**Part Number:**THVD1410

**Other Parts Discussed in Thread:**SN65HVD78

I am trying to understand where the 4000 foot (1200 m) maximum cable length comes from in the RS-485 standard. I have looked at various application notes and there is only reference to the maximum cable length. From *The RS-485 Design Guide* (SLLA272C), I see that the minimum required driver differential voltage is 1.5 V and the minimum receiver input differential voltage is 200 mV. In AN-1057, section 5, it states that the maximum cable length is the result of the voltage divider that the cable's DC loop resistance and the termination resistance create. I have analyzed this for a 4000 foot cable with 15 mOhm / foot resistance and the resulting differential voltage at the receiver is 750 mV, not 200 mV.

Am I missing something or is this limitation imposed for margin?