Question E7A11:
What does “positive logic” mean in reference to logic devices?  

Choices:  
A: The logic devices have high noise immunity  
B: High voltage represents a 1, low voltage a 0  
C: The logic circuit is in the “true” condition  
D: 1s and 0s are defined as different positive voltage levels    

Correct Answer is  B: High voltage represents a 1, low voltage a 0  

Correct answer: B — High voltage represents a 1, low voltage a 0.

Why B is correct:
- Positive logic defines the mapping between voltage level and logic state as:
  - $V_H \Rightarrow 1$ (TRUE)
  - $V_L \Rightarrow 0$ (FALSE)
- In other words, higher voltage means logical 1, lower voltage means logical 0.

Why the other choices are incorrect:
- A: “High noise immunity” is a characteristic of a logic family (e.g., CMOS vs TTL), not of “positive logic” itself. Positive vs negative logic only defines the mapping of voltage to logic states.
- C: “The logic circuit is in the ‘true’ condition” describes a state, not the mapping. Positive logic doesn’t say whether the circuit is currently true; it says what voltage corresponds to “true.”
- D: “1s and 0s are defined as different positive voltage levels” is not required by positive logic. Positive logic does not require both levels to be positive; for example, 0 V (low) and +5 V (high) still qualifies. The key is the mapping high→1, low→0, not that both are positive voltages.

Useful rules of thumb:
- Positive logic: $V \ge V_T \Rightarrow 1$, and $V < V_T \Rightarrow 0$ for some threshold $V_T$.
- Negative logic: mapping is reversed: $V_H \Rightarrow 0$, $V_L \Rightarrow 1$.
- “Active-high” signals use positive logic; “active-low” signals (often shown with an overbar or suffix “_L”/“_n”) use negative logic for that control line.