System buses – what is the primary purpose of tri-state or open-collector memory outputs when multiple devices share a common bus?

Introduction / Context:
Many memory chips connect to a shared data bus. If more than one device attempts to drive the lines simultaneously, contention can damage parts or corrupt data. Tri-state and open-collector outputs enable safe sharing by ensuring only one device actively drives the bus at a time.

Given Data / Assumptions:

• Multiple memory or peripheral devices connected to a single data bus.
• Chip select and output enable control which device is active.
• Bus pull-ups (for open-collector/drain) or high-impedance state (for tri-state) are available.

Concept / Approach:
A tri-state output has three states: logic 0, logic 1, and high-impedance (Z). When not selected, the device goes to Z, effectively disconnecting from the bus. An open-collector/drain output can only pull the line low; a shared pull-up resistor brings it high. Wired-OR/AND combinations become possible without active conflicts, enabling multiple devices to interface safely on common lines.

Step-by-Step Solution:

Verification / Alternative check:
Timing diagrams show OE controlling Z vs active drive. I2C uses open-drain sharing with pull-ups, demonstrating conflict-free multi-device operation.

Why Other Options Are Wrong:
They do not address contention prevention: speed, current, or internal simplicity are side effects at best, not the core reason.

Common Pitfalls:
Forgetting to deassert OE/CS on inactive devices; neglecting proper pull-ups for open-drain buses.

Final Answer:
isolate devices connected to a common bus