where CAN bus resistors in a car? - Page 1
Forum section: electronics / general

author: k8943
author_rank: Regular Contributor
timestamp: December 21, 2018, 09:13:31 am
content: Am new to Canbus, have done some reading and am preparing for first project.Gather that require a 120ohm resistor at each "end" of the bus.My setup, though not large, won't be at all linear and duly wondering where to place the resistors. (See image attached: 1metre scale; 28AWG wires; transceivers at each dot.)Set me wondering as to where the resistors are placed in a car with many different CAN transceivers and quite a bit of wiring?

author: Rerouter
author_rank: Super Contributor
timestamp: December 21, 2018, 09:59:53 am
content: That network diagram is a bit of a mess, a Car canbus is meant to be either a stub bus or a piggyback bus, where the ends of the bus have 120 ohm resistors,In your case, you can fit one at your main control node and it will work just fine, The resistors really only matter for drops over 5 meters,

author: k8943
author_rank: Regular Contributor
timestamp: December 21, 2018, 11:44:03 am
content: Well that's easy. Thanks!In cars do they get away with a single resistor at the ECU? Or maybe have a second one at the other end of the car in a box by itself (so as never to be removed if a single device is faulty)?

author: Rerouter
author_rank: Super Contributor
timestamp: December 21, 2018, 12:01:32 pm
content: It depends a little on the application, OBD2 gateway modules always have one built in and expect the diagnostic device to have the other,In passenger buses, its not uncommon for them to have 2 physical resistors at the end of the bus, with stub plugs all along the wiring for things to connect in. so if a device is hooked up that has a resistor built in and cannot be turned off, its generally connected to an end plug with the physical resistor removed in its place.And in trucks, its generally, a physical resistor inside the canbus gateway module (they are becoming quite common, low speed bus + high speed bus + kline of to weird peripherals) and a plug mounted resistor at the end of the line.Most 250Kbps buses can handle up to 3 termination resistors (40 ohm bus) without issue, but some modules transceivers can act intermittently. Mercedes OEM gear is really sensitive to this.Older J1708 buses just relied on every node having a 10K resistance between A and B, and no more than about 32 nodes to a run.The resistors main purpose is to normalize the differential voltage to 0 when the bus is idle, A lot more transceivers are center biased these days, e.g. pulled to a 2.5V rail generated in the chip, rather than some bias divider per wire, but its still not perfect, take a given transceiver, and you may find 40mV difference across them at idle with no resistance,

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 24, 2019, 02:32:02 am
content: I do not now if this is also a thing with CAN, but with RS485 it is common to use low baud rates and "slew rate limited" transceivers if the bus topology is less than ideal. Both these measures mitigate the effect of reflections from the ends of cable stubs.

author: free_electron
author_rank: Super Contributor
timestamp: January 28, 2019, 02:12:31 pm
content: Quote from: k8943 on December 21, 2018, 09:13:31 amThat's not how you wire can ....Most can buses in cars are point to point without side taps. if there are side taps then they use a daisy chain configuration with terminators only at the ends.