Logic probe and pulser diagnosis: A probe on a gate's output shows a dim (weak) indication, and exercising each input with a logic pulser does not change the output indication. What fault does this most strongly suggest?
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AThe dim indication on the logic probe indicates that the supply voltage is probably low.
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BThe output of the gate appears to be open.
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CThe LOW indication is the result of a bad ground connection on the logic probe.
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DThe gate is a tri-state device.
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EThe clock frequency is too high for the probe.
Answer
Correct Answer: The output of the gate appears to be open.
Explanation
Introduction / Context:Logic probes typically display HIGH, LOW, or pulsing states, and some indicate a “weak” or indeterminate level with a dim LED. When a logic pulser applied to inputs fails to elicit any deterministic response at the output, a high-impedance or floating node is suspected.
Given Data / Assumptions:
- Probe indicates a dim/weak output level.
- Inputs are individually driven with a logic pulser.
- No output state change occurs.
- Board power and ground are otherwise present.
Concept / Approach:An “open” output (internally damaged transistor pair or disconnected pin) leaves the node floating. The probe senses neither a firm HIGH nor a firm LOW, resulting in a dim indication. Because the output stage is not actively driving, changing inputs does not propagate to the output node. A genuine tri-state could also float, but it is usually controlled by an enable pin and would be documented; here, the symptom is consistent with an internal open.
Step-by-Step Solution:Confirm probe ground and supply are good (quick sanity check).Pulse each input; observe no change at output → logic not coupling to node.Interpret dim/floating output as high-Z with no active drive.Conclude: output stage is open; replace the IC.
Verification / Alternative check:Scope the output; a floating node may show noise pickup. Ohmmeter to ground/VCC with power off may show abnormally high resistance compared to a known-good gate.
Why Other Options Are Wrong:Low supply would affect many nodes, not just one output, and the pulser would still move levels.
A bad probe ground gives erratic readings everywhere, not reproducible behavior on one gate.
Tri-state requires a disabled enable pin; check OE first, but the symptom here points to damage.
Probe bandwidth limits produce missed pulses, not a constant dim DC indication.
Common Pitfalls:Forgetting to verify output-enable pins on bus drivers; confirm OE is asserted before condemning the IC.
Final Answer:The output of the gate appears to be open.