Bus interfacing: To emulate an open-collector output on a shared bus using a standard logic device, each output should drive a separate external transistor stage acting as the open collector.

Introduction / Context:
Open-collector (or open-drain) outputs allow wired-AND (wired-OR with active-low logic) connections on a bus. If a logic IC lacks native open-collector outputs, designers can create the behavior with external components.

Given Data / Assumptions:

• Need to share a bus line among multiple outputs safely.
• Require the ability for any device to pull the line low while a pull-up resistor returns it high.
• Standard push-pull outputs cannot be tied together safely.

Concept / Approach:
An external NPN transistor (collector to bus, emitter to ground) implements the low-side pull. The logic device drives the base (often via a resistor). A single pull-up resistor on the bus restores HIGH. This mirrors open-collector behavior and supports wired-AND signaling.

Step-by-Step Solution:
Add an NPN transistor per output line.Connect collector to the bus node; emitter to ground.Insert a base resistor from logic output to transistor base.Use a shared pull-up resistor from bus node to Vcc.

Verification / Alternative check:
Scope the bus: multiple outputs can assert LOW without contention, and the pull-up restores HIGH when all are inactive.

Why Other Options Are Wrong:
A series resistor does not prevent opposing drivers from fighting. LEDs and plain CMOS buffers do not provide open-collector behavior. Diode-ORing is not equivalent to open-collector with proper logic levels.

Common Pitfalls:
Forgetting proper base current limiting; omitting the bus pull-up; mixing push-pull and open-collector drivers on the same net.

Final Answer:
discrete transistor