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Which waveform does a healthy high speed bus show?

  • A.Two mirrored traces around a 2.5 volt idle level
  • B.A single trace switching between zero and five volts, which represents the data being carried by the network
  • C.A flat line
  • D.One line rising to about 3.5 volts and the other falling to about 1.5 volts during each dominant bit transmitted

Answer: A

Both lines sit at two and a half volts when idle and separate by a volt each way during a dominant bit. The last option gives exactly those levels, which makes it a correct and more specific version of the same answer. A symptom that appears under one condition and not another is what narrows the diagnosis quickly. Most errors here come from carrying across a result from a neighbouring case where it does not hold. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

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