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SN65HVD75: RS485 Rise Time and Bit Time Ratio

Hao Wang14
Expert 4415 points
Part Number: SN65HVD75

Hi Team,

The output rise time and bit time ratio for 250kbps, 20Mbps and 50Mbps is almost the same with the same load. Is there a maximum output rise time and bit time ratio for RS485 communication with different load? Thanks!

Best Regards!

Hao

  • 0
    TI__Guru 62920 points

    Hi Hao,

    A general rule of thumb is for the maximum transition time to be less than 1/3 of the total bit time.  The 'HVD72, 'HVD75, and 'HVD78 all meet this requirement, and you can see their individual tR/tF specifications given in separate switching characteristics tables (Section 7.7, 7.8, and 7.9 respectively).  These maximum transition times are a function of slew rate limiting that is implemented in the driver circuit.  Because of this, they will not vary much at different loads.  Different load resistances will tend to scale the output amplitude (higher resistance leads to higher amplitude) but the transition rate remains similar.  Different capacitive loads will only have a significant impact at very high capacitances (i.e., high enough to be the dominant factor in limiting the slew rate rather than the driver's slew rate limiting).

    I hope this information is clear and addresses your questions.  Please let us know if not.

    Regards,
    Max