RS-485 Fail-Safe Bias Resistor Calculator
Enter the supply, receiver threshold, cable impedance, topology, and node loading. The calculator chooses standard bias resistors and checks the idle voltage, termination correction, unit-load budget, current, and resistor power.
Recommended pull-up and pull-down resistors464 Ω each (E96)
- Target idle differential voltage
- 0.3 V (300 mV)
- Limiting bias resistance
- 467.59
- Biased-end termination resistor
- 137.822 Ω theoretical; use nearest E96 value 137 Ω at the biased end. Equivalent biased-end load is 119.377 Ω (-0.52%). The unbiased end remains 120 Ω.
- Worst-case achieved idle voltage
- 0.302 V (302.471 mV)
- Achieved noise margin
- 0.102 V (102.471 mV)
- Maximum supported nodes
- 49
- Node-loading check
- 4 UL required, 6.138 UL available; the entered node count fits with 2.138 UL of headroom.
- Idle bias current
- 4.793 mA per bias location; 4.793 mA total.
- Bias-resistor dissipation
- 0.011 W per resistor; use at least 0.063 W with a 4x design margin.
Assumes one termination at each end and the RS-485 32-unit-load common-mode limit represented by 375 ohms.
External bias may not be required when every receiver has integrated idle-bus fail-safe behavior.
Check the transceiver data sheet because A and B labeling conventions can reverse which conductor is pulled high.
How to use this calculator
- Enter the lowest transceiver supply voltage available at the bias network.
- Enter the receiver threshold and the extra differential noise margin you want on the idle bus.
- Choose the cable impedance, single-end or dual-end biasing, transceiver unit load, and node count.
- Select the preferred resistor series, then check the recommended bias resistors and corrected termination value.
How the bias values are calculated
An idle RS-485 bus needs a known differential voltage when no driver is active. The external fail-safe network usually uses one pull-up resistor to the positive supply and one pull-down resistor to ground, with termination still present across the pair. The calculation here follows the common application-note model that treats the worst-case receiver load as 32 unit loads, or 375 ohms common-mode.
The target idle voltage is the data-sheet receiver threshold plus the margin you enter:
Vt = Vit + Vn
The loading term is:
A = 1 / 375 + 4 / Z0
Z0 is the cable characteristic impedance. The topology multiplier k is 1 for one bias location and 2 for biasing both ends. The largest theoretical bias resistor that can still meet the idle voltage is:
RBmax = (k × Vs / Vt + 1) / A
The calculator searches the selected E-series and chooses the largest standard RB at or below that limit while leaving a nonnegative RS-485 unit-load budget. Larger bias resistors draw less current and consume less load budget, but they also produce less idle differential voltage.
Why the termination value changes
At a biased end, the two bias resistors appear as a differential load of about 2 × RB across the line. Leaving a normal 120 ohm terminator in parallel with that load makes the end look lower than the cable impedance. The corrected termination is:
RT = (2 × RB × Z0) / (2 × RB − Z0)
For a single-bias network, this corrected value is used only at the biased end and the far end remains at Z0. For a dual-bias network, both ends are biased and both ends use the corrected value. The result row also reports the equivalent load after rounding to a standard resistor, because a rounded terminator can create mismatch.
What the checks mean
The achieved idle voltage is recalculated from the selected standard resistor:
Vab = k × Vs / (RB × A − 1)
The achieved noise margin is Vab − Vit. Remaining load capacity is estimated as:
nUL = 12000 × (1 / 375 − k / RB)
That available unit-load count is divided by the selected unit load per receiver to estimate maximum supported nodes. Idle bias current is approximated as Ib = (Vs − Vab) / (2 × RB) per bias location, and each pull-up or pull-down resistor dissipates Ib² × RB. The suggested power rating applies a four-times design margin.
What this leaves out
This is a lumped worst-case calculation, not a transmission-line simulation. It does not include cable resistance, propagation delay, stubs, reflections, distributed capacitance, resistor tolerance, temperature drift, ground-potential difference, or transient noise. It also assumes the standard 12 kOhm unit-load model; verify the actual input current of every transceiver on the bus.
Worked example
With the default 4.75 V minimum supply, 0.2 V receiver threshold, 0.1 V margin, 120 ohm cable, single-end biasing, 32 one-eighth-unit-load transceivers, and E96 parts, the target idle voltage is 0.300 V. The loading term is 1 / 375 + 4 / 120 = 0.036, so RBmax = (4.75 / 0.300 + 1) / 0.036 = 467.6 ohms.
The largest E96 value that fits is 464 ohms for both the pull-up and pull-down resistors. The corrected biased-end terminator is 137.8 ohms, so the nearest E96 value is 137 ohms and the far end remains 120 ohms. Rechecking the selected parts gives about 0.302 V idle differential voltage, 0.102 V of noise margin, 49 maximum one-eighth-unit-load nodes, 4.79 mA per bias location, and about 0.011 W in each bias resistor.
Common questions
What values should RS-485 bias resistors have?
They should be low enough to create an idle differential voltage above the receiver threshold plus margin, but high enough that the bias network does not consume the bus unit-load budget or waste current. For a 5 V, 120 ohm, single-end bus, common answers often land near 680 ohms, 560 ohms, or 470 ohms depending on the required margin and worst-case supply.
What is the difference between termination and fail-safe biasing?
Termination matches the cable impedance so signal edges do not reflect strongly at the ends of the bus. Fail-safe biasing creates a known differential voltage when all drivers are disabled. A bus can need both, and the biased-end termination may need correction because the bias resistors add load across the pair.
Should an RS-485 or Modbus bus be biased at one end or both ends?
Single-end biasing is common and keeps current lower, especially on short and medium runs. Dual-end biasing can hold the idle voltage more evenly on long runs, but it consumes twice as much unit-load budget and idle current. Bias should be applied intentionally, not randomly at many nodes.
How many one-eighth-unit-load transceivers can the biased bus support?
The maximum depends on how much of the 32-unit-load RS-485 budget is left after the external bias network. The calculator computes the remaining unit loads and divides by 0.125 for one-eighth-unit-load receivers. Stronger biasing lowers that number because smaller bias resistors draw more common-mode load.
Do modern true-fail-safe transceivers need external bias resistors?
Many modern receivers specify an idle, open, or short-circuit fail-safe output without external bias. External bias may still be used for interoperability or noise margin, but it adds DC loading and current. Check every transceiver data sheet before adding a network by habit.
Which RS-485 line should be pulled high?
The non-inverting line for the desired receiver idle state is pulled high and the other line is pulled low. Unfortunately, A and B labels are not used consistently across manufacturers. Confirm the data-sheet truth table or measure the receiver output before wiring the pull-up and pull-down.