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ISO1042-Q1: Number of nodes supported on ISO1042-Q1

John Louie
Prodigy 80 points
Part Number: ISO1042-Q1
Other Parts Discussed in Thread: ISO1042, ISO1050

Hello TI Team,

I was going through the datasheet and it mentions the number nodes this device is supported.

Is there a way to calculate how many nodes are supported on this device? I read on a NXP PCA82C250 CAN transceiver that can support up to 110 nodes.

Does the transceiver the ISO1042 is communicating with, affect the number of nodes it can support?

Thank you!

  • 0
    TI__Expert 6955 points

    John,

    In section 9.2.2.1 of the datasheet titled  "Bus Loading, Length and Number of Nodes"  there is an explanation for this. Essentially the number of nodes the CAN transceiver can support is a function of the bus load. In a perfect world, the ISO1042 could support 100 nodes but that doesn't account for non-ideal network properties such as line capacitance.

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    Prodigy 80 points

    Hello Lucas,

    Thank you for the prompt response.

    I was looking in the same section for the ISO1050 and it would appear to say the same thing.

    Does TI have a recommended approach to either measuring or predicting the impedance of the network (approximately of course)?

    Best regards,

    John Louie

  • 0 in reply to John Louie
    TI__Expert 6955 points

    John,

    Predicting the network properties will depend highly on how you're constructing the network. Say if you are using twisted pair cables for the bus, then that twisted pair should have a certain amount of capacitance, resistance, and inductance per unit of length. If you know the length then you can calculate those properties. 

    Other then that, you can measure a network with a network analyzer of which would tell you what you're dealing with. You may also benefit from asking questions specific to CAN over on the Interface forum. The isolation forum specializes more the isolation aspect of transceivers and such. 

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    Prodigy 80 points

    Lucas,

    Could you please reassign this chat to the Interface team so that I still have the conversation above as a reference?

    Thank you,

    John Louie

  • 0 in reply to John Louie
    TI__Expert 6955 points

    John,

    Sure thing, Someone from TRX now should be able to help you further with accessing a CAN network and it's properties.

    Respectfully,

    Lucas

  • 0 in reply to Lucas Schulte9
    TI__Guru 59762 points

     Hey John,

    Just to add on to what Lucas said, the amount of nodes is limited to the input resistance of each transistor. To calculate this, it will be a DC measurement and again, is affected by how many transceivers are on the bus at that time.

    Characteristic impedance doesn't affect the node count, but will affect considerations for termination schemes. This is an AC property that needs to be handled with the correct termination in order to mitigate signal reflections, and thus maintain correct signal integrity.

    Please let me know if you have any other questions.

    Regards,

  • 0 in reply to Eric Hackett
    Prodigy 80 points

    Hello Eric,

    Thank you for the response.

    Could you please go into detail or provide an app note which shows how to properly measure the input resistance of each transistor?

    Regards,

    John Louie

  • 0 in reply to John Louie
    TI__Guru 59762 points

    John,

    The input resistance is shown in the datasheet as an electrical parameter under "Receiver Electrical Characteristics". I'm currently working with our design team to understand how this is measured and will get back once I receive an answer.

    Regards,

  • 0 in reply to Eric Hackett
    TI__Guru 59762 points

    John,

    One way to measure this resistance is to measure the recessive voltage on the CANH or CANL pin with no loading. Then force another voltage and measure the current induced by the potential, then calculate the resistance using the voltage drop value and the current.

    Please let me know if you have any questions about this.

    Regards,