Digital I/O architecture (input port) In a typical microprocessor system, which circuit is used at the interface of an input port so that external device data can be conditionally placed onto the CPU data bus only when the port is selected?

Introduction / Context:
Unlike output ports, which must store and continuously drive a value, input ports must share the CPU data bus with many other devices. The hardware must therefore connect the external input to the bus only at the right time to avoid contention.

Given Data / Assumptions:

• Many devices share the data bus.
• Only the addressed input port should drive the bus during a read.
• Control signals include RD̄ and chip-select generated by address decoding.

Concept / Approach:
A tristate buffer presents three states: logic 0, logic 1, and high-impedance (Hi-Z). When not selected, the buffer is disabled and its output is Hi-Z—electrically disconnected from the bus. When selected during a read, the buffer enables and drives the bus with the device's data, preventing bus fights.

Step-by-Step Solution:

Verification / Alternative check:
Logic analyzer traces show only one device driving the bus during a read; bus lines float or are pulled when the port is not selected, proving effective tristate control.

Why Other Options Are Wrong:

• Decoder: Selects the device but does not isolate it from the bus.
• Latch: Stores data, appropriate for outputs; does not provide Hi-Z isolation by itself.
• None of the above: Incorrect because tristate buffer is standard practice.

Common Pitfalls:
Leaving multiple input sources enabled simultaneously causes bus contention. Always gate the buffer with address select and RD̄.

Final Answer:
tristate buffer