Wiring, Termination & EMC
CAN Bus Wiring Requirements
The physical CAN bus cable consists of a twisted pair: CAN_H and CAN_L. The twist is not cosmetic — it is essential for electromagnetic compatibility.
| Parameter | CAN High-Speed (ISO 11898-2) | CAN Low-Speed (ISO 11898-3) |
|---|---|---|
| Max bus length | 40 m at 1 Mbit/s | Up to 1000 m at 40 kbit/s |
| Wire type | Twisted pair, 120Ω characteristic impedance | Single wire or twisted pair |
| Wire gauge | 0.34 mm² to 0.5 mm² typical | 0.5 mm² typical |
| Twist pitch | 30–50 mm recommended | Not critical |
| Shielding | Optional (used in harsh EMC environments) | Not required |
| Max stub length at 500 kbit/s | 0.3 m | Not applicable |
| Max nodes (electrical) | 32 standard transceivers | 32 |
Termination: The 120Ω Resistors
CAN requires exactly two 120Ω termination resistors — one at each physical end of the bus. These resistors are connected between CAN_H and CAN_L at the two nodes that are physically furthest apart on the bus.
Why 120Ω? The CAN bus cable has a characteristic impedance of approximately 120Ω. Placing a matching 120Ω resistor at each end absorbs the signal energy and prevents reflections that would corrupt data. The two 120Ω resistors in parallel give a total bus impedance of 60Ω, which the transceivers are designed to drive.How to Verify Termination
The quickest diagnostic check on any CAN bus:
1. Disconnect the bus from all ECUs (or turn ignition off if ECUs have high-impedance standby)
2. Measure resistance between CAN_H and CAN_L with a multimeter
3. Expected reading: 60Ω (two 120Ω in parallel)
| Measurement | Diagnosis |
|---|---|
| 60Ω | Correct — both termination resistors present |
| 120Ω | One termination resistor missing |
| 40Ω | Three termination resistors (one ECU has an extra — find it and remove) |
| ∞ (open) | Both termination resistors missing or cable broken |
| < 10Ω | Short circuit between CAN_H and CAN_L |
Common Mistake: Never place termination resistors on middle-of-bus ECUs. Only the two nodes at the physical ends of the bus should have them. Incorrect termination placement causes signal reflections at specific points on the bus, leading to intermittent bit errors that correlate with message timing — one of the hardest bugs to diagnose.
EMC Considerations
Automotive CAN buses must pass EMC testing per CISPR 25 (radiated emissions) and ISO 11452 (immunity). Key design practices:
- Keep CAN wires away from high-current paths: Maintain at least 100 mm separation from starter motor cables, alternator wiring, and ignition coil feeds.
- Use common-mode chokes: A ferrite common-mode choke on the CAN lines at each ECU connector filters high-frequency noise without affecting the differential CAN signal.
- Add ESD protection: TVS (Transient Voltage Suppressor) diodes on CAN_H and CAN_L protect the transceiver from electrostatic discharge events.
- Ground the shield (if shielded cable is used): Connect the shield to chassis ground at one end only to avoid ground loops.
Exercise: Measure the CAN bus termination resistance between CAN_H (OBD pin 6) and CAN_L (OBD pin 14) with the ignition off. You should read approximately 60Ω. Document your finding.