RS-485 Termination Resistor
RS-485 requires termination at both ends of the trunk to match cable characteristic impedance (~120Ω). Missing or incorrect termination causes signal reflections, overshoot, and bit errors (CRC failures).
Common Causes
- No termination: trunk is unterminated or only one end has resistor
- Wrong value: using 220Ω, 330Ω, or other non-standard resistance
- Middle nodes terminated: only end nodes should have 120Ω
- Switch/DIP setting wrong: internal termination not enabled where needed
Fix Steps
- Identify the two physical ends of the RS-485 trunk (not logical ends).
- Install 120Ω resistor between A and B at each end; enable if built-in.
- Ensure no middle taps have termination; only end devices.
- Verify with multimeter: between A–B at idle, expect ~200–500 Ω (two 120Ω in parallel).
- If using bias resistors, ensure they do not conflict with termination.