Logic probe diagnostics — dim output and no response to input pulsing A logic probe on a gate’s output shows a dim indicator. A pulser applied to each input terminal does not change the output indication. What is the most probable fault?

Introduction / Context:
Logic probes commonly display three states: LOW, HIGH, and mid/indeterminate (often a dim or flicker). When an output node is floating (open circuit), the probe may show a weak, unstable or dim indication. If input stimulation fails to move the output, the evidence points away from valid gate operation and toward a physical discontinuity at the output pin or inside the device.

Given Data / Assumptions:

• Probe shows a dim (indeterminate/mid-level) reading on the gate output.
• Pulsing each input (with a pulser) does not affect the output indication.
• Assume the probe and pulser are working and ground reference is sound.

Concept / Approach:

An open output results when the output driver is not electrically connected to the node (broken bond, lifted pin, cracked trace) or when an internal transistor pair has failed open. With no solid path to Vcc or ground, the node floats and couples small leakage or capacitive signals, yielding a dim or ambiguous probe reading. Stimulating inputs will not change a truly open output node.

Step-by-Step Solution:

Verification / Alternative check:

Use an ohmmeter (power off) to check continuity from the output pin to the connected net; inspect under magnification for cracked solder joints. Compare with a known-good identical gate on the board.

Why Other Options Are Wrong:

Supply voltage low: would likely affect multiple points and be diagnosed at the power pins, not just one output.

Bad probe ground: would corrupt all readings, including known references; also typically gives erratic, not consistently dim, behavior.

Tristate device: a high-impedance output can float, but input pulsing would toggle the enable if the device were functioning; the stated behavior fits a fault (open) rather than a valid high-Z condition.

Common Pitfalls:

Confusing dim indication with a valid mid-level analog signal; overlooking cracked solder joints or lifted pads; failing to verify probe/calibration on a known-good logic level.

Final Answer:

The output of the gate appears to be open.