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SN65HVD234: Bus Biasing clarification for ISO11898-2

Dan Abbey
Prodigy 590 points
Part Number: SN65HVD234

Hi team, 

Please can you help with the below, 

I am looking at verifying a CANbus interface against ISO11898-2.  We are using the SN65HVD234 and the specific part of the ISO spec I am looking at is Bus Biasing Active/Inactive. 

Looking through the SN65HVD234 datasheet I can’t see anything about whether bus biasing can be turned on/off, or what happens to the biasing when the Rs line is pulled high and the the driver is switched off.

In normal operation the device will set CANH and CANL to nominal 2.3V for the Recessive state, this is Table 5 in ISO11898-2.

When Rs is pulled high the driver is switched off, but do the Recessive state voltages stay at +2.3V (bus biasing presumably remaining active), or do CANH/L drop to nominal 0V (bus biasing inactive), per ISO11898-2 Table 6?

Thanks and best regards,

Dan

  • 0
    TI__Guru 50115 points

    Hi Dan,

    The recessive bias for this transceiver is always active while the transceiver is powered. The bias does not turn off when the CAN driver is disabled (Rs = high). 

    Many older CAN transceivers may not disable this biasing in their low-power modes. I believe that this behavior is a more recent addition to the ISO standard as power management has become more prevalent in automotive settings. The focus previously was maintaining bus stability while some transceivers may be switched into different modes. 
    Also keep in mind that all 3.3V supplied CAN transceivers can only claim compatibility with ISO 11898-2 devices, as the specification itself does not include considerations for these types of devices. Therefore they cannot be compliant with the standard. 

    Let me know if you have any more questions.

    Regards,
    Eric Schott