Troubleshooting a memory with inactive outputs Given: Q0–Q3 outputs are always LOW. Address lines A0–A7 mostly pulse, except A3 is constantly HIGH and A7 is constantly LOW. Two select lines exist: one shows pulse activity and the other is stuck HIGH. The device uses an active-LOW select AND gating scheme. What is the most likely fault and how can you test it?

Difficulty: Medium

One of the inputs to the active-LOW select AND gate may be stuck high for some reason; take both select lines LOW and check for pulse activity on the outputs, Q0–Q3. If the outputs now respond, the problem is most likely in the program or circuitry driving the select lines.
The problem appears to be in the two address lines that never change levels; the problem is probably in the program driving the memory address bus.
The output buffers are probably defective since they are all tied together; the common input line is most likely stuck LOW. Change the output buffer IC.
Since no data appears to be getting through to the output buffers, the problem may be in the X decoder; change the X decoder IC.

Correct Answer: One of the inputs to the active-LOW select AND gate may be stuck high for some reason; take both select lines LOW and check for pulse activity on the outputs, Q0–Q3. If the outputs now respond, the problem is most likely in the program or circuitry driving the select lines.

Explanation:

Introduction / Context:
When a memory’s data outputs remain at a constant logic level, the fault may be within the device or in the external selection/enable logic. Understanding active-LOW chip-select behavior and how it gates the internal read path is key to isolating the issue quickly.

Given Data / Assumptions:

Outputs Q0–Q3 are always LOW.
Address lines A0–A7 show activity except A3 is stuck HIGH and A7 is stuck LOW.
Two select lines exist; one shows pulses, the other is stuck HIGH.
Selection is via an active-LOW AND gate (both selects must be LOW to enable the device).

Concept / Approach:

If any input to an active-LOW select AND gate is held HIGH, the device never asserts its internal output drivers, regardless of address activity. Therefore, a stuck-HIGH select input can fully explain “outputs always LOW.” A practical test is to force both select lines LOW and observe whether Q0–Q3 now show activity; if they do, the device is likely fine and the fault lies upstream in the select-driving logic or code.

Step-by-Step Solution:

Recognize selection requirement: both active-LOW selects must be 0 to enable output.Observation: one select stuck HIGH → device never selected.Diagnostic: manually pull both selects LOW and monitor Q0–Q3 for pulses.If outputs now respond → external logic or firmware error is implicated, not the memory IC itself.

Verification / Alternative check:

Compare by probing the enable pins relative to datasheet truth table; if enable never satisfies the active condition, static outputs are expected. Additional check: inspect OE/CE/WE polarity and timing.

Why Other Options Are Wrong:

Address lines stuck levels (B): a stuck address bit alone would still produce changing data on other addresses; it does not force all outputs permanently LOW if the device is enabled.
Defective output buffers (C): less likely; all outputs LOW with never-enabled chip is simpler explanation.
X decoder failure (D): also possible in rare cases, but the stuck-HIGH select provides a direct cause consistent with the symptom.

Common Pitfalls:

Misinterpreting active-LOW enables—remember that LOW means “asserted.”
Overlooking simple selection faults before replacing ICs.

Final Answer:

One select input stuck HIGH preventing chip enable; force both selects LOW to test. If outputs then respond, the external select-driving circuitry is at fault.

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