Know-how · Networks
CAN FD
Same two wires, same arbitration, but the data phase accelerates once the bus has been won — and a classical controller on the same network will not forgive you.
In short
CAN FD (CAN with Flexible Data rate) extends classical CAN in two ways: the payload grows from 8 bytes to up to 64, and the data phase can switch to a higher bit rate after arbitration completes. Arbitration itself still runs at the slower nominal rate, because it depends on every node being able to see and react to a dominant bit within one bit time. This gives much higher throughput without changing the two-wire physical bus or the priority model. The cost is compatibility: a classical CAN controller cannot interpret a CAN FD frame and will treat it as an error, flooding the bus with error frames. Mixed networks therefore need either CAN FD tolerant hardware throughout, or careful partitioning. CAN FD is specified in ISO 11898-1.
01
What actually changed
Two things. The data length code is reinterpreted so payloads can be 12, 16, 20, 24, 32, 48 or 64 bytes rather than stopping at 8 — note the jumps, which is why a 40-byte payload does not exist and a message of that size is padded to 48. And a bit rate switch flag allows the controller to raise the bit rate for the data and CRC fields, then drop back for the acknowledgement.
What did not change is arbitration. Priority is still decided by identifier, bit by bit, with dominant winning, and that still has to happen at a rate slow enough for propagation across the whole bus within a bit time. The speed-up applies only after a node has won the bus and is talking alone.
- PayloadUp to 64 bytes, in defined steps rather than any length.
- Bit rate switchFaster data phase, nominal rate retained for arbitration.
- CRCExtended and strengthened for the longer frames.
- UnchangedTwo-wire bus, identifier-based arbitration, priority model.
02
Mixed networks, and what breaks
A classical CAN controller receiving a CAN FD frame does not ignore it. It sees a protocol violation and raises an error frame, which is destructive — it corrupts the transmission and triggers retransmission, and a node doing this continuously can take the bus down. This is the single most common surprise when CAN FD is introduced into an existing network.
The usual answers are to make the whole segment CAN FD capable, to use controllers with CAN FD tolerance that stay quiet rather than erroring during an FD frame, or to separate the traffic onto different segments behind a gateway. Which one applies is a vehicle architecture decision, not something a measurement tool can work around.
- Classical controllerTreats an FD frame as an error and disrupts the bus.
- FD tolerantRecognises and ignores FD frames without erroring.
- FD capableFull participation at both rates.
- Logging implicationThe interface must support FD, or the frames never reach the recording.
03
Why it matters for measurement
A 64-byte payload carries signals that previously had to be split across several frames with the reassembly done in the receiver. Fewer frames, less multiplexing, and timestamps that mean more because related signals genuinely arrive together rather than being stitched from separate messages sampled at different instants.
It also moves the practical ceiling on what can be put on the bus for a test. CAN FD is one of the transports XCP runs over, so the same shift raises what a calibration measurement raster can carry before Ethernet becomes necessary.
Standards & references
Check the source.
These primary sources define the formats and regulatory context discussed above. They do not certify or endorse VehicleLab products.
Questions
Frequently asked.
What does CAN FD stand for?
CAN with Flexible Data rate. The flexibility is the ability to switch to a higher bit rate for the data phase of a frame, after arbitration has completed at the nominal rate.
How much data can a CAN FD frame carry?
Up to 64 bytes, compared with 8 for classical CAN. The permitted sizes step from 8 to 12, 16, 20, 24, 32, 48 and 64, so payloads in between are padded.
Why is arbitration still slow in CAN FD?
Arbitration depends on every node on the bus seeing and reacting to a dominant bit within one bit time, which limits the rate by propagation delay across the network. Only after a node wins the bus can the rate safely increase.
Can CAN FD and classical CAN share a bus?
Not safely without care. A classical controller treats a CAN FD frame as an error and emits destructive error frames. The segment needs CAN FD capable or at least CAN FD tolerant controllers, or the traffic must be separated.
Do I need a different logger for CAN FD?
Yes. The interface and logging hardware must support CAN FD, otherwise the frames are not captured and will simply be absent from the recording.
Next step
Open one of your own files with us.
Send a measurement, an A2L or an EHANDBOOK container. We will open it in the instruments on a call, with your engineers in the room.