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ISO1042-Q1: Resistance changing when CAN data is ON

ArTzy
Genius 16350 points
Part Number: ISO1042-Q1
Other Parts Discussed in Thread: ISOM8610

Tool/software:

Hi Experts,

Seeking your assistance on this query using ISO1042QDWRQ1 for CAN interface. It is terminated by a 120 ohm resistance using 2 OptoMOS switches:

When CAN output is ON and 120 ohm termination resistance is checked between CAN_H and CAN_L, the resistance keeps on varying. Continuously between 500 ohm and 30 ohm. Fluctuating random values. When CAN output is turned OFF, the resistance can be measured as 122 ohms. Why is resistance changing when CAN data is ON?

Thank you.

Regards,
Archie A.

  • +1
    TI__Mastermind 23685 points

    Hello Archie, 

    Thank you for reaching out. To start, the CAN bus should always have two termination resistors at the two farthest nodes on the bus. Both termination resistors will be 120-ohm so that the impedance of the bus reads 60-ohms when measured. Please confirm in your application that only two 120-ohm resistors are turned on and that the bus reads 60-ohms. 

    Also, I would like to recommend ISOM8610 to be used as a Opto-Emulator Switch.

    Best,
    Andrew

  • 0 in reply to Andrew J
    TI__Genius 16350 points

    Hi Andrew,

    Thank you for your guidacne.

    If I have 1 device that has configurable CAN termination, what is the ideal way to check CAN termination for such a device? Should data be turned OFF, termination enabled and checked for 120 ohm?

    For any CAN bus, should termination be checked in absence of any device CAN data and once confirmed should data be enabled?

    Regards,
    Archie A.

  • 0 in reply to ArTzy
    TI__Mastermind 23685 points

    Hi Archie, 

    Yes, the impedance of the CAN bus should be measured from CANH to CANL in the recessive state when no node is driving the bus. The impedance of the bus should read 60-oohms (two 120-ohms in parallel). The bus will likely have a fluctuating resistance if the measurement is taken while the bus is switching or a node is driving a dominant, 

    Best,
    Andrew

  • 0 in reply to Andrew J
    TI__Genius 16350 points

    Hello Andrew,

    Thank you for the information.

    Andrew J said:
    Yes, the impedance of the CAN bus should be measured from CANH to CANL in the recessive state when no node is driving the bus. The impedance of the bus should read 60-oohms (two 120-ohms in parallel). The bus will likely have a fluctuating resistance if the measurement is taken while the bus is switching or a node is driving a dominant

    Do we have application notes to follow on this?

    Thank you.

    Regards,
    Archie A.