TTL fan-out reality check: Evaluate — “A typical fan-out for most TTL devices is 9.”
Correct Answer: Incorrect
Introduction / Context:Fan-out in TTL is the number of standard inputs a gate output can reliably drive while meeting logic-level specifications. Historically, standard TTL families specify a fan-out of about 10 in the LOW state (sinking current), a widely quoted rule of thumb in textbooks and datasheets.
Given Data / Assumptions:
- Standard TTL (e.g., 74xx, 74LSxx) defines input current requirements and output drive capability.
- Fan-out is calculated as I_OL(max sink) / I_IL(max per input) for LOW, and I_OH(max source) / I_IH(max per input) for HIGH.
- Many TTL families arrive near 10 for LOW-state drive.
Concept / Approach:Typical TTL fan-out is taught as 10, not 9. Some variations exist by subfamily and temperature, but “9” is not the standard nominal. Therefore the claim is inaccurate in general contexts.
Step-by-Step Solution:
Check TTL data: a driver can sink roughly 16 mA at VOL spec, each input may require about 1.6 mA.Compute LOW fan-out ≈ 16 mA / 1.6 mA = 10.Thus the commonly accepted value is 10, not 9 → statement is incorrect.Verification / Alternative check:Consult standard 74xx and 74LSxx datasheets; many show IOL = 16 mA and IIL = 1.6 mA for the reference load, yielding fan-out ≈ 10.
Why Other Options Are Wrong:
- Correct / subfamily/voltage/rise-time qualifiers: These specifics do not shift the canonical figure to 9 as a typical value.
Common Pitfalls:Confusing worst-case margins or particular device variants with the widely taught nominal; ignoring that HIGH-state fan-out can differ from LOW-state fan-out.
Final Answer:Incorrect