Reliable CAN bus wiring is a physical-layer discipline governed by three hard constraints — topology, stub length, and termination placement:
Key Takeaways
- CAN uses a linear (daisy-chain) bus only — every node taps a single trunk in series, and star, tree, or ring layouts are prohibited because branch reflections corrupt bit sampling.
- At 1 Mbit/s, individual stub length should stay under 0.3 m and total bus length under roughly 40 m; both limits relax as the bit rate drops.
- ISO 11898-2 requires a 120 Ω termination resistor at each physical end of the bus — two terminators total, never one and never three.
- Measuring ~60 Ω across CAN_H and CAN_L with power off confirms correct dual termination; ~120 Ω signals a missing terminator and ~40 Ω signals an extra one.
- Custom CAN harnesses for SAE J1939 and CANopen control stub length at the connector breakout, keeping drops short enough to preserve signal integrity at 500 kbit/s and above.
Engineering rule of thumb: put exactly two 120 Ω terminators at the two farthest ends of the trunk, keep every stub under 0.3 m at 1 Mbit/s, and extend the trunk to reach a node rather than branching off it.
Why CAN Tolerates Only a Linear Daisy-Chain Topology
CAN is a multi-drop differential bus defined by ISO 11898-2 as a single linear trunk. Each node connects to that trunk through a short stub, or drop, rather than through its own branch. The bus relies on every transceiver seeing a clean differential waveform within a single bit time, including during non-destructive arbitration where dominant and recessive states must settle across the full length of the cable.
Star, tree, and ring topologies break this. Each junction is an impedance discontinuity that launches reflections back along the trunk, and those reflections arrive at sampling points as overshoot, ringing, or false edges. A production CAN segment is therefore built as a single custom wire harness trunk with short, controlled breakouts to each node connector — not as a hub with radiating spokes. When a star is unavoidable, an active CAN repeater or hub is required to re-terminate each segment.
Stub Length Limits and Bit Rate
A stub is the unterminated length of cable between the trunk and a node. Because the cable's nominal 120 Ω impedance is interrupted at the open stub end, the stub behaves as a transmission-line discontinuity: a portion of the signal reflects, travels back to the trunk, and superimposes on the live waveform. When the round-trip delay along the stub approaches a meaningful fraction of the signal rise time, that reflection lands inside the sampling window and corrupts the bit.
The faster the bit rate, the shorter the bit time, and the shorter the tolerable stub. At 1 Mbit/s the canonical limit is roughly 0.3 m per stub, with cumulative stub length across the whole bus also capped. The reasoning behind the 120 Ω target and how it differs from 100 Ω Ethernet cable is covered in our guide to the characteristic impedance of CAN bus cable. The table below summarizes ISO 11898-2 / CiA-aligned guidance for common bit rates.
| Bit Rate | Max Bus Length (typical) | Max Individual Stub | Max Cumulative Stub |
|---|---|---|---|
| 1 Mbit/s | 40 m | 0.3 m | ~0.6 m |
| 500 kbit/s | 100 m | 0.6 m | ~1.5 m |
| 250 kbit/s | 250 m | 1.0 m | ~3 m |
| 125 kbit/s | 500 m | 1.5 m | ~6 m |
| 50 kbit/s | 1,000 m | 3 m | ~12 m |
Bus-length figures are propagation-delay limited and well established; the lower-rate stub allowances are industry-typical scaling rather than fixed standard values, so treat them as design ceilings and stay well inside them on EMC-sensitive builds.
Need a CAN Bus Harness Built to Spec?
Termination Placement — Two 120 Ω Resistors, No More
ISO 11898-2 mandates a 120 Ω termination at each physical end of the trunk to match the cable impedance and absorb the signal so it does not reflect. Two 120 Ω resistors in parallel present 60 Ω to the bus, which is why a powered-down, properly terminated segment reads approximately 60 Ω across CAN_H and CAN_L. A reading near 120 Ω means one terminator is missing; a reading near 40 Ω means a third resistor has been added somewhere on the bus.
