RS-485 termination and bias resistors: when are they needed and what values should I use?

Termination exists to stop reflections. The resistor matches the cable's characteristic impedance, about 120 Ω for typical RS-485 twisted pair, and it goes across A and B at the two physical ends of the cable only, never at every node. Whether you need it depends on edge speed against cable delay. Signal travels at roughly 5 ns per metre, so 60 m is about 300 ns one way. A bit at 19200 baud lasts 52 µs, so reflections die out long before the receiver samples. At that speed and length the bus will usually work unterminated.

Biasing solves a different problem. When no driver is active, a terminated pair sits near 0 V differential, which is inside the receiver's undefined band of ±200 mV. The receiver output can then chatter, and the UART sees false start bits. Bias resistors pull A and B apart to a defined idle state: a pull-up on the line that is positive in the idle state and a pull-down on the other.

Many current transceivers have built-in fail-safe receivers that treat an open, shorted or idle bus as a valid idle level. If every node uses one, external bias is not required.

The bias calculation with two 120 Ω terminators: they appear in parallel as 60 Ω. To hold at least 200 mV across 60 Ω you need 0.2 V / 60 Ω, about 3.3 mA. From a 5 V supply the two bias resistors in series must then total no more than (5 V - 0.2 V) / 3.3 mA, about 1440 Ω, so up to 720 Ω each. Values such as 560 Ω or 680 Ω are common. Fit the bias network at one place only, normally at the master.

Without termination the pair is only loaded by the receivers, and much weaker bias is enough.

Two wiring points matter as much as the resistors. Use a daisy chain with short stubs to each node, not a star, because every long stub is an unterminated branch. And provide a common reference: RS-485 receivers work over a common-mode range of -7 V to +12 V, so nodes on different supplies need a ground or reference conductor in the cable, or isolated transceivers if the ground potentials can differ by more than that.

Remember that termination costs drive current and bias current loads the bus like extra nodes, so add them deliberately. A quick check on a finished network: with everything powered and nobody transmitting, measure the DC voltage between A and B. It should be comfortably above 200 mV in the idle polarity. Then look at the waveform at the far end with a scope; ringing that lasts a noticeable part of a bit means the ends need terminating.