Connecting TTL totem-pole outputs Complete the statement: Totem-pole outputs ________ be connected ________ because ________.
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Acan, in parallel, sometimes higher current is required
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Bcannot, together, if the outputs are in opposite states excessively high currents can damage one or both 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 devices
Explanation
Introduction / Context:TTL outputs often use a totem-pole (active pull-up and active pull-down) structure for fast transitions. Designers sometimes ask if two outputs can be tied together to increase current. With totem-pole outputs, this is unsafe.
Given Data / Assumptions:
- Both drivers are totem-pole TTL outputs.
- They may not be perfectly in phase or may drive opposite logic values.
- No explicit wired-OR capability is provided.
Concept / Approach:Totem-pole outputs actively drive HIGH and LOW. If two are connected and one drives HIGH while the other drives LOW, a direct low-impedance path from VCC to GND is formed, causing large contention currents (shoot-through), potential overheating, and damage.
Step-by-Step Solution:
Recognize that wired-OR/wired-AND requires open-collector/open-drain or tri-stated outputs with resistive pull-ups.Identify that totem-poles are not passive when “HIGH” and therefore cannot share a node safely.Conclude: never tie totem-pole outputs together unless specifically designed with bus-friendly tri-state arbitration.Answer the completion: cannot, together, because opposing states create damaging currents.Verification / Alternative check:Datasheets warn against paralleling totem-pole outputs; contention graphs show current spikes when phases differ by even nanoseconds.
Why Other Options Are Wrong:
- Parallel for higher current (option a/d): Unsafe with totem-poles.
- Series for higher voltage (option c): Not a valid digital design technique and would violate VOH/VOL specs.
Common Pitfalls:Confusing totem-pole with open-collector; only open-collector outputs may be safely wire-OR’d using external pull-ups.
Final Answer:cannot, together, if the outputs are in opposite states excessively high currents can damage one or both devices