Flow Ctrl Tech Engineering

RS-485 Transmission Lines: 120-Ohm Termination Resistors & Fail-Safe Biasing Guide

Field guide to solving RS-485 serial communication failures: characteristic impedance matching, eliminating standing-wave signal reflections, and pull-up/pull-down line biasing.

  • Termination resistors (120 Ohm) must ONLY be installed at the two absolute physical ends of the daisy-chain cable.
  • Installing termination on middle nodes drags total bus resistance too low, causing transmitter driver overload.
  • Line biasing holds lines in an idle MARK state (>200mV differential) when all transmitters are in high-impedance tri-state.

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