Two schemes are common. Standard termination places a single 120 Ω resistor at each end. Split termination divides each terminator into two 60 Ω resistors in series, with a capacitor — typically 4.7 nF — to ground at the midpoint, which shunts common-mode noise and lowers radiated emissions on long industrial runs.
| Termination Scheme | Configuration | When to Use | Common-Mode Behavior |
|---|---|---|---|
| Standard | One 120 Ω resistor at each bus end | Automotive and short industrial runs | No common-mode filtering |
| Split | Two 60 Ω resistors in series at each end, 4.7 nF to ground at the midpoint | Long runs and EMC-sensitive nodes | Filters common-mode noise, lowers emissions |
CAN Wiring in Practice: Automotive, Heavy Equipment, and Industrial
In light vehicles, the drivetrain and OBD-II diagnostic networks run CAN at 500 kbit/s over a twisted pair, and the entire ECU network is built as a daisy-chained automotive wire harness with terminators integrated into the two end modules. SAE J1939 governs heavy-duty and commercial-vehicle networks, historically at 250 kbit/s and at 500 kbit/s under J1939-14.
Off-highway and agricultural equipment add sealing requirements, so J1939 backbones commonly terminate in Deutsch DT and DTM connectors rated for vibration and ingress; a sealed Deutsch wire harness keeps the trunk continuous while breaking out short stubs to each controller.
On the factory floor, CANopen (per CiA 301) and DeviceNet deploy the same physical layer through M12 5-pin or DB9 connectors per CiA 303 pin assignments, often in continuous-flex drag-chain cable. A drag-chain-rated industrial wire harness must hold stub discipline through the flexing section, where a long or shifting drop will degrade signal integrity faster than a static installation would.
Common Questions About CAN Bus Wiring
What is the maximum stub length for CAN at 500 kbit/s?
At 500 kbit/s, keep each unterminated stub under roughly 0.6 m and the cumulative stub length under about 1.5 m. These are design ceilings derived from the bit time and signal rise time, not hard standard limits, so shorter is always safer on noisy or long buses.
Can a CAN termination resistor go in the middle of the bus?
No — the two 120 Ω terminators must sit at the two physical ends of the trunk, not in the middle. A mid-bus terminator splits the cable into two unterminated segments whose open ends reflect signals, and it adds a third resistance in parallel that drops bus impedance below the matched value.
What happens if a CAN bus has three terminators?
Three 120 Ω resistors in parallel drop the effective bus impedance to roughly 40 Ω, which overloads the transceivers and weakens the differential voltage swing. The bus may still pass at low bit rates but will show rising error counts as speed or temperature increases.
Does CAN support star or branch topology?
CAN is specified for linear daisy-chain topology only; native star and ring layouts are not allowed. A star is only viable through an active CAN hub or repeater that re-terminates and re-drives each leg as an independent terminated segment.
How do you source a custom CAN harness with controlled stub lengths?
A custom CAN harness controls stub length at the connector breakout and integrates the two 120 Ω terminators into the end nodes or end connectors. Specify the bit rate, total bus length, node count and spacing, connector family (Deutsch, M12, or DB9), and whether split termination is required, and the build can be validated against the stub and length budget before production.
CAN reliability is decided at the harness, not in firmware: a single linear trunk, stubs held under the bit-rate ceiling, and exactly two 120 Ω terminators at the physical ends. Specify those three constraints correctly and the bus tolerates noise, vibration, and distance; get any one wrong and the failures appear as intermittent, hard-to-trace bit errors under load.
About the Author
Michael Wang
Senior Technical Engineer
As the technical lead at TeleWire, Michael bridges the critical gap between complex engineering requirements and precision manufacturing. With deep expertise in Design for Manufacturing (DFM) and signal integrity, he oversees the technical validation of custom interconnect solutions for mission-critical automotive, industrial, and medical applications.
