Typical logic supply on PC circuit boards — What DC voltage rail powers most TTL/CMOS logic on older motherboards and add-in cards?

Introduction / Context:
Classic desktop PCs used multiple rails from the PSU: +12 V, +5 V, −12 V, and later +3.3 V. Most legacy digital logic on motherboards, ISA cards, and many peripherals historically ran from +5 V. Recognizing this helps with diagnostics and interpreting schematics or power issues.

Given Data / Assumptions:

• Legacy platforms and add-in cards before widespread 3.3 V logic dominance.
• Standard TTL/CMOS logic families (LS/HC/HCT).
• Question asks the most common logic rail.

Concept / Approach:

While motors, drives, and fans commonly use +12 V, and some serial interfaces used ±12 V signaling (via regulators), the heart of earlier digital logic operated at +5 V. Many ICs and memory devices expected regulated +5 V.

Step-by-Step Solution:

Verification / Alternative check:

Motherboard silkscreens and PSU labels show +5 V as a primary rail. Bench measurements confirm logic IC Vcc pins near +5 V in legacy systems.

Why Other Options Are Wrong:

−12 V is for serial interfaces; 0 V is ground, not a supply; +12 V powers motors and some analog stages; “None” is incorrect because +5 V is the standard answer for the era.

Common Pitfalls:

Confusing modern +3.3 V and +1.x V core rails in newer systems with older PC generations; assuming the presence of −5 V (obsolete) is still required.

Final Answer:

+5 V.