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RS-485

A differential, multi-drop physical layer for serial data — think "UART that survives long cables and noise." A transceiver turns the MCU's UART into a balanced A/B pair; many nodes share one bus, usually half-duplex.

Pros & cons​

ProsCons
Long distance (up to ~1200 m)Usually half-duplex (one talker at a time)
Noise-immune (differential)Needs direction control (DE/RE)
Multi-drop (32+ nodes, more with low-load parts)No built-in addressing — needs a protocol
Simple, cheap transceiversTermination + bias resistors required

Use cases​

  • Industrial sensor/actuator networks, Modbus RTU, building automation, long factory-floor cable runs.

Hardware requirements​

  • An RS-485 transceiver (e.g. MAX485, THVD).
  • Direction control: a GPIO driving DE/RE (or an auto-direction transceiver).
  • 120 Ω termination at both ends, plus fail-safe bias resistors so an idle bus reads a defined level.
  • Twisted pair; common ground or isolation between distant nodes (ground offsets are real).

Software requirements​

  • Assert DE before transmitting, and release it after the last byte has shifted out (watch the timing — releasing early truncates the frame).
  • An addressing/framing protocol on top (e.g. Modbus): node addresses, CRC, turnaround timing.
Top RS-485 failures

Forgetting to toggle DE (or releasing it too early), missing termination/bias, two nodes driving at once, or a ground potential difference between ends — use isolated transceivers for long runs.

See also​