CAN bus physical-layer interface of an ECU: the transceiver, termination, choke and ESD protection between the CAN controller and the twisted pair, sized by signalling rate, bus and stub length, and loop delay
The CAN physical layer of a control unit · transceiver, 120 Ω or split termination, common-mode choke and ESD clamp, and the bus length, stub length and loop-delay budget they have to fit
In a control unit, the CAN controller inside the microcontroller produces logic-level bits; everything between that and the two-wire bus is the physical-layer interface. On the board it is a transceiver chip and, where fitted, a common-mode choke and an ESD clamp at the connector, and at the ends of the bus a termination network; off the board it is the twisted pair, its length and the stubs to each node. Whether the network works at the chosen bit rate depends on this chain: the signal must reach the farthest node and come back within the bit, reflections from stubs must die out before the sample point, and noise picked up along the cable has to be kept out of the receiver.
The CAN and CAN FD physical-layer interface of an electronic control unit or other node: transceivers, bus termination (single 120 Ω or split), common-mode chokes and ESD/transient protection on the bus lines, cable, connectors, bus and stub lengths, galvanic isolation and the propagation-delay budget. Engine control units, their network counts and housings are covered on the Electronic control unit (ECU) page; stand-alone CAN controller ICs on the CAN bus controller page; RS-485 transceivers on the Line driver / transceiver page; Ethernet PHY chips on the Ethernet PHY chip page; stand-alone common-mode chokes on the Common-mode choke page; TVS diodes on the TVS diode page; interface cards and modules on the Communication interface board and Communication module pages.
CNFX editorial research · Sources checked October 13, 2026 · 3 sources
Bus length, stubs and termination: 40 m at 1 Mbps, 0.3 m stubs, and 120 Ω at each end or split into two 60 Ω halves
TI's application report on CAN physical-layer requirements summarises ISO 11898 high-speed CAN as 1 Mbps over 40 m with at most 30 nodes, a maximum unterminated stub of 0.3 m, and a shielded or unshielded twisted pair of 120 Ω characteristic impedance terminated by 120 Ω resistors at both ends, not on a node, so the bus keeps its termination if a node is unplugged. [1]
For buses over 100 m, TI's rule of thumb keeps signalling rate (Mbps) × bus length (m) at or below 50; it reports that lab tests on 1,000 m of 120 Ω twisted pair confirmed about 50 kbps. The standard's tolerated ground offset is a −2 V to 7 V common-mode range. Split termination replaces each 120 Ω resistor with two matched 60 Ω (±1 %) resistors and a capacitor to ground, with CL = 4.7 nF (typ.) giving a −3 dB point at 1.1 Mbps; transceivers with a Vref or Vsplit pin can stabilise the common-mode voltage through it. TI warns that a 1/4 W termination resistor would burn out if a bus line shorted to a 24 V supply. [1]
Stub length follows from the driver's edge rate: with a 50 ns transition time and 5 ns/m cable delay, the down-and-back delay is 10 ns/m, the critical length 5 m and the maximum unterminated stub one-third of that, about 1.67 m; slowing the edge to 160 ns with a 10 kΩ slope resistor on an HVD230 extends it to 16/3 ≈ 5.3 m. [1] Editorial calculation, with an example bus: a 250 m bus under the ≤ 50 rule allows 50 / 250 = 0.2 Mbps, between the table's 200 m and 500 m rows. [1]
Bus length (TI SLLA270, Table 1)
Signalling rate
40 m
1 Mbps
100 m
500 kbps
200 m
250 kbps
500 m
100 kbps
1,000 m
50 kbps
State the planned bit rate (and CAN FD data rate), total bus length, the longest stub, number of nodes, cable type, where the two terminations sit, and whether split termination is used.
