CAN-Bus am Oszilloskop
About this app
CAN-H, CAN-L und Differenzsignal am Oszi, 60-Ω-Abschluss stromlos messen, acht Fehlerbilder diagnostizieren.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
How high-speed CAN transmits
High-speed CAN according to ISO 11898-2 transmits every bit over two twisted wires. Recessive, logic 1, means CAN-H and CAN-L both sit at about 2.5 V. Dominant, logic 0, means CAN-H rises to about 3.5 V and CAN-L falls to about 1.5 V. The control units evaluate the difference: about 0 V recessive, about 2 V dominant. From about 0.9 V a bit counts as dominant, below 0.5 V as recessive. Each end of the line carries a 120 Ω terminating resistor, 60 Ω in parallel. At 500 kbit/s one bit lasts 2 µs.
What the app shows
On the left a line bus with five control units, twisted wires, two terminators, a test point, a multimeter and the battery. On the right an oscilloscope with CAN-H, CAN-L and the difference as a math channel, plus the receiver thresholds. An example message with identifier 0x1A0 runs across the screen. Below it the app decodes SOF, identifier, control field, data, CRC, ACK and EOF and marks the stuff bits. The decoder reads the bits from the difference and checks stuffing, checksum and acknowledgement.
Try it yourself
Inject common-mode interference: CAN-H and CAN-L move together, the difference stays clean. Disconnect the battery and measure 60 Ω between CAN-H and CAN-L. Then choose a fault: missing terminator, both missing, CAN-H to ground, CAN-L to battery plus, swapped wires, short between the wires or an open line. Compare scope picture, resistance and symptom each time. At 1 µs/div you see overshoot at the edges.
Frequently asked questions
- Why do you measure 60 Ω between CAN-H and CAN-L?
- Each end of the bus line carries a 120 Ω terminating resistor. Seen from the test point both are in parallel, and two equal resistors in parallel give half. Measure de-energised, with the ignition off and the battery disconnected. If the multimeter shows 120 Ω, one terminator is missing or the line is broken between the test point and the second terminator.
- Does the CAN bus work without a terminating resistor?
- If only one terminator is missing, the bus often keeps running. The open end of the line reflects, however, and the edges show overshoot and ringing. Depending on line length and bus load, sporadic communication faults occur. If both terminators are missing, communication is usually heavily disturbed.
- Why is the CAN signal immune to interference?
- Injected interference acts almost equally on both twisted wires, so both voltages shift together. The receivers only evaluate the difference, in which the common shift drops out. This holds as long as the wires stay within the receivers’ common-mode range, typically about −2 V to +7 V.
- How realistic are the values in the app?
- Levels, resistances and ringing are typical values for high-speed CAN according to ISO 11898-2. Vehicle, control units and message are a generic example. Real values depend on transceiver, line length and bus load; when measuring on a vehicle, follow the manufacturer’s instructions.
Subject: Automotive engineering
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