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RS-485 Termination & Bias Resistor Calculator

Calculate RS-485 bus termination resistors, bias resistors, maximum baud rate for cable length, propagation delay, and bias current consumption

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Formula

Rterm=120 Ω;Rbias≤Vcc⋅Rload/Vdiff−Rload2R_term = 120\,\Omega;\quad R_{bias} \leq \frac{V_{cc} \cdot R_{load} / V_{diff} - R_{load}}{2}
R_term— Termination resistor (match to Z₀) (Ω)
R_bias— Bias/failsafe resistor (Ω)
BR_max— Max baud rate ≈ 10⁸/L kbps (kbps)
t_pd— Cable propagation delay (ns)

How It Works

This calculator determines RS-485 termination resistor values for reliable differential signaling over long distances. Industrial automation engineers and building management system designers use it to eliminate signal reflections that corrupt data. Per TIA/EIA-485-A, RS-485 uses differential signaling with characteristic impedance of 100-120 Ohm (twisted pair cable). Termination resistors at both network ends absorb transmitted energy, preventing reflections that would otherwise return with 2x propagation delay. Without termination, a 100-meter cable at 1 Mbps exhibits reflections arriving 1 microsecond after transmission, causing bit errors when reflection amplitude exceeds 200 mV (the RS-485 noise margin). The specification mandates termination when cable length exceeds 1/10 of the signal wavelength: at 1 Mbps (1 us bit time, ~200 m wavelength), cables longer than 20 meters require termination. A 120 Ohm termination resistor reduces reflection coefficient from 1.0 (open) to under 0.1, eliminating 90% of reflected energy.

Worked Example

A factory floor SCADA system connects 8 RS-485 devices over 300 meters of Belden 9841 cable (120 Ohm characteristic impedance) at 115200 baud. Per TIA-485-A Section 7: Cable electrical length = 300 m / (0.66 x 3e8 m/s) = 1.52 microseconds round-trip. Bit period = 1/115200 = 8.68 microseconds. Since round-trip delay (1.52 us) < bit period (8.68 us), reflections settle within one bit time, but termination is still required for noise immunity. Install 120 Ohm 0.25W resistors at both physical ends (Master PLC and furthest slave). Power dissipation per terminator = V^2/R = (5V)^2/120 = 208 mW maximum (when one driver active). Total network DC load = 2 x 120 Ohm parallel = 60 Ohm, requiring 83 mA driver capability - within the 250 mA limit per TIA-485-A.

Practical Tips

  • ✓Per TIA-485-A, use termination resistors only at the two physical ends of the bus - never at intermediate nodes, as mid-bus termination creates impedance discontinuities
  • ✓For networks with frequent idle periods, use AC termination (120 Ohm in series with 10 nF capacitor) to reduce DC power consumption from 42 mW to under 1 mW
  • ✓Measure cable impedance with TDR if unknown; CAT5/6 is 100 Ohm while dedicated RS-485 cable (Belden 9841, Alpha 6453) is 120 Ohm

Common Mistakes

  • ✗Using 100 Ohm termination on 120 Ohm cable, causing 9% reflection coefficient versus 0% with matched termination - sufficient to cause 1-5% error rate at 1 Mbps
  • ✗Terminating only one end, which reduces reflection amplitude by only 50% and shifts the reflection point to the unterminated end
  • ✗Using 1/8W resistors that overheat when continuously driven - specify 1/4W minimum for reliable operation at 5V supply

Frequently Asked Questions

120 Ohm for standard twisted pair RS-485 cable per TIA-485-A. CAT5/5e/6 Ethernet cable has 100 Ohm impedance, requiring 100 Ohm termination. Always match termination to cable characteristic impedance within +/-10% for reflection coefficient under 0.05.
Exactly two - at both physical ends of the bus. A daisy-chain topology with 8 nodes at 50-meter intervals needs termination only at node 1 and node 8. Star topologies require redesign with repeaters, as multiple unterminated stubs cause cumulative reflections.
Yes, if matched to cable impedance. RS-485 cable: 120 Ohm. CAT5/6: 100 Ohm. Telephone twisted pair: 100-150 Ohm. Mismatched termination by 20% (e.g., 100 Ohm on 120 Ohm cable) creates reflection coefficient of 0.09, causing detectable eye diagram closure at rates above 500 kbps.

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