CAN Bus Termination: Why 120 Ω, When to Split, and How to Test
A CAN network that passes a bench test can still fail on the road. The classic pattern: a machine runs clean for weeks, then floods the log with error frames on hot days or at high bus load. Crews swap sensors and reflash controllers. The real fault costs less than a connector: one missing 120 Ω terminator.
This guide explains why the resistor matters, how to test a bus with a multimeter in 30 seconds, and when CAN FD needs split termination. Calculators are included.
Why a CAN bus needs two 120 Ω resistors
A CAN bus is a transmission line. The twisted pair has a characteristic impedance near 120 Ω. When a signal edge reaches the end of an unterminated cable, the energy cannot leave the wire. It reflects back and collides with the next bits. The receiver samples a mix of real signal and reflection. That mix produces bit errors, error frames, and retransmissions.
ISO 11898-2 defines the high-speed CAN physical layer around this cable impedance. A resistor equal to the cable impedance absorbs the edge energy at each end. Two 120 Ω resistors, one at each extreme end of the trunk, remove the reflection path. Nodes in the middle stay unterminated. Watch the difference:
The 30-second multimeter test
- Power the network down.
- Set the meter to resistance.
- Measure between CAN_H and CAN_L at any node.
| Meter reads | Meaning | Action |
|---|---|---|
| ~60 Ω | Two 120 Ω terminators in parallel. Correct. | None. |
| ~120 Ω | Only one terminator on the bus. | Add a terminator at the open end. |
| ~40 Ω or less | Three or more terminators. A node DIP switch is on. | Remove extras until the bus reads ~60 Ω. |
| Open / megohms | No termination at all. | Install a pair, one at each end. |
Field reports on engineering forums repeat the same two mistakes. Installers leave a DIP-switch terminator on at a middle node, and the bus drops to 30–40 Ω. Or a network runs for months on one resistor and fails only at high temperature or high bus load. Both faults hide well and read instantly on a meter.
The meter answers one question: is the termination right? It does not find chafed wires, corroded connectors, or intermittent shorts along the harness. For those faults, use our SAM CAN Bus Wire Tester (CBWT02). It tests the wiring itself and finds bad wires, connectors, and shorts.
How much signal reflects: the mismatch calculator
The reflection coefficient compares the load at the cable end with the cable impedance. An open end reflects 100% of the energy. Even a wrong-value resistor reflects a measurable share. Change the numbers and watch what the bus sees:
CAN FD and split termination
CAN FD moves data at 2–5 Mbps. Faster edges couple more common-mode noise into the pair, and drives or ignition systems nearby add more. Split termination keeps the same 120 Ω differential load but adds a noise drain. The resistor splits into two 60 Ω halves. A capacitor ties the midpoint to ground. Watch the amber noise leave through the capacitor while the blue data passes untouched:
The capacitor sets the corner frequency of the common-mode filter. Texas Instruments documents the formula in application note SLLA271:
Keep stubs short
The drop cable from the trunk to a node is a stub. A stub is an unterminated branch, so it reflects. The faster the edge, the shorter the stub must stay. A common rule keeps the stub delay under one tenth of the edge rise time. With typical cable at 5 ns/m: classic CAN at 500 kbps tolerates stubs near 1 m. CAN FD at 5 Mbps tolerates roughly 0.2 m. If your CAN FD upgrade fails on a harness that classic CAN accepted, measure your stubs first.
Fit the right terminator
For CANopen networks on RJ45, CiA 303-1 puts CAN_H on pin 1 and CAN_L on pin 2. Our terminators come wired for that map, tested, and marked:
- RJ45, pins 1+2, 120 Ω, 1%, soft silicone — pair, $20 for bench and indoor networks.
- RJ45, pins 1+2, CAN FD split 2×60 Ω + 4.7 nF, shielded, epoxy — pair, $41 for CAN FD and high-EMC installs.
Every order ships as a pair because a bus needs both ends terminated. Each unit is measured before it ships. See all 144 configurations, or request a custom build for DB9, M12, or other values.
Quick answers
Do I need a 120 Ω resistor on my CAN bus?
Yes, two of them. ISO 11898-2 high-speed CAN expects one at each extreme end of the trunk. Short lab setups sometimes run without them. Do not ship that condition.
Should my meter read 60 Ω or 120 Ω?
60 Ω with power off. That is two 120 Ω resistors in parallel. A 120 Ω reading means one terminator is missing.
Can a CAN bus work with only one resistor?
Often, at low speed and short length — until temperature, load, or cable length pushes it over the edge. The failure is intermittent, which makes it expensive to find.
Does every node need a terminator?
No. Only the two ends of the trunk. Extra terminators overload the transceivers and cut the signal amplitude.
My termination is correct and the bus still fails. Now what?
Suspect the wiring. Chafed insulation, corroded pins, and crushed segments produce the same intermittent symptoms. The CAN Bus Wire Tester checks the harness directly.