Totem-pole output connection rules: Consider two TTL devices with totem-pole outputs. Can these outputs be tied together, and if not, why is such a connection discouraged?
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Acan, in parallel, sometimes higher output current is required
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Bcannot, together, if the outputs are in opposite states excessively high currents can damage one or both of the devices
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Cshould, in series, certain applications may require higher output voltage
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Dcan, together, together they can handle larger load currents and higher output voltages
Answer
Correct Answer: cannot, together, if the outputs are in opposite states excessively high currents can damage one or both of the devices
Explanation
Introduction / Context: TTL totem-pole outputs use complementary transistors to actively drive both HIGH and LOW. Unlike open-collector or tri-state outputs, they are not intended for direct wire-OR or parallel connection. Misconnecting them can cause severe contention currents.
Given Data / Assumptions:
- Totem-pole outputs actively source and sink current depending on the logic state.
- No built-in provision isolates one driver from another when both are enabled.
- We assume standard TTL without tri-state or open-collector configuration.
Concept / Approach: If two totem-pole outputs are tied together and one tries to drive HIGH while the other drives LOW, a direct low-impedance path from VCC to GND is created through the output stages. This “shoot-through” causes excessive current, potential overheating, incorrect logic levels, and device damage.
Step-by-Step Solution:
Identify output type: totem-pole = actively drives both 1 and 0.Tie two together: risk of opposite states creating a short path.Conclusion: Do not tie totem-pole outputs together; use open-collector or tri-state with proper bus control instead.Verification / Alternative check:
Application notes recommend tri-state for shared buses or open-collector with pull-ups for wired-logic.Why Other Options Are Wrong:
can, in parallel / can, together: Encourages unsafe contention and is not supported by device design.should, in series: Series connection does not yield a defined logic function or safe increase in voltage.Common Pitfalls:
Confusing totem-pole with open-collector; only the latter is intended for wired-logic with a pull-up.Overlooking enable/disable control required for tri-state busing.Final Answer:
cannot, together, if the outputs are in opposite states excessively high currents can damage one or both of the devices