02
The transceiver: CAN FD at 2, 5 and 8 Mbps, ±58 V bus-fault protection, a 1.2 ms dominant time-out and loop delay up to 210 ns
TI's TCAN1044A-Q1 is an automotive (AEC-Q100 Grade 1) CAN FD transceiver meeting ISO 11898-2:2016: it supports classical CAN and CAN FD at 2, 5 and 8 Mbps, 12 V and 24 V battery systems, ±12 V receiver common-mode input and ±58 V bus-fault protection, with undervoltage protection, thermal shutdown, a low-power standby mode with remote wake-up, high-impedance bus pins when unpowered, and I/O from 1.7 to 5.5 V on the V variant. [2]
The dominant time-out disables the driver if TXD is stuck dominant, so a failed node cannot block the bus; because CAN allows at most eleven successive dominant bits, it also sets the lowest data rate, 11 bits / 1.2 ms = 9.2 kbps. [2] With 1.7 V I/O the loop delays rise to 165–180 ns (typ.) and 255 ns maximum. [2]
TI's physical-layer guide totals the round trip a bit must make in a 10 m isolated network — isolator, driver, 10 m of bus, receiver, isolator, controller, and back again — as 618 ns, using 50 ns for the controller, 17 ns per digital isolator, 100 ns each for driver and receiver and 5 ns/m of cable, and lists optocouplers at up to 140 ns. [1] Editorial calculation: at 1 Mbps a bit lasts 1,000 ns and, with the sample point at 75 % of the bit as in TI's description, 750 ns are available; 618 ns uses about 82 % of it before oscillator tolerance and other variables are allowed for. [1]
TCAN1044A-Q1 parameter
Value
Loop delay TXD→RXD, recessive to dominant (VIO 2.8–5.5 V)
Specify the CAN FD data rate needed, supply (12 V or 24 V system), logic voltage, required bus-fault and ESD levels, standby and wake-up behaviour, and the loop-delay figure to use in the timing budget.
03
Keeping noise and ESD out: common-mode chokes on the line and a 24 V, 30 pF clamp at the connector
TI reports induced voltages of 10 V or more on industrial buses and shows unshielded twisted pair picking up noise from fluorescent lighting; it names a TDK choke, ZJYS81RS-2PL51(T)-G01, developed for CAN, as a remedy, along with shielded cable grounded at one point and single-point grounding of unused wires. [1]
At the connector, Nexperia's PESD2CAN protects both bus lines in one SOT23 package: reverse standoff voltage 24 V, diode capacitance 25 pF (typ.) and 30 pF maximum per line at 1 MHz, leakage below 1 nA (typ.), 10 nA maximum, at 24 V, peak pulse power 230 W and 5 A (8/20 µs), clamping at 34 V for 1 A and 41 V for 5 A, and ESD to IEC 61000-4-2 level 4 (more than 15 kV air, more than 8 kV contact), up to 30 kV, AEC-Q101 qualified. [3]
Editorial note: the clamp adds its capacitance to each bus line at every node, and TI's guide treats node capacitance as a limit on how closely nodes can be spaced; the clamp's 41 V clamping level also sits well inside the TCAN1044A-Q1's ±58 V bus-fault rating. [1][3][2]
List the protection on the bus lines at each node (choke, clamp, split-termination capacitor), the clamp's standoff voltage against the system supply, its capacitance per line, and the ESD and transient levels the ECU must pass.
04
CAN interface details to send
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Network: [CAN or CAN FD; arbitration and data rates]
Bus: [total length; longest stub; node count; cable]
Termination: [120 Ω or split; location]
Transceiver: [supply system 12/24 V; logic voltage; standby/wake]
Protection: [choke; ESD clamp standoff and capacitance; ESD/transient levels]
Isolation: [isolator type and delay]
Timing: [loop-delay budget; sample point]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
How long can a CAN bus be?
TI's guide lists 40 m at 1 Mbps, 100 m at 500 kbps, 200 m at 250 kbps, 500 m at 100 kbps and 1,000 m at 50 kbps, and keeps Mbps × metres at or below 50 beyond 100 m. [1]
What termination does CAN need?
120 Ω at each end of the 120 Ω twisted pair, or split termination of two matched 60 Ω resistors with a capacitor to ground, 4.7 nF (typ.) giving a −3 dB point at 1.1 Mbps. [1]
What does a CAN FD transceiver have to withstand?
TI's TCAN1044A-Q1 is rated for ±58 V bus faults, ±12 V receiver common mode, ±10 kV HBM and ±8 kV contact ESD on the bus pins, and CAN FD at up to 8 Mbps. [2]
Which ESD clamp suits a CAN bus?
Nexperia's PESD2CAN protects both lines with a 24 V standoff, 30 pF maximum capacitance per line and IEC 61000-4-2 level 4 ESD protection. [3]
Sources and what still needs a direct check
Sources checked 2026-10-13. Three sources from two publishers: two Texas Instruments documents, the application report SLLA270 on CAN physical-layer requirements and the TCAN1044A-Q1 CAN FD transceiver datasheet, and Nexperia's PESD2CAN CAN bus ESD protection diode datasheet.
