Introduction / Context:
Open-collector TTL outputs allow multiple gates to share a wire with a single pull-up resistor. Understanding the logic effect of this connection is vital for building simple wired interconnects, interrupt lines, or bus arbitration signals.
Given Data / Assumptions:
- Each open-collector output can pull the line LOW but cannot drive it HIGH.
- A common pull-up resistor brings the line HIGH when no gate is pulling it LOW.
- Positive-logic convention applies: LOW means 0, HIGH means 1.
Concept / Approach:
If any one output pulls the line LOW, the shared line reads 0. The line goes HIGH only if all outputs release it. In positive logic, this is exactly the logical AND of the outputs’ “release” conditions, commonly referred to as a wired-AND. (In negative logic, it is a wired-OR.)
Step-by-Step Solution:
Tie multiple open collectors together with one pull-up.If any gate outputs 0 (sinking), the line is 0 → dominant LOW.Line is 1 only if all outputs are “open” (not sinking) → logical AND.Therefore the outputs are ANDed together in positive logic.
Verification / Alternative check:
Truth-table construction confirms that the combined line is 1 only if all individual outputs are 1 (non-sinking).
Why Other Options Are Wrong:
usually burn out: Open-collector is designed for this connection; it is safe with proper pull-up.produce more voltage: The voltage is set by VCC and the pull-up, not “more voltage.”produce more fan-out: Fan-out is an output drive/input load relationship, not increased by tying collectors.
Common Pitfalls:
Forgetting the required pull-up resistor.Confusing wired-AND (positive logic) with wired-OR (negative logic naming).
Final Answer:
are ANDed together
Discussion & Comments