Full-duplex wiring and the 400 kbps data rate
Full-duplex operation means the SN65HVD71DGKR uses two twisted pairs — one for transmit, one for receive — so the bus can send and receive simultaneously without a direction-control pin. At 400 kbps, the practical cable length on a typical RS-485 segment is around 4000 feet (1200 meters) before signal degradation sets in, assuming proper termination. The 70 mV hysteresis on the receiver input rejects common-mode noise that couples onto the pair in a motor-drive or PLC environment.