLA arrays)

2. Which arrays are programmable in a PAL?
   A. Programmable AND array, fixed OR array
   B. Fixed AND array, programmable OR array
   C. Both programmable
   D. Both fixed
   Answer: A. A PAL programs the AND array only, so each function must be simplified to SOP first. (Key point: PAL structure)

3. What is the simplified form of A = Σ(2,4,5,7)?
   A. xy' + yz
   B. xy' + xz + x'yz'
   C. x + yz'
   D. xz + x'y
   Answer: B. m4, m5 give xy'; m5, m7 give xz; m2 stays alone as x'yz'. (Key point: Simplified A)

4. Why does the unused sixth product term not change output B?
   A. It is connected to Vcc so it is always 1
   B. It is a duplicate of term 5
   C. All its fuses are blown so it gives 1
   D. Both x and x' stay connected so the term is always 0
   Answer: D. With a variable and its complement both in the term, the AND output is 0 and the OR is unaffected. (Key point: Unused product term)

## Related in Digital Logic

- [Memory devices; PAL vs PLA](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apal-vs-pla) — Digital Logic, chapter 4
- [RAM vs ROM; functions using PROM](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Aram-rom-prom-functions) — Digital Logic, chapter 4
- [PLA for F1 = Σm(3,5,6,7), F2 = Σm(0,2,4,7)](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apla-two-functions) — Digital Logic, chapter 4
- [PLA and full subtractor using PLA](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apla-full-subtractor) — Digital Logic, chapter 4

## Related in other subjects

- [Merits of functions; actual vs formal parameters](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Audf-merits-demerits-parameters) — Computer Programming, chapter 6