CAN-H · CAN-L · termination
CAN bus wiring diagram: CAN-H, CAN-L and 120 Ω termination
A CAN bus is one twisted pair that runs past every module, with a 120 Ω resistor at each end. Get the topology and the termination right and the bus is very robust; get them wrong and modules drop off at random.
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The layout
- One line, not a star: CAN-H and CAN-L run from the first module to the last, and every other module taps in with a short stub (well under a metre).
- The two wires are twisted together (about 30–40 twists per metre) so interference hits both equally and cancels out.
- A 120 Ω resistor between CAN-H and CAN-L at each physical end of the line — two in total. Many ECUs have one built in or switchable.
- Every module also needs its own power and ground; the bus does not carry power.
Checking the bus with a multimeter
| Measurement | Healthy | If not |
|---|---|---|
| CAN-H to CAN-L, power off | ≈ 60 Ω | 120 Ω: one terminator missing · ≈ 40 Ω: three terminators · 0 Ω: H and L shorted |
| CAN-H to ground, bus idle | ≈ 2.5 V | 0 V or 12 V: short to ground or to + |
| CAN-L to ground, bus idle | ≈ 2.5 V | Same as above |
| With traffic (meter averages) | H a little above 2.5 V, L a little below | Both stuck at one value: no module is talking |
Common faults
- CAN-H and CAN-L swapped at one module — that module never appears.
- Long untwisted stubs or a star layout — reflections and errors at higher bit rates.
- A terminator at a module in the middle of the line — the bus reads about 40 Ω and becomes unreliable.
- A module with a bad ground — it may pull the bus with it or drop off when the load changes.
Draw it in WireForge
- WireForge’s simulator works with power, ground and currents — it does not simulate CAN messages. Use it for the documentation and for the power side.
- Place the ECU and the modules (Custom block with your own pins: CAN-H, CAN-L, +12 V, GND).
- Draw CAN-H and CAN-L as a pair in two colours (e.g. yellow and green) and add Text notes for the 120 Ω terminators at the two ends.
- Wire each module’s power through its fuse and simulate it: the simulator checks that every module gets power and ground.
Questions
Why exactly 120 Ω?
It matches the characteristic impedance of the twisted pair, so the signal is absorbed at the end instead of reflecting back. Two in parallel give the 60 Ω you measure.
Do I need shielded cable?
Usually no: a well-twisted pair is enough in a car. Shielded cable is used in very noisy places; ground the shield at one end.
Which colour is CAN-H?
There is no universal standard; aftermarket ECUs often use yellow or white for CAN-H and green or blue for CAN-L. Follow your ECU’s documentation and keep it consistent.