Module 05 ~45 min Interactive

Termination & Fail-Safe Biasing

Two small resistor networks separate a reliable RS485 bus from an intermittent one: the 120 Ω terminators that kill reflections, and the fail-safe bias resistors that keep the idle bus in a defined state. Watch reflections form and vanish, then size a bias network yourself.

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A signal edge travelling down a twisted pair sees the cable's characteristic impedance (~120 Ω for typical RS485 cable). When it reaches the far end, any impedance mismatch reflects part of the energy back — ringing that can be misread as extra bits. A 120 Ω resistor at each end matches the line so the energy is absorbed instead of bounced.

End termination resistance120 Ω
Matched at 120 Ω — the edge is absorbed cleanly, no ringing.
Where to place them: exactly one terminator at each physical end of the bus — not on every node. Two 120 Ω across the pair present ~60 Ω total DC load to the drivers, which is by design.

On a half-duplex bus, there are moments when no driver is enabled — the bus is idle. With only terminators, the differential voltage collapses toward 0 V, which sits in the receiver's ±200 mV indeterminate zone. The receiver output then chatters randomly, and the UART sees phantom bytes.

Fail-safe bias fixes this with a pull-up on the non-inverting line (B/D+) and a pull-down on the inverting line (A/D−). These gently force the idle bus to a valid mark (> +200 mV), so an idle line reads as a clean logic 1.

Fail-safe bias network Vcc GND R_up B (+) 120Ω A (−) R_dn
Pull-up on B, pull-down on A, with the 120 Ω across the pair: idle bus is forced to a valid mark.

The bias resistors and both terminators form a divider from Vcc to GND. To guarantee a fail-safe mark, the idle differential voltage across the parallel termination must exceed the receiver threshold (use 200 mV; many designs target 250–300 mV for margin). Enter your values:

Idle differential voltage —
Fail-safe margin (need > 200 mV) —
Model: the two bias resistors (2R total in series with the termination) drop Vcc across the effective termination Rt. Idle Vdiff ≈ Vcc · Rt / (2R + Rt). Smaller bias resistors give a stronger mark but load the bus more and raise idle current — a trade-off.

RS232 point-to-point links need no bus termination. The cable is short relative to the (slow) edge rate, so reflections settle long before they matter. Termination and biasing are a differential-bus concern.

Symptom Likely cause Fix
Occasional glitches / extra bytes at high speed or long cable Missing or wrong termination → reflections 120 Ω at each end
Random garbage on an idle bus No fail-safe bias → indeterminate idle Add pull-up/pull-down bias
No communication at all, no damage A/B swapped Reverse the pair (Module 3)
Signal fine on scope but data corrupt Common-mode range exceeded Isolation / shared ground (Module 2 & 8)

Answer to reveal instant feedback.