Module 3: Differential Buses: CAN & Ethernet
CAN levels and termination, Ethernet PHY/auto-neg, physical-layer debug.
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1conceptOn a healthy CAN bus, what differential voltage (CANH − CANL) distinguishes a recessive bit from a dominant bit?
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Correct. Recessive idles with both lines near 2.5 V so the difference is ~0 V; dominant drives CANH to ~3.5 V and CANL to ~1.5 V for a ~2 V differential.
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This swaps the two states: dominant is the driven ~2 V differential, not recessive.
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3.5 V and 1.5 V are the single-ended line voltages in the dominant state, not the differential values, and recessive is not 3.5 V.
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2.5 V is the single-ended common-mode bias of each line, not the differential between them.
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2conceptA single CAN node is powered up and transmitting onto a bus with no other nodes online. What state does it end up in?
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Without an ACK from any other node, the transmit error counter climbs past the thresholds: it cannot stay error-active indefinitely.
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Correct. With no peer to drive the ACK slot, each attempt increments the TX error counter; crossing 128 makes it error-passive and crossing 256 takes it bus-off.
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Bus-off is reached after the error counter crosses 256, not on the very first frame; the counter must accumulate first.
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A normally-transmitting node is not in listen-only mode, and its error counter does increment on missing ACKs.
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3calcA CAN bus is terminated with a 120 Ω resistor at each physical end. With the bus unpowered, what resistance should an ohmmeter read across CANH–CANL, and what does ~120 Ω indicate?
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The two 120 Ω terminators sit in parallel across the pair, not in series, so the healthy reading is ~60 Ω, not 120 Ω.
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Correct. Two 120 Ω resistors in parallel give ~60 Ω for a healthy bus; reading ~120 Ω means only one terminator is present, so the other end is missing or open.
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240 Ω would be the series value, which is not how the terminators are connected; healthy is ~60 Ω.
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0 Ω indicates a CANH-to-CANL short, not a healthy bus; a healthy idle bus reads ~60 Ω.
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4conceptOn a copper Ethernet link, the PHY's link LED is solidly on but the host receives no usable frames. What is the most reliable next check?
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A link LED says a partner was detected and auto-negotiation completed; it says nothing about whether frames decode correctly.
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Correct. Link-up does not equal good data; querying the PHY over MDIO reveals the actually-negotiated speed/duplex and CRC/error counters that expose a bad data path the LED hides.
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The LED can be lit while cabling is marginal enough to corrupt data, so an off LED is not the only cabling symptom; checking the PHY registers is more diagnostic.
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The magnetics can be intact (LED lit, link established) while data still fails for other reasons; swapping them blindly is a guess.
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5scenarioAn intermittent 1000BASE-T link drops under vibration; you suspect a partial cable break. Which physical-layer tool localizes the fault to a distance along the cable?
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An eye/mask test grades signal-integrity quality at the connector; it does not tell you how many meters down the cable the break is.
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Correct. TDR launches a pulse and times the reflection from the impedance discontinuity (the break), converting that round-trip time into a distance to the fault.
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A BER count quantifies how bad the link is but gives no location along the cable.
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Auto-neg over MDIO tells you the negotiated speed/duplex, not where a physical break sits.
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