---
title: "Positive and negative logic"
subject: "Digital Logic"
chapter: 2
key: "semester_3/digital_logic:positive-negative-logic"
source: https://www.bctengineer.com/q/semester_3/digital_logic/positive-negative-logic
site: BCT Engineering
---

# Positive and negative logic

Digital Logic · Chapter 2: Digital Logic

- Asked in: 2082 Shrawan, 3 marks; 2081 Baisakh, 2 marks (asked 2 times)
- Video: [Positive and Negative Logic](https://www.youtube.com/watch?v=gtucOKD1pHA) — 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/positive-negative-logic
- Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-2

## Question

Explain a positive logic and the negative logic with suitable examples.

## Short answer

Positive logic assigns HIGH voltage to logic 1 and LOW to 0; negative logic assigns HIGH to 0 and LOW to 1. For example, a gate outputting HIGH only when both inputs are LOW is an AND in positive logic and an OR in negative logic.

## Answer

In a digital circuit the two logic states are shown by two voltage levels. Which level is called 1 decides whether the system uses positive or negative logic.

- **Positive logic:** HIGH voltage = logic 1, LOW voltage = logic 0. Example: in TTL, +5 V = 1 and 0 V = 0. Most systems use positive logic.
- **Negative logic:** HIGH voltage = logic 0, LOW voltage = logic 1. Example: +5 V = 0 and 0 V = 1; or in a –5 V system, –5 V = 1 and 0 V = 0.

```text
  Positive logic       Negative logic
  +5V ---- 1           +5V ---- 0
   0V ---- 0            0V ---- 1
```

**Example:** a circuit has this voltage table:

| A | B | Y |
|---|---|---|
| L | L | L |
| L | H | L |
| H | L | L |
| H | H | H |

- In positive logic (H = 1): Y = 1 only when A = B = 1, so it is an **AND gate**.
- In negative logic (H = 0): Y = 0 only when A = B = 0, so it is an **OR gate**.

So **positive AND = negative OR**, and similarly positive OR = negative AND, positive NAND = negative NOR, positive XOR = negative XNOR. The physical gate is the same; only the meaning of the levels changes.

## Check yourself

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

1. In positive logic:
   A. HIGH = 0
   B. LOW = 1
   C. Only -5V = 1
   D. HIGH = 1, LOW = 0
   Answer: D. Most systems use this. (Key point: Positive)

2. A gate with Y high only when A and B high is?
   A. AND in both
   B. OR in positive logic
   C. AND in positive logic, OR in negative logic
   D. NAND in negative logic
   Answer: C. Positive AND = negative OR. (Key point: AND to OR)

3. Positive NAND equals?
   A. Negative NAND
   B. Negative NOR
   C. Negative AND
   D. Negative XOR
   Answer: B. Positive XOR = negative XNOR as well. (Key point: Duals)

## Related in Digital Logic

- [Positive/negative logic; XNOR using NAND](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-negative-logic-xnor-nand) — Digital Logic, chapter 2, asked 2×
- [Positive/negative logic; Ex-NOR using NOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-negative-logic-xnor-nor) — Digital Logic, chapter 2, asked 2×
- [Positive/negative logic; verify 3-variable De Morgan](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Ademorgan-three-variable-verify) — Digital Logic, chapter 2
- [Universal gates; positive and negative logic](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Auniversal-gates-positive-logic) — Digital Logic, chapter 2
- [Positive XOR equals negative XNOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-xor-negative-xnor-define) — Digital Logic, chapter 2, asked 3×
- [Prove positive XOR equals negative XNOR](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Apositive-xor-negative-xnor-prove) — Digital Logic, chapter 2, asked 3×

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