Why are 120-ohm termination resistors needed, and what happens without them?

Each end of the CAN bus must be terminated with a 120-ohm resistor connected between CAN_H and CAN_L. These resistors serve two purposes, both essential for reliable operation:

Signal reflection prevention: The CAN bus is a transmission line. When a signal reaches the end of an unterminated bus, it reflects back and interferes with the original signal (constructive or destructive interference depending on the round-trip delay). At 1 Mbit/s with a 40-meter bus, the reflected signal arrives approximately 400 ns later — within the same bit period — causing the transceiver to see ringing or an incorrect voltage level. Termination resistors absorb the signal energy at the bus ends, preventing reflections. The 120-ohm value matches the characteristic impedance of a standard CAN twisted-pair cable, providing maximum absorption.

DC bias and recessive state definition: The two 120-ohm resistors in parallel create a 60-ohm load between CAN_H and CAN_L. In the recessive state, the transceiver's internal bias drives both lines to 2.5V through this load, establishing a well-defined common-mode voltage. Without termination, the recessive voltage is poorly defined and susceptible to noise.

Symptoms of missing or incorrect termination:

• Missing both terminators: the bus may appear to work at low speeds and short distances but fails intermittently under load or at higher speeds. CAN_H and CAN_L show ringing on an oscilloscope.
• Missing one terminator: reflections from the unterminated end cause bit errors that increase with bus length and speed.
• Wrong value (e.g., 60 ohm instead of 120 ohm): the total bus impedance is too low, overloading the transceivers' output drivers and causing voltage levels to fall outside the specification.
• Extra termination in the middle: creates impedance discontinuities that cause partial reflections.

A practical diagnostic: measure the resistance between CAN_H and CAN_L with the bus powered down and all ECUs disconnected except the two end nodes. You should read 60 ohm (two 120-ohm resistors in parallel). This is the first check when debugging a new CAN bus that is not working.