PC Logic Supply Levels On most digital circuit boards inside a PC, which DC voltage rail is the primary supply used by TTL/CMOS logic ICs and many controllers?

Introduction / Context:
Personal computer power supplies provide multiple DC rails for different purposes. Recognizing which voltage powers the bulk of logic ICs helps in diagnostics and power testing. This question asks for the most common logic rail on traditional motherboards and add-in cards.

Given Data / Assumptions:

• Legacy AT/ATX designs with rails including +12 V, +5 V, +3.3 V, and negative rails.
• Many classic TTL/CMOS parts and older controllers use +5 V.
• Modern chipsets often use local regulators to derive lower voltages, but +5 V remains common for legacy logic and peripherals.

Concept / Approach:

Historically, +5 V is the primary logic supply for TTL families and numerous peripheral ICs. +12 V powers motors and some analog circuits; 0 V is ground reference; −12 V supports certain serial interfaces or analog sections. Therefore, the best single choice for “most circuit boards” is +5 V.

Step-by-Step Solution:

Verification / Alternative check:

Board silkscreens, regulator inputs, and PSU label diagrams confirm +5 V as a primary logic feed in many systems.

Why Other Options Are Wrong:

−12 V is niche; 0 V is ground, not a supply; +12 V is typically for drives and fans. “None of the above” is unnecessary.

Common Pitfalls:

Assuming newer low-voltage cores negate +5 V usage entirely; many interfaces and legacy devices still rely on +5 V.

Final Answer:

+5 volts