CAN and CAN-FD: topology, termination, bit timing

1. Engineering problem

CAN connects motor drivers, batteries and sensors over a shared pair of wires with built-in error detection. It works only if the bus is terminated and every node samples bits at compatible times.

2. When is this needed?

• Connecting motor controllers, a BMS or distributed actuators.
• When a CAN bus shows error frames or bus-off events.
• When configuring a microcontroller CAN peripheral.

3. Selection and design decisions

Sourced facts (Wikipedia, citing ISO 11898): high-speed CAN (ISO 11898-2) uses a twisted pair with a nominal 120 Ω characteristic impedance on a linear bus, terminated at each end with a 120 Ω resistor; the bit speed is up to 1 Mbit/s on classic CAN and 5 Mbit/s on CAN FD; a classic frame carries up to 8 bytes and a CAN FD frame up to 64 bytes; in CAN FD, data length codes 9 to 15 mean 12, 16, 20, 24, 32, 48 and 64 bytes.

4. Advantages and disadvantages

• Classic CAN is simple and universal; CAN FD carries more data per frame at higher speed and needs FD-capable nodes everywhere.
• Higher bit rate shortens the maximum bus length.
• More nodes increase capacitance and bus load.

5. Requirements and constraints

• Exactly two termination resistors, at the physical ends.
• Cable is twisted pair with the right impedance.
• Bit rate, sample point and (for FD) data-phase rate agree across nodes.

6. Architecture

node A ──stub──┬────────────── twisted pair (CANH/CANL) ──────────────┬──stub── node D
   120 Ω ───────┘   node B      node C                               └─────── 120 Ω

7. Implementation

1. Choose the bit rate from the bus load and length.
2. Lay out a linear bus; terminate both ends and nothing else.
3. Choose prescaler and segment lengths so the bit rate is exact (below).
4. Set the sample point (commonly 75 % to 87.5 %: check the device and transceiver guidance).
5. Measure the resistance between CANH and CANL with power off and view the waveform.

def bit_timing(clk, prescaler, seg1, seg2):
    tq = 1 + seg1 + seg2
    return clk / (prescaler * tq), (1 + seg1) / tq * 100
for p, s1, s2 in ((5, 13, 2), (4, 16, 3), (8, 7, 2)):
    rate, sp = bit_timing(40e6, p, s1, s2)
    print(f"prescaler {p}, seg1 {s1}, seg2 {s2}: {rate / 1000:.1f} kbit/s, sample point {sp:.1f} %")
DLC = {9: 12, 10: 16, 11: 20, 12: 24, 13: 32, 14: 48, 15: 64}
print("CAN FD DLC 0-8 =", "0-8 bytes; DLC 9..15 ->", [DLC[k] for k in sorted(DLC)])

Printed (checked by the site’s tests):

prescaler 5, seg1 13, seg2 2: 500.0 kbit/s, sample point 87.5 %
prescaler 4, seg1 16, seg2 3: 500.0 kbit/s, sample point 85.0 %
prescaler 8, seg1 7, seg2 2: 500.0 kbit/s, sample point 80.0 %
CAN FD DLC 0-8 = 0-8 bytes; DLC 9..15 -> [12, 16, 20, 24, 32, 48, 64]

Reading it: with a 40 MHz CAN clock all three settings give exactly 500 kbit/s (clock ÷ (prescaler × time quanta per bit)) but with sample points of 87.5 %, 85 % and 80 %. Choose per the transceiver and bus length. The DLC mapping line shows that CAN FD data lengths above 8 bytes are not linear (9 → 12, 15 → 64). The clock rate is illustrative.

8. Real examples

• AMR: wheel motor drivers and BMS on a 500 kbit/s CAN bus, terminated at the controller and at the last driver.

9. Failure scenarios

• Termination at every node (low resistance) or none at all, causing errors.
• A star topology with long stubs.
• Two nodes with different sample points that never agree.

10. Troubleshooting

1. Error frames: check termination, ground and bit rates.
2. Bus-off: look for a node with wrong timing or a shorted line.
3. Intermittent errors at speed: shorten stubs, add shielding or lower the rate.

11. Validation

• The example prints the lines shown (executed by the site tests).
• Resistance between CANH and CANL is about 60 Ω with power off.
• Error counters stay at zero over a long run with the motors active.

12. Engineering checklist

• Linear topology
• 120 Ω at both ends only
• Twisted pair
• Same bit rate and compatible sample point
• Bus load computed
• FD support on all nodes if used

13. Further references

• Wikipedia, CAN bus and CAN FD, read 2026-11-06 (secondary source; the standards are ISO 11898, which was not read).

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