Schottky Ttl 1970s Compare Propagation Delay Against Ecl the Other Answer

# Schottky TTL (early 1970s): Step 15 — Compare Propagation Delay Against ECL, the Other Answer to Saturation

Compare Propagation Delay Against ECL, the Other Answer to Saturation. Step 4 drew the qualitative contrast between Schottky TTL's clamping approach and ECL's avoidance approach; this step puts final numbers on both sides of that fork using the fastest commercial parts each family reached by the mid-1970s.

## 1. Revisiting the Fork from Step 4 with Numbers
Step 4 drew the qualitative contrast between Schottky TTL's clamping approach and ECL's avoidance approach; this step puts final numbers on both sides of that fork using the fastest commercial parts each family reached by the mid-1970s. 74S's typical propagation delay of roughly 3 ns sat close to first-generation MECL's 2-4 ns range (the ECL series' Step 15), meaning the fastest Schottky TTL parts actually matched the earliest ECL parts on raw speed. But ECL's evolution did not stop there: MECL II (ECL Step 16) reached roughly 1-2 ns, and MECL 10K (ECL Step 17) pushed further while also improving manufacturability, so by the time Schottky TTL was in volume production, the fastest ECL parts had pulled ahead again — the clamping approach closed most, but not quite all, of the original 1960s saturation-delay gap.

## 2. Real Diagram: Three-Family Delay Timeline, Standard TTL to 74S to ECL

Propagation Delay Timeline: Standard TTL, 74S, and ECL Generations typical gate delay, log-ish scale for comparison std. TTL ~10ns 74S ~3ns MECL I ~2-4ns MECL II ~1-2ns MECL 10K speed (taller = faster) 74S closed most of the ECL speed gap but ECL's own generations kept advancing too

## 3. Why the Remaining Gap Did Not Matter to Most Designers
That ECL retained a speed lead did not make Schottky TTL a lesser achievement for its intended market, because the two families were never really competing for the same design slot except at the margins. A designer choosing between 74S and ECL for a speed-critical path was weighing a modest further speed gain against ECL's non-standard supply, continuous power draw, and transmission-line interconnect discipline (ECL Step 12); for the vast majority of applications where Schottky TTL's 3-10 ns range was already more than fast enough, the comparison never arose at all, and the real competitive question for Schottky TTL's future, taken up in Step 18, was not ECL but CMOS.

## 4. Companion Diagram: Stage Sequence and Trend Summary

Schottky TTL vs. ECL Delay Comparison how close 74S came to ECL speed without negative supply overhead 74S SCHOTTKY (3ns) Planar Silicon Substrate BJT Base Overdrive Excess Stored Charge Saturation Risk 74S gate delay reached 3ns, within striking distance of ECL 10K (2ns). INTERCONNECT OVERHEAD Planar Silicon Substrate PtSi Schottky Barrier V_f ≈ 0.35V < 0.7V Anti-Saturation Clamp 74S runs on standard +5V single supply; ECL requires -5.2V and -2V lines. ECL 10K (2ns) Planar Silicon Substrate Fast Totem-Pole Delay: 3ns (74S) / 9ns (74LS) Instantaneous Recovery For all but the largest mainframes, 74S avoided ECL's packaging complexity. Key Circuit Architecture Takeaway 74S pushed TTL performance close enough to ECL that only extreme supercomputers justified the added cost of current-mode logic.

## Compare Propagation Delay Against ECL, the Other Answer to Saturation's Place in the Process Lineage
This is step 15 of 20 in the Schottky TTL (early 1970s) series, closing the numerical loop on the ECL comparison introduced qualitatively in Step 4.

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