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Positive and negative logic

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.

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

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.
  Positive logic       Negative logic
  +5V ---- 1           +5V ---- 0
   0V ---- 0            0V ---- 1

Example: a circuit has this voltage table:

ABY
LLL
LHL
HLL
HHH
  • 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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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