I am using ISO1042BDWV in my board. The schematic is given below. I know that the characteristic impedance of the CAN bus is 120 ohm.
The layout team routed CANH and CANL as 90 Ω differential. The CAN bus is operating at maximum speed of 1 Mbps only.
May I know will this 90 Ω makes any trouble?
The CANH and CANL lines will be terminated in a connector and from there it will travel through cable assembly.
Do I need to keep a 120 Ω resistor at the end of cable assembly?
May I know will this 90 Ω makes any trouble?
CAN is usually quite forgiving on trace impedance mismatches, especially when the transmission distances are short compared to the max "allowed" distances for the data rates (e.g. suggested max cable length is 40 metres for 1 MBaud).
But the extra 10-Ohm resistors (R607 & R608) and the 100p caps (C280 & C282) look a bit unusual to me. Note that having an RC network means limiting the rise time of the signals which then results in the maximum usable cable length. Typical rise time for a 1 MBaud bus is about 30 ns so the bandwidth is about 11.67 MHz. Make sure the RC network do not reduce the total bandwidth.
Also, you have a CM choke in series so it's better to put caps on both sides i.e. not on the bus-side only but on the driver-side as well.
Do I need to keep a 120 Ω resistor at the end of cable assembly?
That depends on the design of the whole network. CANbus normally allows an unterminated stub length of ~1 ft = 30 cm for 1 MBaud. If the network has 120 Ohm terminations on both ends and if the cable length of your equipment is less than 30 cm then you may not need any termination resistors. See the diagram from National Instruments below:
When I designed CANbus devices for Mercedes-Benz and MAN coaches I never used termination resistors because those German specs clearly defined the max stub (cable) length as well as the termination resistor requirement. They probably used termination resistors on both ends of the transmission line and since the stub lengths were short enough they didn't need termination resistors on nodes as well.
You will need the 120 Ohm resistor if your node is the last one on the bus.
This resistor prevents reflections.
I have doubt about those 10 Ohm series resistors and capacitors. Those are not required, and they may limit the capabilities. (R607 & C280)
I have never put special care in CAN bus routing. PCB traces are very short. CAB bus runs through single cores in a car wiring harnesss. Don't worry too much about that with normal speed CAN.
