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THVD1450: RS-485 cable length vs. data rate graphs (difference between vendors)

Conor
Guru 12965 points
Part Number: THVD1450


Tool/software:

Hi,

I would like to confirm my understanding regarding RS-485 transmission distance.

In Analog Devices’ AN-960, the commonly cited graph shows that at 10 Mbps the achievable distance is only about 15 ft. On the other hand, in TI’s THVD1450 datasheet and application note SLLA431, the graphs and measured data show that 10 Mbps operation can be achieved over several tens of meters (e.g. 50 ft or more), and in some test conditions even close to 100 m.

Our customer is asking why there is such a large difference between ADI’s chart and TI’s chart.
My current understanding is:

  • ADI’s chart represents a conservative rule-of-thumb range derived from the RS-485 standard

  • TI’s chart represents measured results under specific cable/termination conditions, showing what can realistically be achieved

Therefore, the apparent difference comes from different assumptions rather than a contradiction.
Could you please confirm if this interpretation is correct?
Also, it would be helpful if you could clarify how TI officially positions the “standard guideline” versus the “measured results” shown in datasheets.

Thanks,

Conor

  • 0
    Guru 318760 points

    The AD chart comes from the RS-422 specification (TIA/EIA-422-B):

    This curve is based upon empirical data using a 24 AWG, copper conductor, unshielded twisted-pair telephone cable with a shunt capacitance of 52.5 pF/meter (16 pF/foot) terminated in a 100 ohm resistive load.

    The TI chart is probably based on a different termination (120 Ω at each end for RS-485) and RS-485 transceivers, but the details are not known. Only the application report bothers to mention what cable is being used.

    Also see figure 9-1 of SLLA272.

  • 0
    TI__Guru* 77381 points

    Conor,

    I don't have a 100% certain answer for you but my guess is there may be a difference in the acceptable jitter range. Where TI uses a larger value for it's base compared to ADI. ADI is more conservative probably targeting 0.1% jitter while TI is probably using the 1% jitter. In most systems you could probably get away with less than 5% jitter so I think the TI one is likely a better reference since being too pessimistic or conservative can make you set lower limits that can realistically be exceeded.

    -Bobby