Terminating open-collector TTL outputs: Evaluate the statement: “A pull-down resistor must be used with open-collector TTL circuits.”

Introduction / Context:
Open-collector outputs are a classic TTL feature used for wired-AND logic and level interfacing. Because the transistor can sink current but cannot source it, an external resistor is required to define the logic HIGH level when the transistor is off. This question checks whether you know which direction that resistor goes.

Given Data / Assumptions:

• Open-collector output can pull the line LOW (sink), but cannot drive it HIGH.
• An external resistor determines the inactive line level.
• Loads may include multiple OC outputs sharing a bus (wired-AND).

Concept / Approach:
The correct termination for an open-collector line is a pull-up resistor to the positive supply. When the transistor is off, the resistor pulls the node HIGH; when on, the transistor sinks current and forces the node LOW. A pull-down resistor would not create a valid HIGH level and would fight the intended operation.

Step-by-Step Solution:

Verification / Alternative check:
Application notes and interface schematics consistently show pull-up resistors on open-collector outputs, often with values sized to meet rise time and current specs.

Why Other Options Are Wrong:

Common Pitfalls:
Choosing too-strong a pull-up that wastes power or too-weak that causes slow rise times; balance with bus capacitance and speed.

Final Answer:
Incorrect