What Is CAN Bus Termination?
A CAN bus that works on the bench and fails in the field is very often a termination problem, and it is one of the easiest faults to confirm with a multimeter once you know what to measure. Termination is the pair of resistors that make a two-wire CAN network behave like a proper transmission line instead of an antenna that rings. This page explains why CAN needs exactly two 120 ohm terminators, where they belong, how to check them with a resistance measurement, and what the classic symptoms of wrong termination look like.
CAN Bus Termination in one line: CAN bus termination is a 120 ohm resistor fitted across the CAN_H and CAN_L wires at each of the two physical ends of the bus. The two resistors match the cable's characteristic impedance so signal transitions do not reflect back and corrupt data. Because the two 120 ohm terminators sit in parallel, a powered-down bus measured end to end reads about 60 ohms, which is the quickest field check that termination is correct.
Why Two 120 Ohm Terminators Are Required
A CAN network is a transmission line, and every transmission line reflects energy at an unterminated end the way a rope flicks a wave back from a fixed post. Those reflections superimpose on the real signal and, at CAN's fast edges, can turn a clean dominant or recessive level into an ambiguous one that a receiver misreads. A terminating resistor equal to the cable's characteristic impedance absorbs the arriving energy so nothing bounces back, which is why the standard calls for 120 ohms, matching typical twisted-pair CAN cable.
The resistors go at the two extreme ends of the linear bus, not in the middle and not on every node. A CAN bus is meant to be a single backbone with short stubs, so it has exactly two ends and therefore exactly two terminators. Putting a terminator on a device in the middle of the run, or on every node, over-loads the bus and drops the level the transmitters can achieve; leaving an end unterminated lets reflections through. Both mistakes degrade margin and both get worse as the bus gets longer or faster.
Measuring Termination With a Multimeter
The fastest confirmation is a resistance measurement, and it must be done with the bus powered down so you are reading resistors and not fighting live drivers. With power off, measure resistance between CAN_H and CAN_L at any convenient point. Two correct 120 ohm terminators appear in parallel, so the meter should read close to 60 ohms. That single number tells you a great deal before you ever plug in an analyzer.
The failure readings are just as informative. Roughly 120 ohms means only one terminator is present, so an end is missing its resistor or a terminator has failed open. A reading near 40 ohms suggests a third terminator has been added somewhere. An open circuit or a very high reading points to a break in the bus or both terminators missing, and a near-zero reading points to a short between the two conductors. Because the number is so diagnostic, checking termination resistance is the first thing to do on a CAN bus that will not communicate.
One caution: some devices contain a switchable or permanent internal terminator, so the 60 ohm expectation assumes exactly two are active on the whole segment. When commissioning, confirm which devices have termination enabled rather than assuming, because two field devices each quietly enabling their internal terminator plus an external pair is how a bus ends up over-terminated. This is the same discipline you apply to a terminator on any other fieldbus segment.
Symptoms of Wrong Termination in the Field
Missing or wrong termination rarely produces a clean, total failure, which is what makes it frustrating. More often it shows up as intermittent errors that rise with bus length, temperature, or traffic: sporadic error frames, occasional dropped messages, and nodes that go bus-off under load but seem fine when lightly loaded. A short cable in a warm cabinet can tolerate poor termination that the same design cannot tolerate on a long outdoor run, so the symptom appears only after installation.
Because the fault is marginal, it interacts with everything else on the physical layer. A bus that is slightly over-terminated has less drive margin, so it is more sensitive to a marginal ground, a long stub, or a noisy environment, and the reported error can look like a grounding or shielding problem when the root cause is an extra terminator. Confirming the 60 ohm reading first removes termination from the list of suspects so you are not chasing the wrong physical-layer fault.
When these physical-layer errors reach a monitoring system through a gateway, they present as increased retransmissions, missing periodic tags, or a node that periodically disappears and reappears rather than as a labelled termination fault. That is why the resistance check at the wire stays the ground truth: the trend in a time-series historian tells you which node is dropping and when, and the multimeter tells you whether termination is why.
Frequently Asked Questions
Why does a CAN bus read about 60 ohms end to end?
Because a correctly terminated CAN bus has one 120 ohm resistor at each of its two ends, and those two resistors sit electrically in parallel. Two 120 ohm resistors in parallel give 60 ohms. Measuring between CAN_H and CAN_L with power off should read close to 60 ohms. A reading near 120 ohms means one terminator is missing, and a reading near 40 ohms means an extra terminator has been added somewhere on the bus.
Can I put a terminator on every CAN node?
No. A CAN bus is a linear backbone with exactly two ends, so it needs exactly two terminators, one at each end. Adding terminators to middle nodes over-loads the bus, lowers the signal level the transmitters can drive, and reduces noise margin. Many devices ship with a switchable internal terminator, so during commissioning you should confirm that only the two end devices have termination enabled.
What are the symptoms of missing CAN termination?
Missing or wrong termination usually causes intermittent trouble rather than a clean failure: sporadic error frames, occasional lost messages, and nodes going bus-off under heavy traffic while working fine when lightly loaded. The symptoms get worse with longer cable, faster baud rates, or higher temperature, so a design can pass on the bench and fail after installation. A resistance check for the 60 ohm value confirms it quickly.
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