CAN Bus on a PCB (Please help)

Discussion in 'General Electronics Chat' started by custm-1, Mar 28, 2011.

  1. custm-1

    Thread Starter New Member

    Mar 28, 2011
    I'm new to designing PCB's.
    I don't really understand the highspeed requirements on a PCB.

    My question is if I am adding a trace for a CAN Bus signal on my PCB is there anything special I need to do?

    My CAN Bus will be the standard ISO 11898.
    I need to go from one point on the PCB to a DB-25 connector located about 6 inches of PCB trace in length.

    I downloaded Z-calc and I kind of understand how to input my PCB data to get a specified Differential impedance.

    What I'm asking is: What should my Differential impedance be for the CanBus ISO 11898?
  2. ifixit

    Well-Known Member

    Nov 20, 2008

    The CAN bus data lines are normally terminated with 120Ω at each end of the bus. The resistors go between the CANL and CANH lines.

    If your PCB has a ground layer under the tracks then use the formula that takes that into account. They look like two 60Ω tracks with respect to ground.

    If the tracks are isolated from ground then only the differential impedance of 120Ω applies. Use the formula for two rectangular, parallel conductors.

    The 1MHz speed is quite slow by todays standards, so the layout is not too critical. However, you should do the best you can to minimise the effects of crosstalk from other signals. Run the traces parallel to each other and maintain the required spacing everywhere along the route.

    Read the ebooks on this site.
    Google: pcb trace impedance calculator
    Google: can bus tutorial