Open-collector TTL/CMOS output requirement To obtain a valid logic HIGH or LOW from an open-collector gate, what external element must be connected between +VCC and the collector terminal?

Introduction / Context:Open-collector (TTL) and open-drain (CMOS) outputs are widely used for wired-OR connections, level interfacing, and bused signals. Unlike totem-pole outputs, an open-collector stage cannot drive a logic HIGH by itself; it only sinks current when asserted LOW. Understanding the needed external component is essential for correct operation.

Given Data / Assumptions:

• An open-collector output contains an internal transistor whose collector node is brought to the pin.
• When the transistor is ON, it pulls the output LOW by sinking current.
• When OFF, the node is otherwise floating unless externally biased.

Concept / Approach:To create valid logic levels, a passive element must pull the output node HIGH when the internal transistor is OFF. The standard solution is a pull-up resistor connected from +VCC to the collector. The resistor supplies a small current to raise the node to HIGH, while still allowing the internal transistor to sink the node LOW when it turns ON.

Step-by-Step Solution:

Verification / Alternative check:Datasheets for open-collector devices specify VOL at certain sink currents given a pull-up resistor; app notes show wired-OR by tying multiple open collectors to one pull-up.

Why Other Options Are Wrong:

Common Pitfalls:Choosing too small a resistor (excessive current and power) or too large (slow rise time, susceptibility to noise).

Final Answer:pull-up resistor