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sn65hvd78 instead sn65hvd75 for 20msps and 60m cable length

Other Parts Discussed in Thread: SN65HVD78, SN65HVD75


We need data transmission rate upto 20msps and we need a transmission line length of 60-70meters with cat6 cable. The appropriate ic seems to be sn65hvd75. However, we have many sn65hvd78 with us so we prefer using sn65hvd68 instead sn65hvd65 in our application. the max rate of sn68hvd78 is 50msps and max length is 50m. When we use sn65hvd78 with 20msps rate, will it work for transmission length of 60-70m ? 


  • Mahmut,

    The maximum achievable data rate on an RS-485 bus is ultimately constrained by the length of the bus.  The stated speed ratings of the SN65HVD75 and SN65HVD78 (20Mbps vs 50Mbps) are simply the result of their difference in driver output rise and fall times.

    So for a given data rate (in your case 20Mbps), the maximum achievable bus length when using the SN65HVD78 will be at least as long as that when using the SN65HVD75.

    The downside is that, even though the SN65HVD78 is only sending data at 20Mbps, its driver output rise and fall times are the same as they would be if it were sending data at 50Mbps.  That is to say, faster than they need to be.  Therefore, at 20Mbps, the SN65HVD78 will radiate more high frequency noise than will the SN65HVD75, due to its higher slew rates.

  • Thank you Bart for your valuable contribution.