Nominal logic supply rails What is the nominal dc supply voltage used by classic TTL and many CMOS families in 5 V systems?

Introduction / Context:
Digital logic families are associated with standard supply voltages. Classic 5 V logic remains common in legacy and mixed-signal designs even as lower-voltage CMOS (3.3 V, 1.8 V) has proliferated.

Given Data / Assumptions:

• Question refers to “TTL and CMOS” without qualifiers, implying the ubiquitous 5 V era.

Concept / Approach:
Standard TTL (74xx, 74LSxx) is specified at 5 V ± tolerance. Early CMOS 4000B series operated over a wide range (3–15 V) but was frequently used at 5 V for compatibility. Many 74HC/HCT families also target 5 V systems.

Step-by-Step Solution:
Identify the conventional supply level shared across many TTL and CMOS parts: +5 V.

Verification / Alternative check:
Look at datasheets for 74LS00 or 74HC00: recommended VCC ≈ 5 V.

Why Other Options Are Wrong:
+3 V: common in modern low-voltage CMOS but not “nominal” for TTL.+9 V and +12 V: used in analog or power domains, not standard for TTL logic rails.

Common Pitfalls:
Assuming all CMOS is 3.3 V; many CMOS families are 5 V compatible or specified at 5 V.

Final Answer:
+5 V