Multiplexing/CAN Bus and Vehicle Networks
What the exam tests
Vehicle network types, CAN voltage levels, termination and network diagnosis.
Core
- CAN is a differential bus on twisted pair. CAN-H and CAN-L carry opposite signals; the difference is read by transceivers, giving immunity to noise.
- Recessive (logic 1) is about 2.5 V on both lines. Dominant (logic 0) drives CAN-H to about 3.5 V and CAN-L to about 1.5 V. A dominant bit always wins arbitration.
- Powertrain CAN commonly runs at 500 kbit/s; body CAN at 125 kbit/s. The bus needs termination, usually two 120 Ω resistors, giving about 60 Ω measured across CAN-H and CAN-L with power off.
- LIN is a single-wire, low-speed (up to 20 kbit/s) master-slave bus used for simple body devices.
- A gateway connects networks; a fault on one bus can produce symptoms across many systems.
- Test resistance with the battery disconnected, voltages with key on, and use a scope for distortion or shorted lines.
Worked example
Several modules cannot communicate. Measure resistance across CAN-H and CAN-L at the DLC with the battery disconnected: 120 Ω means one terminator is missing or the bus is open on one side. 0 Ω indicates a short between the lines. About 60 Ω suggests termination is correct, so scope the bus for stuck dominant or noise. Split the network at connectors to isolate the segment.
Traps
Measuring CAN resistance with the battery connected; probing data lines with a test light; assuming one non-communicating module is the cause; ignoring sleep mode; confusing LIN and CAN voltages.