Open-collector TTL outputs: What external component is required for proper logic-level operation on the output node?

Introduction / Context:
Open-collector TTL outputs present an NPN transistor collector at the pin. The device can pull the line LOW (sink current) but cannot drive it HIGH. This topology enables wired-AND connections and level shifting, but it requires an external component to establish a valid logic HIGH when the transistor is off.

Given Data / Assumptions:

• Open-collector output transistor can only sink current.
• No internal path to VCC is provided in this mode.
• Logic HIGH must be created externally.

Concept / Approach:
To produce a HIGH when the transistor is off, a resistor connects the output node to VCC, supplying a weak current that pulls the node up. When the transistor turns on, it sinks current through this resistor, forcing a LOW. The resistor value must balance rise time, fan-out, and power dissipation.

Step-by-Step Solution:
Identify output type: open-collector has no active pull-up.Add external element to create HIGH: a pull-up resistor.Confirm behavior: transistor off → HIGH; transistor on → LOW.

Verification / Alternative check:
Datasheets specify recommended pull-up ranges to meet VOH and rise-time requirements given bus capacitance and fan-out.

Why Other Options Are Wrong:
Pull-down resistor would hold the line LOW, defeating logic-high generation.“No resistor” cannot generate HIGH; the node would float.Arbitrary resistor or diode does not guarantee valid logic levels.

Common Pitfalls:
Using a pull-up that is too strong (excessive current when LOW) or too weak (slow rise time).

Final Answer:
a pull-up resistor