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ISO1050: CAN bus 30M @ 1Mbps question.

Part Number: ISO1050


We use ISO1050 for CAN bus application got 30M @ 1Mbps question.

Please kindly let me know why R1 = 120 Ohm / R2 = 120 Ohm is not work. And R1 = 60 Ohm / R2 = Open is Work.

The test condition is transmissions 1000 times. CAN bus SPEC termination resistors is R1/R2 = 120 Ohm or not?

R1 = 120 Ohm / R2 = 120 Ohm @ 1Mbps test fail. CH1 and CH2 is CANH and CANL

R1 = 100 Ohm / R2 = 100 Ohm @ 1Mbps test fail. CH1 is ISO1050 Rx. CH3 and CH2 is CANH and CANL.

R1 = 60 Ohm / R2 = Open @ 1Mbps test Pass. CH1 is ISO1050 Rx. CH3 and CH2 is CANH and CANL.

  • In reply to AJ Cheng:

    Dear AJ,

    Hi and good day to you as well.

    Generally, we don't see or recommend the use of single 60ohm resistors - unless the numbers of nodes are very small **and** the distance between them is also small. The simple reason for this is to control and minimize reflections from a long cable and for good signal integrity.

    We will be glad to help you with the layout, but to do that: we need the following info as well:
    1) System schematic
    2) How many nodes in total ? (1 node = 1 CAN transceiver + 1MCU)

    A schematic review/discussion is best handled over email and or a conference call. This is to protect your IP. Is it okay for me to contact you at the email address you have used for the my.ti account ?

    Best regards,

  • In reply to Abhi Aarey:

    Dear Abhi.
    Good day. Thank you for your reply and help.
    my.ti account email address is OK or you can provide your email address.
    I will provide those information for you and Thank you for always helping~~
  • In reply to AJ Cheng:

    Hi AJ,

    I will reach out over  to your email. Once we are done in email,  I will post a high-level summary here.

    Also, for completeness, I wanted to share an additional scope shot (courtesy Daniel Kisling; thanks Dan).

    This is with two ISO1050 EVMs, 120ohms on each side and separated by 20m cable with a lot of stubs (= unterminated sections) in between to deliberately make this a difficult test. Device 1 transmitting and listening and Device 2 listening. As you can see,  there is small noise (due to stubs), but despite this, the ISO1050 has no issues communicating.

    This addresses the termination side of the question as well as the cable length.

    Purple = CANH,

    Red/pink = CANL

    Yellow = RX1  (receiver output on device 1)

    Green = RX2  ( receiver output on device 2)

    This data leads us to believe that there may be a USB2CAN related issue or a software config issue.

    We can double confirm this when you can also test with two ISO1050s.

    Thank you. As mentioned I will reach out separately for schematic and layout review discussions. 


  • In reply to AJ Cheng:

    Dear AJ, Gary

    1) Schematics reviewed - sent you feedback

    2) Answered all test/debug questions in your email.

    3) Video : sent you feedback over email. Essentially - appears to be error frames being sent in the middle. Maybe a controller config (software issue), because if this were to be a hardware issue, then you will see this in every few bits and not just in the middle.

    4) Layout: Hard to review like this in general (best done over a call , selecting layers , traces and components) but in general , we can recommend the following:

    • supply and ground planes are clean
    • Decoupling caps (100nF) is placed as closed to the device as possible, including paying attention to the VIAs between power  and ground planes. (Vcc1-Gnd1; Vcc2-Gnd2)
    • CANH and CANL lines/traces are roughly equal/symmetric
    • CANH, CANL lines are not routed next to high-speed switching lines from Micro (can cause interference).

    Doing the tests you plan to do with PCB, EVM and USB2CAN module is a good idea.

    Please let us know if this addressed all your questions. Do let us know how it goes with the customer.  If there is flexibility to use both ISO1050s or both ISO1042, that will be best of course.

    If USB2CAN **has** to be used, and if there are only 2 nodes to be used, *and* if extensive data testing shows shows good performance without any bit errors (with effective termination = 60ohm), then that termination setup can be acceptable as well. It is not the normally recommended solution but can be made to work.

    Best regards,