Standard TTL output drive (HIGH-state sourcing ability): When a standard TTL gate output is HIGH, what current can it source and to how many typical TTL inputs can it reliably fan out?

Introduction / Context: TTL outputs have asymmetric drive: they can sink substantially more current when LOW than they can source when HIGH. Correctly estimating fan-out requires using the guaranteed IOH/IOL and the corresponding IIH/IIL of the receiving inputs.

Given Data / Assumptions:

• Standard TTL IOH(min) ≈ -400 µA (negative sign denotes sourcing).
• Standard TTL IIH(max) ≈ 40 µA per input at HIGH.
• Standard TTL IOL(max) ≈ 16 mA; IIL(max) ≈ -1.6 mA per input at LOW (for reference).

Concept / Approach: Fan-out at HIGH is the number of inputs supported by total available sourcing current divided by the worst-case HIGH input current per gate. With 400 µA available and 40 µA per input, HIGH-state fan-out ≈ 10. This is a canonical figure in many TTL references.

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