Schottky Ttl 1970s Characterize Input Output Loading and Fan Out Rules
# Schottky TTL (early 1970s): Step 12 — Characterize Input/Output Loading and Fan-Out Rules
Characterize Input/Output Loading and Fan-Out Rules. TTL families, Schottky variants included, specify fan-out using the unit load convention: one unit load equals the standard input current a single standard-TTL input presents to a driving gate, and a family's fan-out rating states how many unit loads its output can drive while keeping V_OH and V_OL within specified limits.
## 1. Fan-Out in Unit Loads
TTL families, Schottky variants included, specify fan-out using the unit load convention: one unit load equals the standard input current a single standard-TTL input presents to a driving gate, and a family's fan-out rating states how many unit loads its output can drive while keeping V_OH and V_OL within specified limits. Standard TTL and 74S both specified a drive capability of roughly 10 standard unit loads for most gates. 74LS, because its lower internal currents meant each 74LS input itself presented a smaller load (roughly 0.25 to 0.5 of a standard TTL unit load, depending on the specific input), effectively supported driving significantly more 74LS inputs per output even with comparable or lower output drive current in absolute terms — a case where lower power on both the driving and driven side compounds favorably rather than simply canceling out.
## 2. Real Diagram: Fan-Out Loading Comparison Across Families
## 3. Mixed-Family Loading Rules
Mixing families on one board, as described in Step 10, required designers to convert across each family's own unit-load definition rather than assuming a single universal number, since a 74S output driving 74LS inputs, or vice versa, has to be checked against the actual output current capability (in milliamps, per the data sheet) against the actual input current draw of the specific receiving family, not against a generic fan-out count that implicitly assumes same-family loading. TTL data books of the era (Texas Instruments' and others') published these cross-family loading tables explicitly, because getting this wrong — exceeding a driving gate's actual current capability by mixing in more low-impedance loads than its rated unit-load count assumed — was one of the more common board-level design errors in mixed-family TTL systems.
## 4. Companion Diagram: Stage Sequence and Trend Summary
## Characterize Input/Output Loading and Fan-Out Rules's Place in the Process Lineage
This is step 12 of 20 in the Schottky TTL (early 1970s) series, working through how Schottky TTL's power profile translates into concrete fan-out and loading rules for board designers.