Digital electronics – tri-state outputs on shift registers When the output of a tri-state (three-state) shift register is disabled, what electrical state is presented at the output pins?

Introduction / Context:
Tri-state (three-state) outputs are common on shift registers and bus-oriented digital ICs. They allow multiple devices to share a data bus without interfering with each other. Understanding what “disabled” means at the electrical level is essential for reliable system design and troubleshooting.

Given Data / Assumptions:

• A shift register with tri-state outputs (for example, OE or G control).
• Output enable (OE) can enable the drivers or place them in the disabled condition.
• No external pull-ups or pull-downs are assumed unless stated.

Concept / Approach:
Tri-state logic has three possible output conditions: logic HIGH, logic LOW, and high-impedance (often called Hi-Z). When the output is disabled via the OE control, the internal output transistors are turned off so that the pin neither sources nor sinks current. This makes the pin effectively disconnected from the internal driver, allowing another device to drive the line, or allowing the line to float to a level set by external resistors.

Step-by-Step Solution:
Identify output control: If OE is inactive, internal P- and N-devices are OFF.With both devices OFF, the output driver does not drive HIGH or LOW.Therefore, the pin is in a high-impedance (Hi-Z) state, suitable for bus sharing.

Verification / Alternative check:
Consulting typical datasheets, the disabled output is specified as Hi-Z with only leakage current limits noted; logic thresholds are undefined because the IC is not actively driving the level.

Why Other Options Are Wrong:
Float state / float or high-impedance state: “Float” is colloquial and ambiguous; the formal and correct electrical term is “high-impedance state.”LOW state / pulled to VCC internally: A disabled tri-state driver does not force a defined logic level unless external biasing is provided.

Common Pitfalls:
Assuming a disabled output equals logic LOW; in bus systems this can cause contention if multiple devices attempt to drive simultaneously. Always use proper OE control and, if needed, external pull-ups or pull-downs to define a default level.

Final Answer:
high-impedance state